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May 9, 2018 | Author: Rae Steven Saculo | Category: Mass To Charge Ratio, Electron, Ion, Atoms, Chemical Elements


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The Greek Concept of Atomos: The Indivisible AtomI continue to grow in my knowledge. Atomistic theory is prominent in some of the Hindu teachings in India. Around 440 BC, Leucippus of Miletus, in his lost book "The Greater World System," originated the atom concept. He and his pupil, Democritus (c460-371 BC) of Abdera, refined and extended it in future years. There are five major points to their atomic idea. Almost all of the original writings of Leucippus and Democritus are lost. About the only sources we have for their atomistic ideas are found in quotations of other writers. Democritus is known as the "Laughing Philosopher" because of his joyous spirit. He was a big man (relatively speaking) and enjoyed life tremendously. He also was very widely traveled, having reportedly visited Athens. One point: teachers often think that Democritus developed the atom concept. This is incorrect. In fact, Democritus wrote his version in a (now lost) book called "Little World System." More than likely, he titled it so out of deference to his teacher. So, be prepared for your teacher to want Democritus to be the correct answer. Want some advice? Don't argue with your teacher based on what some guy on the Internet said. This map shows the important towns of Greece, Turkey and Asia Minor around the time the atom concept was developed. It is about 250 miles as the crow flies between the Abdera and Miletus. At this time Greek philosophy was about 150 years old, having emerged early in the sixth century BC, centered in the city of Miletus on the Ionian coast in Asia Minor (now Turkey). The earliest known Greek philosopher was Thales of Miletus. The work of Leucippus and Democritus was further developed by Epicurus (341-270 BC) of Samos, who made the ideas more generally known. Aristotle (384-322 BC) quotes both of them extensively in arguing against their ideas. Much of what we know about their ideas comes to us in a poem titled "De Rerum Natura" (On the Nature of Things) written by Lucretius (c95-55 BC). This poem, lost for over 1000 years, was rediscovered in 1417. On the left is Aristotle and to the right is Epicurus. Point #1 - All matter is composed of atoms, which are bits of matter too small to be seen. These atoms CANNOT be further split into smaller portions. Democritus quotes Leucippus: "The atomists hold that splitting stops when it reaches indivisible particles and does not go on infinitely." In other words, there is a lower limit to the division of matter beyond which we cannot go. Atoms were impenetrably hard, meaning they could not be divided. In Greek, the prefix "a" means "not" and the word "tomos" means cut. Our word atom therefore comes from atomos, a Greek word meaning uncuttable. Democritus reasoned that if matter could be infinitely divided, it was also subject to complete disintegration from which it can never be put back together. However, matter can be reintegrated. Even though matter can be destroyed by repeated splitting, new things can be made by joining simpler pieces of matter together. The process of disintegration & reintegration is reversible. The idea of reversibility means that there must be a lower limit to the splitting of matter. If matter can be split infinitely, there is nothing to stop it from going on forever and destroying all matter. Only with a definite and finite lower limit to splitting do we keep a permament foundation of ultimate particles with which to build up everything we see. As Epicurus says: "Therefore, we must not only do away with division into smaller and smaller parts to infinity, in order that we may not make all things weak, and so in the composition of aggregate bodies be compelled to crush and squander the things that exist into the non-existent...." Epicurus also insisted on an upper limit for atoms - they are always invisible. Although no reason is given, it seems obvious enough: all matter that can be seen by humans is still divisible, therefore cannot be atoms. Point #2 - There is a void, which is empty space between atoms. Aristotle quotes Leucippus: "Unless there is a void with a separate being of its own, 'what is' cannot be moved- nor again can it be 'many', since there is nothing to keep things apart." In other words, there is empty space between atoms. In modern times, we would use the word vacuum, although the Greeks did not. Given that all matter is composed of atoms (the ultimate and unchanging particles), then all changes must be as a result of the movement of atoms. However, in order to move there must be a void--a space entirely empty of matter--through which atoms can move from place to place. Aristotle was opposed to the idea of the void and he based it on his concept of motion, today called the Aristolelian law of motion. This law held that the velocity of a body was directly proportional to the motive power and inversely proportional to the resistance of the medium the body was moving through. Another way to express this: the velocity of a body is proportional to the force acting on it divided by the resisting force of the medium. What this means is that, as the medium the body is passing through becomes more and more "void-like," there is progressively less and less resisting force. Therefore, the body moves faster and faster, because the resistance (remember, it is in the denominator) becomes smaller and smaller. In this example, assume that the motive force remains constant. Since the void, as conceived by Leucippus and Democritus, was completely empty, there was zero resistance and the moving speed of the body became infinite. Since, as Aristotle maintained, an infinite speed was impossible, there could be no void. By the way, Aristotle's ideas of motion were incorrect. It would not be until Issac Newton in 1687 that the correct laws of motion were given. Point #3 - Atoms are completely solid. It then follows that there can be no void inside an atom itself. Otherwise an atom would be subject to changes from outside and could disintegrate. Then, it would not be an atom. We know this is incorrect. In 1911, Ernest Rutherford discovered the nucleus, demonstrating in the process that a single atom is mostly empty space. Point #4 - Atoms are homogeneous, with no internal structure. The absolute solidity of the atoms also leads to the notion that atoms are homogeneous, or the same all the way through. Another way to express this is that an atom would have no internal structure. Although there was speculation about sub-atomic structure in the 1800's after John Dalton introduced the atom idea on a solid scientific basis, it was not until 1897 and J.J. Thomson's discovery of the electron that the atom was shown to have an internal structure. Point #5 - Atoms are different in ... 1) ...their sizes. See the Democritus quote just below. 2) ...their shapes. According to Aristotle: "Democritus and Leucippus say that there are indivisible bodies, infinite both in number and in the varieties of their shapes...." Democritus says of atoms: "They have all sorts of shapes and appearences and different sizes.... Some are rough, some hook-shaped, some concave, some convex and some have other innumerable variations." 3) ...their weight. Again from Aristotle: "Democritus recognized only two basic properties of the atom: size and shape. But Epicurus added weight as a third. For, according to him, the bodies move by necessity through the force of weight." Concluding Remarks The idea of the atom was strongly opposed by Aristotle and others. Because of this, the atom receeded into the background. Although there is a fairly continuous pattern of atomistic thought through the ages, only a relative few scholars gave it much thought. Due to complex circumstances beyond the scope of this lesson, the Catholic Church accepted Aristotle's position and came to equate atomistic ideas with Godlessness. For example, "Democritus of Abdera said that there is no end to the universe, since it was not created by any outside power." It was not until 1660 that Pierre Gassendi succeeded in separating the two and not until 1803 that John Dalton put the atom on a solid scientific basis. The atom concept is often presented as laying fallow between Democritus and Dalton. Atomic Structure from Democritus to Dalton In 1803, John Dalton of England introduced the atomic idea to chemistry (and is called the Father of Modern Atomic Theory for his efforts). However, it would be false to assume that atomic ideas disappeared completely from the intellectual map for over 2000 years. For, although atomic thinkers between the Greeks and Dalton were few, there is a fairly continuous line from the Greeks to John Dalton. Much of the following is based on these articles: 1) "The Origins of the Atomic Theory" by J.R. Partington. Annals of Science, vol 4, no. 3 (July, 1939) 2) "The Atomic View of Matter in the XVth, XVIth, and XVIIth Centuries" by G.B. Stones. Isis, vol. 10, part 2, No. 34 (January 1928). I. Atomism in Antiquity The atomic ideas of Leucippus and Democritus (from about 440 BC) were opposed by Aristotle about 100 years or so later. Those who acknowledged Aristotle as their master opposed atoms. Since Epicurus was an atomist, he was opposed by his rivals, the Stoics. Cicero, Seneca and Galen all spoke against atoms. Hero of Alexandria (150 A.D.?) makes use of atoms to explain compression and rarefaction (to thin something out; become less dense). Hero denied the existence of an extended vacuum, but allowed for a vacuum between atoms. One proof he cited was that fire could enter into a material, showing that it had openings, i.e., a vacuum. the Church also did the same thing with Ptolemy in astronomy.D. Interestingly. But by actual division we arrive at an actually indivisible part which I call an atom. the Catholic Church had long ago elevated Aristotle's works to Scripture.He pointed out that the pores of a diamond were too small to let in fire and so the stone was incombustible.) At age 21. Giles of Rome (ca. in Latin translations of Arabic translations from the original Greek. reprinted in 1486) and became the prime source (still true today) for the ideas of Leucippus and Democritus. Atomism in the Middle Ages Isidore. he was found guilty of heresy." His opponents could not just appeal to .) are names cited by Partington. (In the 1700's.D. cites three known to have existed in William's lifetime. the Chemist does not know how the process took place. he supposed the existence of minute indivisible substances which convey the disease. he called these semina. Lactantius (died 324 A. the Catholic Church began to elevate Aristotle's writings to the same level as Scripture and had associated atomic thinking with Godlessness. William of Conches (1080-1154) and Vincent of Beauvais (died ca.) De Rerum Natura was rediscovered in 1417 (and printed in 1473. In a different book. Other copies certainly also existed at that time. William openly taught about the ideas of Democritus. 1264-8) both showed knowledge of atomic thinking in their writing. both were considered to be infallible. Lucretius (in Book VI) refers to seeds helpful to life and seeds which cause disease and death. the Venerable Bede (672-735). Atomism in the Renaissance A) Nicholas of Cusa (1401-1464) wrote: "What dost thou understand by an atom? "Under mental consideration that which is continuous becomes divided into the ever divisible. (Remember. He also investigated the nature of the vacuum using a clepsydra (a water clock) and a siphon. both Lavosier and Priestly were able to burn diamonds with large lenses that concentrated the sunlight. Over time. For an atom is a quantity. Much scholastic discussion followed among such people as St.) Important figures within the Church spoke against atoms.)." B) Girolamo Fracastoro (1478-1553) was a physician who wrote about atomism. but it did. Scattered about the libraries of churches in Europe were a few copies of De Rerum Natura. II. Vincent wrote a great encyclopedia. This implies an atomic theory of matter. but only gave short quotations about atoms. showing that the void exerted a force of suction. Bishop of Seville (560-636). Thomas Aquinas (1225-74) and Roger Bacon (1214-92).) and Augustine (354-430 A.D. (Quite frankly. 1247-1316) taght that there are natural minima below which physical substances cannot exist. Dionysios (Bishop of Alexandria 200 A. In fact. C) Peter Ramus (1515-1572) broke with Aristotle early in his life. In essence. he presented a thesis based on this idea: "all that Aristotle has said is false. in his article. which on account of its smallness is actually indivisible. and the multitude of parts progresses to infinity. You may ask how William of Conches knew of Democritus. The works of Aristotle were rediscovered by Western Europe about 1200. When Galileo opposed the Church (in the 1630's?). Only recently (around the late 1980's-early 1990's) has the Church formally admitted its error. On a side issue. In discussion the mechanism of infection. Stones. and Hrabanus Maurus (776-856) all used the word "atom" to refer to discontunities in bodies. the phrase "seeds of disease" is asociated with his name. III. Fracastoro indicates his agreement with Democritus and puts forward an atomistic point of view concerning chemical reactions. . structures BUILT UP by bringing atoms together. if not crippling. G) Partington dates the real beginning of the revival of atomic thinking to the invention of the barometer in 1634 by Evangalestia Torricelli. especially by separating a belief in atomism from athesism. D) In 1588. His home was pillaged and his library burned. it was just on the fringes. The division of natural things attains the smallest and last parts which are not perceptible by the aid of human instruments. Ramus resumed teaching and was appointed professor in 1551. since that would be begging the question. the silver is dissolved and the gold remains. Although it appears that Ramus did not write about atomism as such.the authority of Aristotle to refute him. For example. Francis I." E) Partington lists five other names of people alive through in the 1500's and 1600's who wrote about atoms. Another example was that a large volume of vapor yielded a small drop of liquid. J. After attacking his ideas for a whole day and being refuted. What we might call a molecule today. Bartholomew in Paris in 1572. melt some pure gold and pure silver together until completely mixed. He used the fact that vapor from wine penetrated 4 layers of paper to show the smallness of atoms. he wrote two books (aganist Aristotle) that provoked violent reaction. Their publication was banned. 1517. atomism never really went away. the books were burned. who wrote of particles of the first. but as you have seen. However. IV. he was in the forefront of the attack on the authority of Aristotle. Both Bacon and Descartes. An important position of Aristotle (and the Church) was that the vacuum did not exist. and Ramus was silenced by order of the Pope. Sennert taught that there must be atoms of more than one type and that atoms joined together to form composite bodies (I think he called these secondary atoms. He returned eventually. were not too convinced about atomism. He also taught that atoms retain their essential form.C. he embraced the Reformed faith (Martin Luther had nailed his "95 Theses" to church door at the University of Wittenberg on October 31. Above the mercury of the barometer was a vacuum. Giordano Bruno wrote: "The division of natural things has a limit. This invention (and the air pump by Otto von Guericke in 1654) dealt a severe. After the Pope died a year later. an indivisible something exists. Ramus was finally awarded his degree with honors.) and was forced to flee from Paris. Of interest is Sebastin Basso. and third order. F) Daniel Sennert (1572-1637) was an atomist during the time Rene Descartes (1596-1650) and Francis Bacon (1561-1626) were alive. Magnenus attempted to calculate the size of an atom. On treating the mixture with nitric acid. However. Gassendi was successful in making atomism more widely known and acceptable. but ultimately died in the massacre of St. that is to say. blow to the non-existence of the vacuum. but I am not sure). second. although intellectual giants of that era. In 1543. Pierre Gassendi (1592-1655) Gassendi is considered by many to be the reviver of atomism. Mach died. and help to constitute Bodies of very different Natures both from It and from one another. or work upon each other ." In it. sorry Bob. and Ethics. and nevertheless be afterwards reduc'd to the self-same Numerical. but Dei gratia (as a gift of God). In 1649 he published his major work on atomism: Syntagma philosophiae Epicuri. a Body of this or that denomination is producd. but confessedly mixt Bodies. Gassendi allows for the union of atoms to form groups. The following year. physical substances rather than the "principles" the alchemists thought of (the "principle of salt". Gassendi taught that atoms are not just geometric points. . although it is very small. so that according as the small parts of matter reccede from each other. Ostwald and Mach were both respected scientists. they are solid. In 1915. aged 78. It is divided into three sections: Logic. From Gassendi to Dalton: Just Under 150 years Robert Boyle (1627-91) was an atomist. The atoms move not a se ipsis (of themselves). but that they have a definite size. Before even discussing atoms. although he liked the word "corpuscle." Later on in the book. Ponderous. Boyle says: "I can easily enough sublime gold into the form of red Chrystalls of a considerable length." In 1661. This is the idea which freed atomism from athesism. .) In 1908. Mach was still writing in an anti-atomistic way. Physics. Ostwald explicitly stated his belief in the reality of atoms in the introduction to his textbook Outline of General Chemistry. and many other ways may Gold be disguis'd. . but were made by God. he says: "the difference of Bodies may depend meerly upon that of the schemes whereinto their Common matter is put . he insists that the chemical elements must be actual. which he calls moleculae or corpuscula. the Parliment of Paris had issued a decree that anyone holding or teaching a position opposed to Aristotle (including atomism) was liable to be put to death. corpuscles) of gold: "though they may not be primary Concretions of the most minute Particles of matter. .Before going into his teachings. or having their cohesion violated by the divorce of their associated parts or ingredients. and cannot be subdivided." Incidently. these groups are not held together by attractive forces. published the "Sceptical Chymist. so he was left alone. but by mechanical forces such as hooks-and-eyes or antlers. V. He dwells on Torricelli and his experiments at length. He describes the Greek position: atoms cannot be created nor destroyed. However. they have weight. he says of atoms (oops. Again. Fixt. Gassendi has influential friends. and Malleable Gold as it was before its commixture. two of the last non-believers in the reality of atoms were Wilhelm Ostwald and Ernst Mach. he differs from the Greeks in that atoms have not been in existence forever. However. are able to concurre plentifully in the composition of several very differing bodies without losing their own Nature or Texture. the "principle of gold" and so on). it is interesting to note that in 1624. Yellow. . (I am not including those who are not in the mainstream of science. Gassendi devotes three chapters to discussing the void and its necessity. 1803. and indestructible particles called atoms. However.. Modern scholarship has identified four basic ideas in Dalton's chemical atomic theory. study and ponder about. This basic idea goes back to the Greeks. On the 23rd of October the same year[I r]ead my Essay on the absorption of Gases [by Water] at the conclusion of which a series of atomic [weights] was given for 21 simple and compound elements . He published his theories on the atmosphere and gas behavior in a book titled A New System of Chemical Philosophy. he said: "A series of Essays read before this society and afterwards published in the 5th Vol. there are three more-or-less contemporary descriptions of how Dalton developed his ideas. as such. Nash points out.' in which the atomic symbols I still use [were] introduced. These atoms maintain their identity through all physical and chemical changes. Only in the last few pages (Chapter III) did he discuss his atomic theory. 2:3 and so on. it turns out that the three contemporary descriptions are mutually contradictory and none are consistent with information available elsewhere. . This includes Dalton's own words on the subject. The path that Dalton took to the chemical atomic theory is complex. in a paper read to the Manchester Literary and Philosophical Society. gas solubility and gas mixtures. of their Memoirs gradually led me to the consideration of ultimate particles or atoms & their combinations. is not a new idea to Dalton. no one of any scientific substance has questioned the reality of atoms. In 1830. And then. of course. whole number ratios such as 1:1. discrete. 1) chemical elements are made of atoms 2) the atoms of an element are identical in their masses 3) atoms of different elements have different masses 4) atoms only combine in small. I want to draw a difference between physical and chemical atomism. Add to this the several theories advanced over the years by historians and there is a lot to read. . I intend to develop this topic in the future. John Dalton (1766-1844): The Father of the Chemical Atomic Theory Before delving into Dalton. 1) elements are made of atoms Elements are made up of minute. please keep in mind that atoms. I find in my notebook 'Observations on the ultimate particles of bodies and their combinations. indivisible. " t was not until 1805 that the above essay was published and it was not until 1808 that Dalton himself discussed his methods for atomic weight determination. 1:2. were not part of the chemical mainstream in the early 1800's. .Since then. The Chemist believes that the most satisfactory answer to Dalton's path to the chemical atomic theory was by way of his studies on vapor pressure. Under the date of Sept 3d. John Dalton wrote his first table of atomic weights in his notebook dated September 1803. As Leonard K. This. So. Also. . here is where we meet the original contribution of Dalton. Dalton never ruled out the possibility of subatomic structure. Daltonian atoms are usually taught as being similar to featureless billiard balls. heat. please do not think that Berthollet was a total loser.an element is a chemical substance that cannot be decomposed further by chemical means (i. . Berthollet resisted the idea of the atom: that elements combine in small." so to speak. Incidently. it was not until the mid-1860's that chemical equilibrium began to be explored in depth. 3) atoms of different elements have different masses Although this idea is implicit in Dalton's theory. in 1798. 2 to 3. .. the concept of chemical combination in 1803 was much. whole number ratios Chemical combination between two or more atoms occur in simple. . while it was claimed atoms of different elements had different weights. it is not original with him. which distinguishes his chemical atomic theory from earlier work. the Greeks. how weak soever its affinity for another maybe." Even many years later. electricity. Atoms of different elements have different properties. Thackray says (his original is in italics): "The particular development of Dalton. just not for determining atomic weights. so the idea was "in the air. Even into the late 1800's." In other words. This idea was even discussed in chemical textbooks of Dalton's time. However. how strong soever the affinity of that third is. The idea that all atoms of a given chemical element weigh the same is known today to be incorrect. Although Dalton was well-known at the time. but in 1803 the concept of isotopes was just over 100 years in the future.e. the most authoritative chemist of the period was Claude Louis Berthollet and his ideas were phrased thus: "Berthollet has shown also. etc. reacting with another chemical). there were French chemists who used their authority to punish lesser colleagues and students who publically supported the chemical atomic theory. He was the first. provded it be applied in sufficient quantity. Once again. is capable of abstracting part of that other from a third. was his devising of an effective system to obtain these relative particle weights from currently available chemical data. that every body. 1 to 2. Dalton was the first to do so.). such as weight. This definition traces to Lavoisier. Some of the negative reaction to his anti- atom stance seems to have spilled over into unjusly ignoring his other work. 2) the atoms of an element are identical in their masses Atoms of the same element have the same properties. whole number ratios that are fixed. developed this general idea. Arnold W.Dalton's idea of an element is what we believe today . 4) atoms combine in small. He just knew that the state of the art in the early 1800's did not allow the physical structure of an atom to be probed. 1 to 1. no one could figure out what the different weight values were. exactly how did he determine atomic weights? I intend to develop this topic in the future. In truth. On the contrary.e. much different than what Dalton was proposing. to observe a reversible reaction and ideas like the ones expressed in the above quote worked perfectly well for chemical substances reacting. including a different weight. numerical ratios (i. Consequently. They can be recovered by decomposing the substance.This point gives immediate explanation to the Law of Definite Proportions. 3) The unit for atomic weight was called a "dalton" for many years. Still today. and joining those that were previously at a distance.J. as in "The atomic weight of that protein is 35. The law. is: Atoms of the same element can unite in more than one ratio with another element to form more than one compound." As with most of Dalton's theory. announced by Joseph Louis Proust in 1797. Dalton discovered this law while studying some of the oxides of nitrogen. As Dalton says: "We might as well attempt to introduce a new planet into the solar system." he proposed standard symbols for the elements. A fifth idea implicit In Dalton's theory. as to create or destroy a particle of hydrogen. Dalton discovered the Law of Multiple Proportions. An element's atoms do not change into other element's atoms by chemical reactions. In modern times. this idea is not original to Dalton. a few Dalton factoids to close: 1) In "A New System of Chemical Philosophy. He. you most often hear it used in biochemical circles. consist in separating particles that are in a state of cohesion or combination. He was the first to do so. himself. or to annihilate one already in existence.I will defer discussion of this work to a future time. 2) 2) He was the first to identify color-blindness. "daltonism" is often used to name this problem. All the changes we can produce.000 daltons. nitrogen and oxygen atoms stay as themselves even when combined. suffered from red-green color- blindness. Thomson and the Discovery of the Electron . Finally." J. It is Lavoisier who is responsible for the Law of Conservation of Mass in chemical reactions. but usually not discussed is this: atoms can be neither created nor destroyed. in modern terminology. For example. another law which is easily explained by his atomic theory. During his research. Cathode Ray Tube History . twice the speed of sound . Other people had measured the e/m ratio or suggested that the cathode rays were composed of particles. Kaufmann was a follower of a philosophy called "positivism. until he spoke of it again in 1899. This was reaching quite far beyond what he had actually discovered. 1897 lecture in which he first announced his results. Recollections and Reflections. this suggestion was published as a commentary to the publication of Thomson's April 30. Thomson himself continued to use the term corpuscle until 1913.we have in the cathode rays matter in a new state.. In fact. Walter Kaufmann deserves special mention before leaving this subject.On April 30. Joseph John (J.' " (J. 1897. and so on). had suggested that Thomson's 'corpuscles' making up the cathode ray were actually free electrons. G.J. he wrote: ". I was even told long afterwards by a distinguished physicist who had been present at my [1897] lecture at the Royal Institution that he thought I had been `pulling their legs. Kaufmann was convinced by 1901 of the electron's existence and became a leading experimenter working to determine more about it. and he claimed that these corpuscles were the things from which atoms were built up. Since submicroscopic particles were not seen by the human senses.' He also announced that they had a mass about 1000 times smaller than a hydrogen atom. However. So it was that Kaufmann missed out on a great discovery and become a footnote to history. etc. By the way.-is of one and the same kind.) Thomson's corpuscle hypothesis was not generally accepted.J. he had better data than Thomson and had it months before him. Positivism allowed explanations of events which were based on sensory experience only. but he was proved right and for his courage he is remembered as the discoverer of the electron.Mach two. 341. matter derived from different sources such as hydrogen. Kaufmann could not bring himself to the "corpuscle hypothesis" that Thomson announced.J.) He had leaped to the conclusion that the particles in the cathode ray (which we now call electrons) were a fundamental part of all matter." Philosophical Magazine 44. 295. but rather inferred from the data. As he was to recall much later: "At first there were very few who believed in the existence of these bodies smaller than atoms." championed at that time by Ernst Mach (whose name is used today to signify the speed of sound - Mach one.. Thomson (1897). Thomson (1936). "Cathode Rays. It was a risky thing. George Francis FitzGerald (1851-1901). but Thomson was the first to say that the cathode ray was a building block of the atom. By this time. Bell and Sons: London. an Irish physicist. Later in 1897. p. even by British scientists. In 1897. this matter being the substance from which the chemical elements are built up. a state in which the subdivision of matter is carried very much farther than in the ordinary gaseous state: a state in which all matter-that is.) Thomson (1856-1940) announced that cathode rays were negatively charged particles which he called 'corpuscles." (J. oxygen. ) Thomson first becomes interested in the discharge of electricity through a gas a low pressure. He also finds that there is an extended glow on the walls of the tube and that this glow is affected by an external magnetic field.F. this leads in 1897 to discovery that cathode rays are composed of electrons.9 x 107 cm/sec. Thomson announces that he has found that the velocity of cathode rays is much lower than that of light. become the first to produce cathode rays.J. allowing the rays to be studied in the open air. later these will be found to be ions that have had electrons stripped in producing the cathode ray. as compared to the value 3. Philipp von Lenard develops a cathode-ray tube with a thin aluminum window that permits the rays to escape.0 x 1010 cm/sec for light. 1874 George Johnstone Stoney estimates the charge of the then unknown electron to be about 10-20 coulomb.away from the anode. radiation that travels in the opposite direction . (He used the Faraday constant (total electric charge per mole of univalent atoms) divided by Avogadro's Number. the Faraday dark space grows larger. cathode rays. Hittorf finds that a solid body put in front of the cathode cuts off the glow from the walls of the tube. 1858 Julius Plücker shows that cathode rays bend under the influence of a magnet suggesting that they are connected in some way. shows that the rays can penetrate thin foils of metal. This was in response to the prediction by Lenard that cathode rays would move with the velocity of light. 1869 J. Sprengel improves the Geissler vacuum pump. 1871 C. 1865 H. suggesting the idea that there is a smallest unit of electricity. . 1881 Herman Ludwig von Helmholtz shows that the electrical charges in atoms are divided into definite integral portions. leading eventually to the discovery of the electron (and a bit farther down the road to television). which would seem to indicate (incorrectly) that cathode rays cannot be charged particles. by 1897." 1876 Eugen Goldstein shows that the radiation in a vacuum tube produced when an electric current is forced through the tube starts at the cathode. Crookes proposes that they are molecules that have picked up a negative charge from the cathode and are repelled by it. as modified by Sir William Crookes. Establishes that "rays" from the cathode travel in straight lines. these tubes.J. which he takes to support the wave hypothesis.1855 German inventor Heinrich Geissler develops mercury pump . He obtained the value of 1. these rays are called canal rays because of holes (canals) bored in the cathode. Goldstein introduces the term cathode ray to describe the light emitted. he changes the name to "electron. close to the modern value of 1. In 1891. that is to say. he distrusts this measurement.) Stoney also proposes the name "electrine" for the unit of charge on a hydrogen ion. but he did not accept the idea that electricity is composed of particles. James Clerk Maxwell had recognized this method soon after Faraday had published. However. Joseph John (J.6021892 x 10-19 coulomb. 1892 Heinrich Hertz who has concluded (incorrectly) that cathode rays must be some form of wave. 1890 Arthur Schuster calculates the ratio of charge to mass of the particles making up cathode rays (today known as electrons) by measuring the magnetic deflection of cathode rays.W. Plücker uses Geissler tubes to show that at lower pressure. 1886 Eugen Goldstein observes that a cathode-ray tube produces. Varley is first to publish suggestion that cathode rays are composed of particles.produces first good vacuum tubes. in addition to the cathode ray. 1894 J. 1883 Heinrich Hertz shows that cathode rays are not deflected by electrically charged metal plates. In fact. . will be awarded the Nobel Prize in Physics in 1937 for showing that the electron is a wave.Special Note: At this time there was great rivalry between German and British researchers. The image (23K GIF) below is of J. J. J.J.J. Lorentz uses Zeeman's observations of the behavior of light in magnetic field to calculate the charge to mass ratio of the electron in an atom. 1896 Pieter P. Thomson's Cathode Ray Tube J. Thomson and a cathode ray tube from around 1897. Thomson used results from cathode ray tube (commonly abbreviated CRT) experiments to discover the electron. the year he announced the discovery of the electron. Zeeman discovers that spectral lines of gases placed in a magnetic field are split. both will be proven correct. refuting Hertz's concept of cathode rays as waves and showing they are particles. As events unfold over the next few decades. Thomson will be awarded the Nobel Prize in Physics in 1906 for proving the electron is a particle and his son. a phenomenon call the Zeeman effect. whereas the British tended to believe that the cathode ray was a particle. Only the end of the CRT can be seen to the right-hand side of the picture. 1895 Jean-Baptiste Perrin shows that cathode rays deposit a negative electric charge where they impact. As concerning the nature of the cathode ray.J. Hendrik Antoon Lorentz explains this effect by assuming that light is produced by the motion of charged particles in the atom. the Germans tended to the explanation that cathode rays were a wave (like light).J. George Paget Thomson. a year before electrons are discovered and 15 years before it is known that electron are constituents of atoms. The amount the cathode ray bent from the straight line using either the electric field or the magnetic field allowed Thomson to calculate the e/m ratio. which were placed on either side of the straight portion of the tube just to the right of the electrical plates. 1) If an object is placed in the path of the cathode ray. Th diagram (6K GIF) below appeared in an article by J. Everett who helped to greatly increase Thomson's experimental range. This showed that the cathode ray carried energy and could do work. You may wish to compare it to the photo. The discovery of this effect in 1855 predates by some ten years the unification of electricity and magnetism by James Clerk Maxwell. 2) The cathode ray can push a small paddle wheel up an incline. This allowed him to use either electrical or magnetic or a combination of both to cause the cathode ray to bend. This showed that the cathode rays traveled in straight lines. Thomson in 1897 announcing the discovery of the electron. 3) The cathode ray is deflected from a straight line path by a magnetic field. General Cathode Ray Tube Results There were a number of results gathered over the years by cathode ray tube researchers. suggesting that the two were related in some way. Thomson was a very unhandy person. Thomson also could use magnets.J. The two plates about midway in the CRT were connected to a powerful electric battery thereby creating a strong electrical field through which the cathode rays passed. About 1894 he acquired an excellent glassblower named E. The long glass finger (in the photo) projecting downward from the right-hand globe is where the entire tube was evacuated down to as good as a vacuum as could be produced. then sealed. It is about one meter in length and was made entirely by hand. against the force of gravity. . He was very fumble fingered and had a tendancy to break things. Incidently.The image (7K GIF) below of a CRT used by Thomson in his experiments. a shadow of the object is cast on the glowing tube wall at the end. However. The cathode rays bend toward the positive pole. .4) Although there was some speculation that the cathode rays were negatively charged. there is one problem. A different experiment would have to be carried out. However. Therefore. The ISBN is 0-7167-1488-4. please see "The Discovery of Subatomic Particles" by Steven Weinberg. He did so in 1897. Often. Reversing the above figures and using grams rather than kilograms gives a value of 5. By carrying out the experiments and measuring the proper values.J. Usually.109 x 10-31 kilograms.H. The e/m ratio is important because that is as far as Thomson could get with his cathode ray tubes. grams are used rather than kilograms giving a numerical value of 1. the modern value for the e/m ratio is 1. just two years before Thomson announces the electron. books round off the 1.759 x 1011 C/kg.686 x 10-9 g/C. Elsewhere you will find discussion of how the value for 'e. Many textbooks and articles use the m/e ratio. Just below are GIFs of each formula. Freeman and Company. he could calculate what the charge-to-mass (e/m) ratio was for the cathode ray. Thomson had developed formulas based on the deflection of the cathode ray by the electric field and by the magnetic field. it is not shown to be true by experiment until 1895.759 portion to 1. the two formulas above could not give either the charge or the mass by itself.759 x 108. Thomson is the first individual to succeed in deflecting the cathode ray with an electrical field. Knowledge of the value of 'e' or of 'm' would be needed to get to the other once you knew e/m. (May 1996: This will be made a link when that section is written.76. The modern value for the charge on the electron (to four significant places) is 1.' the charge on the electron was determined. that is the mass-to-charge ratio. What is the e/m ratio? e/m ratio stands for charge-to-mass ratio of the electron. which Thomson did know.) Thomson's Calculations For a fuller discussion of the below. It was published in 1983 by W. confirming that cathode rays is negatively charged. 5) J.602 x 10-19 coulombs and the electrons mass is 9. Thomson had measured the cathode rays' velocity. That value was then inserted along with the other values into the deflection formulas shown above. but if we substitute for hydrogen some unknown primordial substance X. Thomson discovered the electron. The internal structure of the atom had been a source of speculation for thousands of years. In the form in which this hypothesis was enunciated by Prout. Although Dalton did allow for the fact that there might be a sub-atomic structure of which he was unaware. He wrote: "The explanation which seems to me to account in the most simple and straightforward manner for the facts is founded on a view of the constitution of the chemical elements which has been favourably entertained by many chemists: this view is that the atoms of the different chemical elements are different aggregations of atoms of the same kind. in this precise form the hypothesis is not tenable. as did Dalton. and if these corpuscles are charged with electricity and projected from the cathode by the electric field. in the very intense electric field in the neighbourhood of the cathode. leaving us with: Since Thomson knew both the electrical and magnetic field strengths as well as the amount of deflection. Thomson faced two major problems: (1) how to account for the mass of the atom when the electron was only about 1/1000 the mass of the hydrogen atom (the more modern figure is 1/1836) and (2) how to create a neutral atom when the only particle available was negatively charged. the molecules of the gas are dissociated and are split up. Then. If. there is nothing known which is inconsistent with this hypothesis. both equations can be used as a ratio if the deflections by the two fields are made to be equal. but into these primordial atoms. charge and both lengths cancel. I. the mass.) However. The Thomson Model of the Atom In 1897.Notice that both equations depend on knowing the velocity of the cathode ray.J. An easy calculation gave the charge-to-mass ratio. The first seed of the model we are discussing appear in his famous 1897 announcement of the discovery of the electron. which we shall for brevity call corpuscles. not into the ordinary chemical atoms. The Greeks taught that the atom was solid. he could easily solve for the velocity. " . which is one that has been recently supported by Sir Norman Lockyer for reasons derived from the study of the stellar spectra. Leadup to Thomson's 1904 Model of the Atom Thomson had been in the business of proposing atomic models since at least 1881. He also was the first to attempt to incorporate the electron into a structure for the atom. His solution was to rule the scientific world for about a decade and Thomson himself would make a major contribution to undermining his own model. which is when he proposed his "vortex" model of the atom. (Several years previous to 1897. the first subatomic particle. the atoms of the different elements were hydrogen atoms. J. We will not go into details about it. but he grew to distrust the results. they would behave exactly like the cathode rays. The Chemist likes to call it the "blueberry muffin" model. .. yet when they are assembled in a neutral atom the negative effect is balanced by something which causes the space through which the corpuscles are spread to act as if it had a charge of positive electricity equal in amount to the sum of the negative charges of the corpuscles. Here is a quote from the 1904 article: We suppose that the atom consists of a number of corpuscles moving about in a sphere of uniform positive electrification . a negative electron is removed leaving behind a positive atom. these corpuscles are equal to each other. good." This last portion is interesting in that it proposes the correct mechanism for ionization. and has led to no new results. . All those round little blueberries surrounded by the bread of the muffin. COLD milk. The detached corpuscles behave like negative ions.. II. this assemblage of corpuscles forms a system which is electrically neutral. Though the individual corpuscles behave like negative ions." Arrhenius goes on to several criticisms of the Thomson Model. However.. while the part of the atom left behind behaves like a positive ion with the unit positive charge and a mass large compared with that of the negative ion. this is often referred to as Thomson's "plum pudding model. Savante Arrhenius (the 1903 Nobel Prize winner in Chemistry) had this to say about Thomson's model in 1907: "This conception has hitherto remained only a formal one." However. Thomson is only able to make calculations where all the corpuscles are limited to roatating in a ring. Oh my. .. . . Some butter on top of a muffin hot from the oven and some nice. Here is what he then said in 1899: "I regard the atom as containing a large number of smaller bodies which I will call corpuscles.. he does not go into the presence of a positive force. In the normal atom. . The first half of the article is filled with detailed calculations about the stability of corpuscles moving about in a positive environment. By the way. Moving from ring to sphere proves too difficult a challenge. ." where the pudding represents the sphere of positive electricity and the bits of plum scattered in the pudding are the electrons. . but the problem will soon become the electrons and their mass. although he must have been aware of its necessisity. each carrying a constant negative charge which we shall call for brevity the unit charge. In fact. That seems pretty straighforward.. Thomson's Mature Model His next statement on the structure of the atom comes in a 1904 article. not everyone is convinced this is the right answer.And a few paragraphs later: "If we regard the chemical atom as an aggregation of a number of primordial atoms. Ummmm. but he modified the slit the alpha rays traveled through. would win the 1915 Nobel Prize in Physics). Sometimes teachers. but rather discussed his results with colleagues. and even textbooks. These efforts came to nothing and the Thomson Model assumed its place in history as the first modern attempt to construct a theory of atomic structure. Interest in the Thomson Model fell off rapidly after 1911. I want to make a point about how the Thomson Model is presented today. The open side of the resulting photographic image was sharp and the mica side was diffuse. the mica taking the place of the air. III. There are NO protons in the Thomson Model of the atoms. Rutherford's Experiment . January to June 1906 Rutherford did not work in a vacuum (although most of the experiments were!). He took a wire coated with radioactive material and passed the alpha rays through a narrow slit. will represent the Thomson Model as a mixture of protons and electrons. Be careful. although in 1914 and 1915 attempts were made to resurrect it. (Those teachers sure are evil. circular pencil-shaped beam. However. like on the right-hand side of this image: Make sure you have the correct idea firmly in mind. The Thomson Model has negative partices (electrons) and a sphere of positive charge. together with his son William L. Rutherford wrote ".Part I: 1906 to Early 1911 I. the greater width and lack of definition of the air lines show evidence of an undoubted scattering of the rays in their passage through air. Half the slit was left open and half was covered by a thin (0. June 1906: Scattering by Mica Rutherford used his wire coated with the alpha-emitting radioactive element. Rutherford's response was to perform a more definitive experiment.Before leaving this topic. including William H. the narrow slit showed perfectly sharp edges on a photographic plate the alpha rays hit. . a teacher might try to trip you up on a test question. In vacuum. This resulted in a narrow. 1906. the edges of the slit became diffuse and widened. January 1906 Rutherford announced the discovery of alpha particle scattering by air in Jan. Bragg. rectangular beam rather than a narrow. publishing the results in June 1906.003 cm) mica plate. . when the beam was passes through air. ." II. aren't they??) The Thomson Model will hold sway for a few years. until Ernest Rutherford announces the nuclear model of the atom in 1911. The experiment was done in the vacuum. Bragg was not happy with Rutherford's conclusion and suggested an alternative explanation. Bragg (who. In light of future events. remember that Rutherford had been Thomson's student and the two were very close. Was it a singly- charged hydrogen molecule or a doubly-charged helium atom? The e / m ratio was consistent with either. more importantly for our story. In 1907. (J. "the scattering is the devil. he sought additional confirmation. This was in response to Bragg's alternative explanation which involved electrons (the exact details are not necessary).He also subjected the alpha beam to a magnetic field. . from his childhood.J. Geiger started on this work even before the counting experiment was done. Since electrons bend the opposite way from alpha particles. a satisfactory counting device was designed and built. VI. had been totally discredited by Thomson and his allies. IV. but speculated on in print by Thomson as early as 1899) is still the only model of the atom acceptable to the British. The "Saturnian model. Or so they thought!! Also. the scattering of the alpha particle was wreaking havoc on their results. V. Moving to Manchester All this time." announced by Nagaoka in May 1904. and in June 1908 he announced some preliminary results. while in Canada he had tried an experiment which needed to count the number of alpha particles.'s model is correct. of Rutherford and his wife over for dinner." By this he meant the Thomson model of the atom. More Results from the Mica Experiment Careful measuring of the images allowed Rutherford to deduce that some alpha particles had been scattered by 2° from a straight-line path. 1908 One of the central goals of Rutherford's work was to determine the nature of the alpha particle. It was a failure. Although in 1907 he was confident it was the latter. Rutherford had been in Canada (since 1898) at McGill University in Montreal.) So it is quite natural for Rutherford to believe that J. To this end.J. a position came open at the University of Manchester and Rutherford applied it and was selected. The vacuum side was sharp and the mica side was diffuse. In another very interesting statement given future events.'s son George has many memories. However. with Geiger now involved. The counter worked fine. Keep in mind that the Thomson model (announced in early 1904. he makes the interesting statement that other particles may have been deflected "through a considerably greater angle." It was evident to both Rutherford and Geiger that an accurate picture of alpha particle scattering was required. he says this result "brings out clearly the fact that the atoms of matter must be the seat of very intense electrical forces -. The result? The image was the same as when there was no magnetic field. but as Rutherford put it in a letter to a friend." He also calculates that a field strength of 100 million volts per cm is required to bend the alpha particles through 2°. Now he was back in England and. any electrons produced by the mica as the beam went through it would be swept away. However. he hooked up with Hans Geiger. Manchester. Rutherford had proved conclusively the alpha particles could be scattered.a deduction in harmony with the electronic theory of matter. as Marsden remembers it 50 years later.." In other words. As this work proceeded. It almost goes without saying. I examined scattering quantitatively in several experiments. At first we could not understand this at all. Marsden Enters the Scene To assist in these experiments. In addition to this. twenty-year-old Ernest Marsden joined Geiger. and did research with Rutherford's direction and collaboration. As Geiger himself says. this work was done in a very dark room. some thirty years later: "In the electric counting of alpha-particles it was seen that the small residuum of gas in the four- metre long tube . neither of them thought the alpha particles were being directly REFLECTED from the walls or the metal foil. and M is a microscope to view the flashes with. They could gain only a rough estimate of the most probable angle of deflection for a given metal. which were far outside the normal variations. he came into the lab one day. According to Marsden (many years later). Z is the zinc sulfide screen that flashed when struck (it was called the scintillation method). .Above is Geiger's 1908 apparatus. "quite an appreciable angle. "Don't you think that young Marsden whom I am training in radioactive methods ought to begin a small research?" Rutherford agreed and. but experimental difficulties persisted." He proposed to continue with as many metals as possible "in the hope of establishing some connection between the scattering and stopping powers of these materials. in other words he was seeing scattering by greater angles than he had expected. They (by the early spring of 1909) decided the trouble was somehow related to scattering by the tube walls and a series of washers installed in the tube solved the problem. turned to Marsden and said "See if you can get some effect from alpha particles directly reflected from a metal surface. It was at this time that he said (according to Rutherford). ." Marsden goes on to say about his own thoughts: "I did not think he expected any such result. . but it was one of those hunches that perhaps some effect might be observed . Later on. The most important observation was the appearance of isolated instances of extremely large angles of deflection. . As Geiger put it. R is the source.D. The tube was improved and lengthened. he found that gold foil scattered through a greater angle than aluminum. Rutherford makes the Critical Suggestion Hans Geiger was a scientist in his own right." . was a teacher on staff. We attributed this to a slight scattering of the alpha- particles. what was first a problem now bcame the subject of study. . VII. influenced the result. S is the thin metal foil which scatters the particles. He held a Ph. Geiger began to notice what he termed a "notable" scattering." VIII. came into my room and told me that he now knew what the atom looked like. Rutherford himself recalled what then happened: "Two or three days later Geiger came to me in great excitement and said: "We have been able to get some of the alpha particles coming backwards. but more recent experiments seem to show that it consists mainly of primary β- particles. The Experiment of 1909 Marsden first used the following set-up in his search for reflected alpha particles: The alpha emitter was placed at A on top of a lead plate which prevented direct access of the particles to the counter located at S. his greatest scientific achievement still lay two years into the future. This is the data they reported in that same paper: 3. Sometime in late 1910/early 1911. It was from this experimental set-up that Geiger reported two or thre days later that some alpha particles had been reflected back. the counter came to life. Z. This is the opening paragraph of Geiger and Marsden's paper of May 1909: When β-particles fall on a plate." For Ernest Rutherford. when one thin gold foil was placed at R.The year before he died. he switched roles with Geiger. who remembers it from 27 years later: "One day Rutherford. 1. For α-particles a similar effect has not previously been observed. 2. A A/Z. This radiation is regarded by many observers as a secondary radiation. Number of scintillations Metal Atomic weight. obviously in the best of spirits. per minute. However. winner of the 1908 Nobel Prize in Chemistry." IX. 4. and is perhaps not to be expected on account of the relatively small scattering which α-particles suffer in penetrating matter. which have been scattered inside the material to such an extent that they emerge again at the same side of the plate. a strong radiation emerges from the same side of the plate as that on which the β-particles fall. Lead 207 62 30 Gold 197 67 34 Platinum 195 63 33 Tin 119 34 28 Silver 108 27 25 Copper 64 14·5 23 Iron 56 10·2 18·5 . With nothing placed at position R the counter did not record any hits. 14. in the same paper. was the nuclear atom and it made Rutherford unique among all other Nobel Prize winners.B. he says. the alpha particle encounters one gold atom after another and each encounter deflects the alpha a bit more. that is. Rutherford is thinking about multiple scattering. Over time. under the described conditions. so that the sum of all the deflections is to make the alpha particle come flying out 90° or more from the direction it went in. two events important to our story happen. Boltwood. Aluminium. 1909 Fades into 1910 and then 1911 Now. 1) On Feb. Crowther (a student of J. However a series of letters he wrote at this time allow a glimse into when he started to put everything together. "It does not appear profitable at present to discuss the assumption that might be made to account for [it]. 17. He is the only one to do his greatest work after receiving the Nobel Prize. 27 3·4 12·5 For platinum. Geiger reads a paper in which he determines the most likely angle of deflection for any one alpha particle to be about 1°. He follows up with more data in June and then December. In other words. an American chemist) in which Rutherford says he has been doing "a good deal of calculation on scattering." It seems safe to say that 10 months after the discovery of large-angle scattering. To keep these dates in perspective. Thomson) publishes a theory of beta-particle scattering in March 1910. He uses the 'multiple scattering' idea. to be published in May 1911. 19.D. 1910 (to B. Such a result brings to mind the enormous intensity of the electric field surrounding or within the atom. Bragg) will become convinced that this is the incorrect explanation for large angle scattering." it's plural. Ernest Rutherford was confronted with the challenge of explaining the results. The very first mention of the atom occurs in a letter dated Dec." Scattering and the atom figure in nine letters between then and Feb. 1910. However.J. almost all historians of science call the discovery of the nucleus Rutherford's greatest scientific work. The papers which bear his calculations are undated." Notice the word "encounters. just 6 months after the discovery of large-angle scattering: "Geiger and Marsden observed the suprising fact that about one in eight thousand α particles incident on a heavy metal like gold is so deflected by its encounters with the molecules that it emerges again on the side of the incidence. 2) J. His answer.H." X. 1911. Here is what he says in a lecture at Clark University in September. Each deflecion is caused by the particle encountering an atom. Geiger (and Rutherford) do not have a clue as to its explanation. In 1910. You see. It is not possible to put a precise date on when Rutherford hit on single scattering as the answer. . Rutherford (in discussion with his friend W. many small deflecions which add up to one large angle. 1909. Geiger and Marsden reported: "Three different determinations showed that of the incident α-particles about 1 in 8000 was reflected. remember that Rutherford's published paper appeared in Philosophical Magazine for May 1911. 87° in his 1911 paper. 2) Some of the alpha particles were deflected only slightly. 'single scattering' refers to one encounter of the alpha particle with an atom. so a very dark room was required. (The plate without any foil was studied and no deflecions were found. he discovered them. Geiger found that an alpha particle was.) There were three major findings: 1) Almost all of the alpha particles went through the gold foil as if it were not even there. for the foil. alpha particles were part of the family. Get to the Answer. The person doing the viewing had to sit in the dark for about an hour before beginning the experiment.) This is a diagram incorporating the three findings. Those alpha particles. What was the behavior. It was transparent to the alpha particles. Big Guy . very few (1 in 8000 for platinum foil) alpha particles were turned through an angle of 90° or more. deflected about 1/200th of a degree by each single encounter with a gold atom. The flashes on the screen were very faint. R is the source of alpha particles and F is the foil that scatters the alpha particles. In fact.) Geiger cites a thickness of 8. To him.6 x 10¯6 cm. the foil was so thin that it had to be supported on a glass plate. What Confronted Rutherford? Ernest Rutherford had been studying alpha particles since 1898. exactly? Hans Geiger and Ernest Marsden aimed a stream of alpha particles at a thin gold foil for several months in 1909. One single incident is sufficient to deflect the alpha more than 90°. Rutherford's Experiment . (Rutherford cites 1 in 20.) 3) A very.Part II: The Paper of 1911 I. (Rutherford cites a figure of 0. II. In 1909 he was confronted with some rather bizzare alpha-particle behavior that he had to explain. Just to review. on average. (They would continue studying scattering until 1913. OK. In fact. but Rutherford was having serious problems making multiple scattering fit. continued on a straight-line path until they hit the detector screen. usually 2° or less. It does seem a bit fantastic that only one encounter with an atom was sufficient. of course. The most probable angle of deflection for one gold foil turned out to be about 1°.000 for gold in his 1911 paper. All Rutherford had to do was explain how it all fit together. to ensure maximum eye sensitivity. M is the microscope used to look at the detector screen which was attached to the front of the microscope. No serious challenge has arisen to the nuclear model of the atom.and small-angle scattering. (It might be helpful to remember that the gold nucleus and the alpha particle are both positively charged. on one occasion was even able to turn a potential enemy into a co-worker. It was already well established that the atom had a radius of about 10¯8 cm.a great." In 1912. this occasionally results in the alpha being turned around 90° or more. just letting the science take them where it would. Rutherford writes the following: "The question of the stability of the atom proposed need not be considered at this stage. he considered the nucleus to act as a point: "We shall suppose that for distances less that 10¯12 cm the central charge and also the charge on the alpha particle may be supposed to be concentrated at a point. What Rutherford did was put most of the mass of the atom at the center of the atom. by pure. by pure random chance. the life of the party -." Rutherford never used the word "nucleus" in his paper. His solution to the enigma of explaining both large. so they will repel each other as they come close together. However. They both loved doing pure research. The alpha. Upon meeting people like Einstein. He found a real soul-mate in Marie Curie. outgoing and vigorous. but essentially goes nowhere. in a space much. indeed! 2) Some alphas. as you probably know. The very heavy nucleus recoils a bit from the impact. early in his paper. His phrase was "charge concentration. will pass near some gold atom nuclei during their passage through the foil and will be slightly deflected. he devotes a few pages to the nuclear model and uses the word nucleus once. By pure chance. and on the motion of the constituent charged parts. he was able to turn them into immediate best buddies. and it does so in a hyperbolic path. Depending on various factors. He was also a hard-working scientist who loved science for itself and never tired of playing in the laboratory." Rutherford is not prepared to take the next step. So. big. Those alphas will emerge slightly deviated (say one or two degrees) from a straight-line path. Lorentz. penetrates the atom and gets very close to the nucleus. was the nucleus. how does the nucleus account for the three major findings by Geiger and Marsden? 1) The nucleus is so small that the odds are overwhelmingly in favor of a given alpha particle motoring right on through the gold foil as if nothing were there. . for this will obviously depend upon the minute structure of the atom. He seldom had problems with people. some or all of the small deflections will add up and shove the alpha particle off a straight-line path. will hit a nucleus almost head-on. . For the purposes of his 1911 paper. traveling at 10% the speed of light.) 3) A very. and Planck for the first time. with no purpose other than to discover new and exciting things. which is to determine how the electrons are arranged in the atom. III. very few alphas.And Rutherford was a big guy. That's It. over- grown child. in a book he published. much smaller that the atom itself -. The Thomson model of the atom spread the entire mass of the atom throughout that space.this is the nucleus. It turns out that the atom is a very empty place. the repulsion between the alpha and the atom nucleus is so great that the atom flings the alpha back out. . However . random chance. Folks Rutherford closed the door on the basic structure of the atom. However. He was fun. Thomson and two co-workers (J. in May 1911. Under the influence of gravity.J. 1.E. II. Townsend's work depended on the fact that drops of water will grow around ions in humid air. Wilson). having just spent a bit more than 6 months in J.J. then multiplied by the mass of one droplet to get the value for e. reproduced from his 1913 article: . Here is a diagram of his apparatus. starting in 1897.A. the total mass of all water droplets (found by measuring the acid's increase in weight) He determined the e/m ratio of the droplets (2 divided by 4).D. Pre-History The charge on electron was first measured by J. accelerating until it hit a constant speed. Townsend's work will be described as an example. the drop would fall. Ernest Rutherford has discovered the nucleus and now it's time to take a well-earned rest. and Wilson each obtained roughly the same value for the charge on positive and negative ions. 27-year-old Niels Bohr (awarded a Ph. the total electric charge carried on all the droplets (this was done by absorbing the water into an acid and measuring the charge picked up. Millikan started his work on electron charge in 1906 and continued for seven years. Townsend.S. Townsend and H. the mass of a water droplet (actually the average mass of many) 2. Determination of the Charge on an Electron I. Robert A. Millikan's Definitive Measurement Robert A. This work continued until about 1901 or 1902. Several items were measured in this experiment. His 1913 article announcing the determination of the electron's charge is a classic and Millikan received the Nobel Prize for his efforts. In March 1912. It was about 1 x 10¯19 coulombs. Thomson's laboratory. Each used a slightly different method.) 3. Thomson. the velocity of the droplet 4. the same month of Rutherford's classic paper) will arrive in Rutherford's laboratory. This constant temperature bath was found essential if such consistency of measurement as is shown below was to be obtained. during an observation. A long search for causes of slight irregularity revealed nothing so important as this and after the bath was installed all of the irregularities vanished. Through the third of these windows. the oil drop is observed. To the three windows g (two only are shown) in the brass vessel D correspond. set at an angle of about 18° from the line Xpa and in the same horizontal plane. The air about the drop p was ionized when desired by means of Röntgen rays from X which readily passed through the glass window g. The brass vessel D was built for work at all pressures up to 15 atmospheres but since the present observations have to do only with pressures from 76 cm. THE EXPERIMENTAL ARRANGEMENTS. . Complete stagnancy of the air between the condenser plates M and N was attained first by absorbing all of the heat rays from the arc A by means of a water cell w. The experimental arrangements are shown in Fig. in general. three windows in the ebonite strip c which encircles the condenser plates M and N. down these were measured with a very carefully made mercury manometer M which at atmospheric pressure gave precisely the same reading as a standard barometer. 1. The atomizer A was blown by means of a puff of carefully dried and dust- free air introduced through the cock e. and a cupric chloride cell d. and second by immersing the whole vessel D in a constant temperature bath G of gas-engine oil (40 liters) which permitted.Here is Millikan's description: 8. long. of course. 02° C. 80 cm. fluctuations of not more than . The value as of 1991 (for the charge on the electron) is 1. In following the oil drop over many ascents and descents.01 mm. he has a highly accurate scale inscribed onto the telescope used for droplet observation and he used a highly accurate clock.60217733 (49) x 10¯19 coulombs. By adjusting the current. He sprayed oil ("the highest grade of clock oil") with an atomizer that made drops one ten-thousandth of an inch in diameter. Oil evaporated much slower than water. The droplet would slow in its fall. rather than a whole cloud. 5. 10.This is a photo dating from the time of the experiment. . 9. It is unlikely that there will be much improvement of the accuracy in years to come. 3. 2. The drops were too small to see. Every time the drop gained or lost charge. 6. One drop of oil would make it through the hole. "which read to 0. These are some points to be made about the experiment: 1. it ALWAYS did so in a whole number multiple of the same charge. which stripped electrons off." Millikan's Improvements over Thomson 1. "correct to about . The two plates were 16 mm across. he could measure the drop as it lost or gained electrons. He could also make the drop move up and down many times. 11. 7. Millikan could study one drop at a time. The plates were charged with 5. 3. The hole bored in the top plate was very small. sometimes only one at a time.002 second. so the drops stayed essentially constant in mass. 8. 4. The 49 in parenthesis shows the plus/minus range of the last two digits (the 33)." 2. What he saw was a shining point of light. The space between the plates was illuminated with a powerful beam of light. This is less than 1% higher than the value obtained by Millikan in 1913. Since the rate of ascent (or descent) was critical. He exposed the droplet to radiation while it was falling. he could freeze the drop in place and hold it there for hours. It took a drop with no charge about 30 seconds to fall across the opening between the plates.000 volts. Had the European War had no other result than the snuffing out of this young life. In it.Interesting Fact about Robert Millikan's Experiment In "The Discovery of Subatomic Particles" by Steven Weinberg there appears a footnote on p. and may have been the first to suggest the use of oil. The Concept of Atomic Weight Leucippus and Democritus (about 440 BC) are credited with the origin of the atom concept. slightly more than 100 years later. Various authoritative chemists of the time prepared competing tables of atomic weights with many values the same. According to Fletcher. Both possibilities had been advanced. but a significant number of differences. There was much discussion and controvery over the next several decades concerning atomic weights. who added weight as a property of atoms. The first tables of relative atomic weights were prepared by John Dalton about 1803. when wide-spread agreement about atomic weight values in the chemistry community started to come together. who was a graduate student at the University of Chicago. 1915. there appeared a remarkable posthumous memoir that throws some doubt on Millikan's leading role in these experiments. he will be remembered as the man who numbered the elements. Moseley showed that the correct ordering of the periodic table is on the basis of the atomic number (the number of positive charges in the nucleus). . a young man twenty-six years old threw open the windows through which we can glimpse the sub-atomic world with a definiteness and certainity never dreamed of before. June 1982. was the first to measure charges on single droplets. as long as our civilization stands. he also showed that there are no elements lighter than hydrogen (atomic number = 1) and that there is no possibility for elements between hydrogen and helium (atomic number = 2). and co-authored some of the early papers on this subject with Millikan. periodic tables were created on the basis of increasing atomic weight (with two exceptions). that alone would make it one of the most hideous and most irreparable crimes in history.Atomic Weights and Periodic Properties Henry Gwyn Jeffreys Moseley was born on November 23. It reads: . Fletcher left a manuscript with a friend with instructions that it be published after his death. skillful in execution. the manuscript was published in Physics Today. As an aside. . he had expected to be co-author with Millikan on the crucial first article announcing the measurement of the electronic charge. Fletcher claims that he was the first to do the experiment with oil drops." A brief summary of atomic weights and periodic properties is in order. I. It was Epicurus. but was talked out of this by Millikan. before he turned 28. II. 97. completed in a six-month span during 1913 and 1914 and published in the last two papers of his life was a tour de force of scientific accomplishment. and illuminating in results in the history of science. Leading up to Moseley . Periodic Properties of Elements . page 43. That work. at Millikan's suggestion worked on the measurement of electronic charge for his doctoral thesis. Said Robert Milikan: "In a research which is destined to rank as one of the dozen most brilliant in conception. with some proposals demanding three elements between H and He. 1887 and would die in battle on August 10. Before Moseley. Harvey Fletcher (1884-1981). . Some issues were not be fully resolved until 1860. However. with Newlands.) Mendeleev did not use the "atomic number" that Newlands had used. The Modern Periodic Table The modern periodic table was developed (discovered? invented?) by Dmitri Mendeleev during the years 1869- 1871. John Alexander Reina Newlands. That work. up to 1800. Also. a man named Carey Foster with no other claim to fame. but not fully developed unil nearly 1830. His tables. Phosphorus was discovered about 1665 and from then.the reverse of the order based on increasing atomic weights. followed what he called the "Law of Octaves. There was an explosion of element discovery starting around 1800. he allowed for no period longer than eight. when ordered by atomic weights." . Finally. In March. 1866 he spoke on his work and one of his listeners. His table of 1865 shows no atomic weights and simply numbers the elements in order from 1 to 56. working after the reform of atomic weights in 1860.Ten elements were known from pre-historical times. bromine and iodine. (Some historians credit others as co-discoverers. He published five triads as well as several "incomplete" triads. which he put in their correct chemical order. but he also correctly allowed for longer periods in the transition and rare earth elements. a young man twenty-six years old threw open the windows through which we can glimpse the sub-atomic world with a definiteness and certainity never dreamed of before. every eighth element showed similar chemical properties. he left gaps for missing elements. "Atomic number" remained a number without any physical meaning. skillful in execution. that alone would make it one of the most hideous and most irreparable crimes in history. pointing out "triads" of elements like lithium. 1915. but we will ignore them. Leading up to Moseley: X-Ray Spectra Henry Gwyn-Jeffreys Moseley was born on November 23. Starting in 1816. The elements were correctly ordered based on the atomic numbers. before he turned 28. 20 more elements were discovered. Mendeleev had periods of eight like Newlands. but his final table of 1865 left no gaps whatsoever. 1887 and would die in battle on August 10. even though no one knew why. as long as our civilization stands. the "atomic number" first enters the scene. III." This meant that. He made a number of correct predictions for missing elements and he had Co/Ni and Te/I in their correct chemical order -. and illuminating in results in the history of science. He ordered the elements on their atomic number except for the two pairs just noted. In his early tables. Newlands' work was not favorably received. rose to facetiously ask if Newlands had ever attempted to classify the elements in alphabetical order. he put two elements into the same position several times. completed in a six-month span during 1913 and 1914 and publishd in the last two papers of his life was a tour de force of scientific accomplishment. Said Robert Milikan: "In a research which is destined to rank as one of the dozen most brilliant in conception. was the first to proclaim a pattern for ALL elements. Johann Wolfgang Döbereiner was the first person to emphasize chemical similarities. he will be remembered as the man who numbered the elements. However. It would be Moseley that finally gave the correct answer to why the elements were reversed from a strict ordering based on atomic weights. sodium and potassium as well as chlorine. However. done in 1864 and 1865. with 27 more elements being discovered by the 1840s. It was simply the numbering of the elements after they had been placed in order by atomic weght. Had the European War had no other result than the snuffing out of this young life. 1895. the secondary X-ray became harder and harder.) In the early years. with many new discoveries being made. Barka measured the absorbance of each secondary radiation.) II. (Please realize that many other discoveries were made about X-rays. "Soft" X-rays means of lower penetrating ability. emits an almost perfectly homogeneous beam of X-rays. It was found that. Co. He found a connection between the atomic weight of the element and its secondary X-rays. He realized the importance of his discovery at once. the penetrating power of which is characteristic of the element emitting it. up to silver which is very penetrating or "hard. others began studying X-rays." Barkla ordered the list of elements above by the penetrating power of the secondary radiation with the Cr called "soft. It turns out that the "hard" radiation (the more penetrating ones) has the shortest wavelength (which also means the highest frequency and the highest energy). heterogeneous." which means not very penetrating. when subject to a suitable primary beam of X-rays.A brief summary of X-ray research is in order. the Ni would come first. I. Barkla. His first efforts in this area were in 1906 (the same year Rutherford discovered alpha-particle scattering) and in 1909 he wrote: "It has been found that each of the elements Cr. In 1909. Cu. since Moseley will use a regular change in the position of lines in the X-ray spectrum of each element to assign a positive charge (the atomic number) to the nucleus of each element. Secondary X-Rays are Characteristic of the Element The next discovery was made by Charles G. so much of the secondary beam is absorbed. (Soft means longer wavelength X-rays which also means lower frequency and lower energy. Zn. As. Ag. If the list were ordered by strict atomic weights. Very quickly. Fe. Barkla published another paper in which he found that the supposedly homogeneous secondary X-rays were. elements below about aluminum could not be studied due to the instruments not being sensitive enough to measure the X-rays after absorption. Ni. He wrote: . in fact. The Discovery of Secondary X-Rays Our X-ray thread starts in the evening of November 8." Notice that the list follows the chemical order of Co then Ni. III. when a primary X-ray beam was directed at a substance. This is the day that Wilhelm Conrad Röntgen discovered X-rays. His "preliminary communication" on X-rays was turned in on December 28. For us.01 cm layer of aluminum and measuring how much of the beam was absorbed. So as the atomic weight increased (with the Co/Ni exception). that substance gave off secondary X-rays. I'm just highlighting the ones which culminate in Moseley's work. Se. He did so by directing the secondary radiation through a 0. A Second X-Radiation is Found Barkla (and his students) continued the detailed study of secondary X-radiation. the next step in our story was made in 1897. He stayed up all night doing experiments and even ate and slept in the laboratory for a time. Since no one could yet measure the wavelengths (or frequencies) of X-rays. 1895 and published before the end of the year. Though a full analysis of the radiations from W. here denoted by the letters K and L*. The lines move towards the more penetrating end of the spectrum with an increase in the atomic weight of the element. Sb. This is very conveniently represented as is [in?] a spectrum of ordinary light." In 1911.. Pt. however. I. The percentage absorptions of the soft radiations from these elements have not yet been determined. The absorptions of the penetrating portions of the beams from each element are shown in fig. . but they are roughly indicated on curve A in fig. and Ba as in fig. "The writer has recently investigated more closely the radiations from Sn. Barkla closed his paper this way: "It has been shown that each element has its own characteristic fluorescent line spectrum in X- rays. I on curve B. etc. 5. I (which have been recorded as elements emitting a radiation of variable penetrating power). he added: "* Previously denoted by letters B and A. Au. except that without a knowledge of the wave-length we are obliged to define the radiations by their absorption in some standard substance. Thus we may represent the known portion of the spectra of elements Sb. Bi. there is strong evidence that the observed radiations from these elements are also principally homogeneous radiations characteristic of the elements emitting them. has not yet been made. Barkla wrote: "It is seen that the radiations fall into two distinct series. It has been found that these consist of a very easy absorbed radiation and a very penetrating homogeneous radiation superposed." In the footnote indicated by the asterisk. The letters K and L are. Pb. preferable as it is highly probable that series of radiations both more absorbable and more penetrating exist. 1. Rutherford was not all that excited by Moseley wanting to study X-rays. Atomic Structure: 1903 . we know that the atomic number gives the number of protons (positive charges) in the nucleus. Another of Rutherford's students -.J.a dense concentration of positive charge with (b) electrons orbiting the nucleus in an unspecified manner. had electrons as negative particles with mass.1911 Exactly where the positive protons (and the negative electrons) were in the atom took time to be worked out.when he started his atomic number work.having arrived in Manchester just weeks before Rutherford published his great nucleus paper -. (1) J. although it has long been known that they occupy an anomalous relative position in the periodic classification of the elements according to atomic weights. Moseley's Discovery .4). It would not be until 1920 that Rutherford proposed the existence of a neutral particle -. It is scarcely too much to say that all the phenomena connected with the transmission of X-rays through matter may be readily explained in terms of a few simple laws expressed with reference to these spectra. Moseley was part of Rutherford's research group -. [You might notice that neutrons have not been mentioned. This holds true for cobalt and nickel. while the positive charge was spread out through the space of the atom. This was the discovery made by Henry Gwyn-Jefferies Moseley." I.The Modern Concept of Atomic Number Today. he wrote: . Bohr took up the question of where the negative electrons are (in the atom) and Moseley studied where the positive charges were. In 1913. physical meaning of "atomic number" was suggested by A. By the way.won the 1935 Nobel Prize for discovering the neutron in 1932.the neutron.James Chadwick -. (3) In 1913. By examination of the wave-length of the characteristic X rays emitted by twelve elements varying in atomic weight between calcium (40) and zinc (65. he has shown that the variation of wave-length can be simply explained by supposing that the charge on the nucleus increases from element to element by exactly one unit. Keep in mind that the electron (the first sub-atomic particle discovered) was not discovered until 1897. He found that certain lines in the X-ray spectrum of each element moved the same amount each time you increased the atomic number by one." Barkla wrote this slightly two years before Moseley would publish his historic papers in December 1913 and April 1914.] Within a few months of Rutherford's nucleus paper being published. (2) In 1911 Rutherford announced his atomic model: (a) a nucleus . van den Broek. Thomson in 1903. but the energy and enthusiasm of the younger man soon wore Rutherford down. Rutherford (in 1914) described Moseley's discovery thus: "Recently Moseley has supplied very valuable evidence that this rule [atomic numbers changing by one from element to element] also holds for a number of the lighter elements. the true. Zeitschr. 21. Moseley needed some function of a nuclear property that increased in the same pattern. 1911. p. 39). Moseley's X-Ray Spectra Work Moseley's problem was to find a linear relationship between the atomic number and a measureable property of the nucleus. the research was carried out and Moseley determined the relationship mentioned above. 'to each possible intra-atomic charge corresponds a possible element. p. with the frequency square root value moving up the same amount for each one unit jump in the atomic number. So Moseley set about to determine the wavelengths of the K radiation using recently discovered techniques by the father-and-son team of W. It was linear. using Moseley's data is graph which shows linear behavior: . and so on).H. "In a previous letter to NATURE (July 20. by one for each element in turn.L Bragg and W.' " II. He found it in the K line of the X-ray spectra of each element. Getting the equipment working so as to give reliable data was probably the most time-consuming task of the entire research he carried out. realized that this meant the X-rays were characteristic of the nucleus. called K and L rays. Why did he choose to study this area for what he needed? We can find the answer in the work of Charles Barkla. The atomic number increased by steps of one (18. It seems to me as I write this that Moseley was pretty confident going into this experiment that all he needed to do was find the proper linear relationship. the number of each element in that series must be equal to its intra-atomic charge. 19.' or that (Physik. It turns out that the square root of the frequency moves by a constant value (let's call it "one unit") for each one unit move by the atomic number. 20. Someone. perhaps Barkla or Bohr or Moseley. These X-rays were independent of the physical or chemical state the element was in.. 78) the hypothesis was proposed that the atomic weight being equal to about twice the intra-atomic charge. Bragg. that is. xiv. 1912. However. He had demonstrated that the elements emitted characteristic X-rays. Here. 'if all elements be arranged in order of increasing atomic weights. The three. The mass number is: The number of protons and neutrons in the nucleus of the atom. Here's another: The atomic number is: The number of protons in the nucleus of the atom.About this data. The number of neutrons is 48 minus 22 = 26. which increases by regular steps as we pass from one element to the next. Here is an example of a nuclear symbol: The element symbol. This quantity can only be the change on the central positive nucleus. subscripted left. is the atomic number and the seven. Here is one last example: The 22 is the atomic number for titanium and 48 is its mass number. . superscripted left. is the mass number. of the existence of which we already have definite proof. the atomic number of the element and the mass number of the specific isotope." The Nuclear Symbol The nuclear symbol consists of three parts: the symbol of the element. Li. Moseley himself said: "We have here a proof that there is in the atom a fundamental quantity. is that for lithium. making sure it is a naturally occuring one. More often than not. each exact atomic weight is multiplied by its percent abundance (expressed as a decimal).10 To calculate the average atomic weight. 2) Suppose you are asked to write a nuclear symbol from scratch and the teacher requires it be a realistic one.90 13 13. add the results together and round off to an appropriate number of significant figures. . Do this: a) Select an element. These values can be looked up in a standard reference book such as the "Handbook of Chemistry and Physics. write the nuclear symbol for the chlorine isotope with 18 neutrons.868) How to Calculate an Average Atomic Weight To do these problems you need some information: the exact atomic weight for each naturally-occuring stable isotope and its percent abundance.000000 98. Then. and all you know is the specific element. Here is the answer: Notice the mass number is rounded off from the atomic weight on the periodic table (107.003355 1.Now. go to a periodic table and find its atomic number." This problem can also be reversed. So. if you need the atomic number. Here are two tips: 1) The element name (or symbol) uniquely determines the atomic number. In the example just above Ti is the only element with an atomic number of 22. This will determine its atomic number. b) Take the element's atomic weight and round it off to the nearest whole number. Study the tutorial below and then look at the problems done in the reverse direction. Write the nuclear symbol for silver. Example #1: Carbon mass number exact weight percent abundance 12 12. this will be the mass number of the most abundant stable isotope Let's try an example. 000000) (0.003074 99.67 30 29.23 29 28. where the isotopic abundances can be calculated knowing the average atomic weight.63 15 15.905085 9.086 This type of calculation can be done in reverse.000108 0.906808 14.976927 92.25 .985042 78.965903 24.77 28 27.37 This is the solution for nitrogen: (14.003074) (0.9963) + (15. Practice Problems Calculate the average atomic weight for: 1) magnesium mass number exact weight percent abundance 24 23.10 The answer for chlorine: 35.011 Example #2: Nitrogen mass number exact weight percent abundance 14 14.23 37 36.00 26 25.84 94 93.0110) = 12.99 25 24.9890) + (13.453 The answer for silicon: 28. This is the solution for carbon: (12.968852 75.976495 4.982593 11.003355) (0.0037) = 14.973770 3.007 Example #3: Chlorine Example #4: Silicon mass number exact weight percent abundance mass number exact weight percent abundance 35 34.985837 10.000108) (0.01 2) molybdenum mass number exact weight percent abundance 92 91. 903440 4. Box 56. but the history of their discovery and the origins of their names. Dunedin.68 118 117.903348 0.92 96 95.36 116 115.79 The answers? Look on a periodic table!! Remember that the above is the method by which the average atomic weight for the element is computed. and Neutron Barrie M.901609 24. have equal quantities of electricity associated with them. Proton.68 97 96. then the atoms of bodies which are equivalent to each other in their ordinary chemical action.906020 9. 9 September 1989.65 115 114. New Zealand Published in the Journal of Chemical Education. if we adopt the atomic theory or phraseology. The existence of a fundamental unit of electricity was first suggested by Michael Faraday in 1834 (1) to account for his results involving the electrolysis of solutions of aqueous acids and salts: " . naturally.22 119 118.902784 0. No one single atom of the element has the given atomic weight because the atomic weight of the element is an average. particularly for the proton. No.902956 7. . he subsequently changed this in 1891 (3) to the name "electron". The Discovery of the Electron. The three fundamental particles making up an atom are introduced.63 124 123. Stoney.905840 15.97 114 113.55 98 97. .902200 32.53 117 116.59 122 121.903310 8.63 3) tin (this one is optional!! Suggestion: set it up as a spreadsheet. pg.904826 0. The observation of a small number of large angle deflections of alpha particles (He2+) incident upon a gold metal foil led Ernest Rutherford in 1911 (4) to suggest that the nucleus of an atom is very small and positively charged. does not appear to be well documented.905406 24.907477 9. 66.) mass number exact weight percent abundance 112 111. Vol." This idea was subsequently extended by other scientists including George J.13 100 99. take it into class and impress your teacher.901747 14. 95 94.904678 16.58 120 119. Two years later he concluded from the results of experiments involving the scattering . specifically called a "weighted" average. 738 Introductory courses in chemistry invariably include an account of the historical development of our ideas on the structure of the atom. who first proposed in 1874 (2) the name "electrine" for the unit of charge on a hydrogen ion.905274 5. Peake University of Otago. 289-306. however. Trans. . (Paragraph 869). 273-276. 1416 1417: 1932. Theoretical Chemistry. O. 3rd ed. Mag. Mag. Compt. 4. Literature Cited 1. Stoney. W. Sourcebook on Atomic Energy. 6. 1412-1414. to alpha rays leads to a highly penetrating radiation. Phys. footnote p 42. 91. Rend.. in 1930. S. In 1931-1932 the Curie-Joliot and Joliot (12) reported that exposure of hydrogen- containing material. These details should enable teachers to complete the historical account of the development of our present knowledge of atomic structure. Roy. 3. Phil. Rutherford. 143. It is interesting to note. 374-400. This occurred at an informal meeting around 1920 of the Physics Section of the British Association (6): "the name proton met with general approval. which appears to be otherwise well presented in many introductory chemistry texts. Curie-Joliot. which he identified with the neutron species first postulated by Rutherford. . Mag. a compound of positive and negative electrons . Footnote (p 282) to Masson. in particular beryllium. 5. 1881. Bothe. E Proc. 1931. Phil. 121. 381. 4. J. 876-877. . Roy. Phil. I. 144. and Samuel Glasstone (7) has noted that it had been used as far back as 1908 or earlier as a general term for a unit from which all elements were built. Soc. G. D. 9. William Draper Harkins first introduced in 1921 (9) the term "neutron": " . an electrically neutral massless molecule".of alpha particles from simple gases (5) that "the hydrogen atom has the simplest possible structure of a nucleus with one unit charge. Marmillan: London. 67. 97. W." The term itself is derived from the Greek protos (first). 663-607.1911 21. a term representing one negative electron and one hydrogen nucleus". Rutherford was also the first to suggest in 1920 (8) that " . 1038-1060 10. 281-285 7. particularly as it suggests the . At the same time Chadwick (13) interpreted both these sets of results in terms of radiation consisting "of particles of mass nearly equal to that of the proton and with no net charge". J. Mag. . . Rutherford. Phil Trans. 1965. Soc. particularly paraffin. Reprinted in Experimental Researches in Electricity. . From observations of the nature of the particles formed from alpha particle scattering from N™. term 'protyle' given by Prout in his well-known hypothesis that all atoms are built up of hydrogen. 11. E. 2. 11. . 8. F. 12. 1911: p 396. Roy. G." It appears that Rutherford was also the first tentatively to suggest the name "proton" for this fundamental particle. 1930. J. 1913 26. Phil. that the same term had also been used by Walther Nernst (10) at least 10 years earlier in the context of ". . Harkins.669-688. Rutherford." In discussing the classification scheme for isotopes. 1891. Dover: New York. forming a kind of nuclear doublet. Soc. Glasstone. 1920. 708-711. Faraday. Chem. W. Macmlllian: London. Joliot. Z. Dublin Soc. 702-712.. to this new radiation lead to the ejection of high- velocity protons. M. (London) 1834. it may be possible for an electron to combine much more closely with the H nucleus. 1921. Nernst. 1921... Am. E.. 43. Such an atom would have very novel properties. 1960. H. Becker. . Sci. Bothe and Becker (11) reported that exposure of light elements. The actual observation of such a fundamental particle had to wait until some time later when. Stoney. While it becomes clear that atoms must exist.. They are: 1) Elements are made up of minute.136. 2) Atoms of the same element have the same properties.C. ultimate particles too small to be individually perceptible to the senses. 1932. hard. 1803 John Dalton postulates the existence of atoms and publishes the first table of atomic weights. 312. Because of its athestic basis as compared to Christianized Aristotle. but it was soon pushed aside (especially by Plato and Aristotle) and did not receive serious consideration until the seventeenth century. J. 1-25 Chronology of Discoveries in Atomic Structure c. “uncuttable”). 2 to 3. but he had no knowledge of their inner structure. These atoms maintain their identity through all physical and chemical changes.) of Adbera develop the materialistic philosophical concept of atoms (from atomos. 4) homogeneous and identical in substance. there is still no knowledge about what the structure of an atom is or how they can connect (or bond) together. Leucippus and his pupil. This long poem was based on atomistic teachings and explained much of its philosophy. God is able to create particles of matter of several sizes and figures. However.C. 460-371 B. This assumption is incorrect. Motion is inherent in the atoms (which move in a void) and they differ in their geometrical and mechanical properties. written by Lucretius (c. To Dalton. with the forces inversely proportional to their distances apart.Roy. 3) eternal..Soc. Aristotle did admit to there being a practical lower limit to a substance being subdivided. This hypothesis enables him to explain many features of chemistry on a simple basis. massy. Proc. 3) Atoms of the same element can unite in more than one ratio with another element to form more than one compound. since Dalton’s results are explainable only on an atomic basis. and 5) there are a finite number of kinds..C. ec. To this day. indivisible. although not a chemical atomist. 1704 Newton wrote. impenetrable. 692-708: 1933.13. 1687 Newton shows that Boyle’s Law (pressure of a gas is inversely proportional to its volume) follows if a gas is made up of mutually repulsive particles. Both Aristotle and Plato rejected the idea of a vacuum or void and the concept of self-moving bodies. 430 B.. 1660 Pierre Gassendi succeeds in freeing atomic theory from godlessness..). but was still able to develop much that was correct in his atomic theory. just as a book arises from different shaped letters and different ordering of the letters.). 1666 Boyle. Examples of Democritus’ atoms 1417 Rediscovery of the poem De Rerum Natura (On The Nature of Things). Chadwick. did think of matter in finite. but consisted of atoms which are 1) solid.C.). “It seems probable to me that God formed matter in solid. numerical ratios (i. Nature l932.55 B.” 1738 Daniel Bernoulli uses the atom idea to correctly account for Boyle’s Law. He called this minima naturalis. From these qualities of size. shape.. but it is historically important for being among the first uses of atoms or particles in an explanation. The general philosophical problem remained. Atoms of different elements have different properties. atomos.” Modern scholarship has identified 4 postulates implicit in Dalton’s work. featureless spheres which must exist. movable particles. Democritus (c. Dalton today is known as the “Father of Modern Atomic Theory. discrete. The general notion of explaining phenomena in terms of particles begins to arise through the efforts of Issac Newton and Robert Boyle..e. Atomism did enjoy some success among followers of Epicuris (c. . 1 to 2. Matter did not form a continuium. and indestructible particles called atoms. atomism had only appealed to a few radical philosophers. 142. and motion our world appears by collision and conglomeration. 95 . 129. very little of Lecuippus’ and Democritus’ writings remain. 4) Chemical combination between two or more atoms occur in simple. how can it be proved that matter is particulate in nature. atoms were hard. 341-270 B. 2) impenetrably hard (or cannot be divided). Dalton thought Newton had proved the gas molecules are mutually repelling. 1865 H. Schuster calculates the ratio of charge to mass of the particles making up cathode rays (today known as electrons) by measuring the magnetic deflection of cathode rays.) Thomson announces that he has found that the velocity of cathode rays is much lower than that of light. which would seem to indicate that cathode rays cannot be charged particles. He also finds that there is an extended glow on the walls of the tube and that this glow is affected by an external magnetic field. He introduces the terms electrode.if we adopt the atomic theory or phraseology.W. then the atoms of bodies which are equivalent to each other in their ordinary chemical action. this leads in 1897 to discovery that cathode rays are composed of electrons. anion.away from the anode. 1890 A. 1881 Herman Ludwig von Helmholtz shows that the electrical charges in atoms are divided into definite integral portions. Goldstein introduces the term cathode ray to describe the light emitted.produces first good vacuum tubes. Hertz shows that cathode rays are not deflected by electrically charged metal plates. Philipp von Lenard develops a cathode-ray tube with a thin aluminum window that permits the rays to escape. he changes the name to “electron. Sprengel improves the Geissler vacuum pump. 1896 Pieter P. refuting Hertz’s wave concept and showing that the cathode ray are particles.. 1871 C. he identified a special. Hittorf finds that a solid body put in front of the cathode cuts off the glow from the walls of the tube. these tubes. but he did not accept the idea that electricity is composed of particles. 1855 German inventor Heinrich Geissler develops mercury pump . suggesting the idea that there is a smallest unit of electricity. Pluecker uses Geissler tubes to show that at lower pressure.” 1838 Faraday studies electric discharges in a vacuum and discovers the Faraday dark space near the cathode. in addition to the cathode ray. leading to the discovery of the electron. allow in the rays to be studied in the open air. (He used the Faraday constant (total electric charge per mole of univalent atoms) divided by Avogadro’s Number. shows that the rays can penetrate thin foils of metal. later these will be found to be ions that have had electrons stripped in producing the cathode ray. 1895 Jean-Baptiste Perrin shows that cathode rays deposit a negative electric charge where they impact. Crookes proposes that they are molecules that have picked up a negative charge from the cathode and are repelled by it. 1892 Heinrich Hertz who has concluded (incorrectly) that cathode rays must be some form of wave. anode. the Faraday dark space grows larger. In 1891. He said. as modified by Sir William Crookes. integral electron-to-atom relationship. 1874 George Johnstone Stoney estimates the charge of the then unknown electron to be about 10-20 coulomb. In other words. these rays are called canal rays because of holes (canals) bored in the cathode.) Stoney also proposes the name “electrine” for the unit of charge on a hydrogen ion.F.J. Varley is first to publish suggestion that cathode rays are composed of particles.. 1886 Eugen Goldstein observes that a cathode-ray tube produces. Establishes that “rays” from the cathode travel in straight lines. Zeeman discovers that spectral lines of gases placed in a magnetic field are . radiation that travels in the opposite direction . cation. He establishes that a definite quantity of electricity is associated with each atom of matter. 1894 Joseph John (J.1833 Michael Faraday discovers quantitative laws of electrochemical deposition. The number of atoms that chemically can react is related to the number of electrons available in the system. “. ion.6021892 x 10-19 coulomb. which he takes to support the wave hypothesis. cathode. 1869 J.” 1876 Eugen Goldstein shows that the radiation in a vacuum tube produced when an electric current is forced through the tube starts at the cathode. close to the modern value of 1. 1883 H. become the first to produce cathode rays. 1858 Julius Plucker shows that cathode rays bend under the influence of a magnet suggesting that they are connected with in some way. have equal quantities of electricity associated with them. electrolyte. James Clerk Maxwell had recognized this method soon after Faraday had published. Stoney introduces the term electron. Faraday measured and quantized electricity. the other 22. H. Magnets and electrical fields can be used to deflect the cathode ray. Therefore. using Charles Wilson’s condensation chamber. showing that a charge was collected only when a magnetic field was used to bend the rays into a path leading to the collector. 1899 Thomson.A. The mass spectrometer is now a fundamental piece of equipment for college-level and above. as the exception. The negative electrons. In fact. 3) he was able to obtain a good value for the charge to mass ratio in two independent ways – from the temperature rise on the charge collector and by balancing the magnetic and electric deflections of the cathode ray beam. homogeneous manner through out the entire spherical volume. which is seen as a jelly-like material. He concludes that canal rays are atoms or molecules of gas that have had electrons knocked out of them and thus are attracted to the negative cathode. a phenomenon call the Zeeman effect. determine the ratio of charge to mass of the particles by deflecting them by electric and magnetic fields. and so on. From the direction and magnitude of the deflection. Thomson discovers the electron. lithium up to 3. a device that separates particles of varying mass by differences in deflection when acted upon by a particular magnetic field. in part because he has better vacuum pumps than were previously available. Knowing the strengths of the magnetic and electrical fields used allowed Thomson to calculate the e/m ratio. Another observation was that most substances tested would release more than one electron as the voltage on the CRT increased. the ions collect on the plate and neutralize the charge on the plates. Most hit the cathode. proves that cathode particles carry the same amount of charge as hydrogen ions in electrolysis. Thomson. Hendrik Antoon Lorentz uses Zeeman’s observations of the behavior of light in magnetic field to calculate the charge to mass ratio of the electron in an atom. In that sense. Lorentz explains this effect by assuming that light is produced by the motion of charged particles in the atom. found its e/m value to differ for every substance tested. Hydrogen. Helium released 1 or 2. about the same time Thomson does. if they are neutral. The development work on the mass spectrometer was continued by Francis Ashton. It is important to underscore the fact that the experiment did yield the SAME charge-to mass ratio (e/m) for every substance tested. the ratios are comparable to e/m ratios of electrically charges atoms. Emil Weichart. it was assumed to contain one electron and one proton (the fundamental positive charge) Since all substances contain electrons. He explained Hertz’s results (see 1883 above) by hypothesizing that gas remaining in the tube was ionized. as measured in electrolysis in solutions. but NOT ‘e’ or ‘m’ alone. and independently. beryllium up to 4. who later won a Nobel Prize for his work. the canal ray tube represented the first mass spectrometer. 2) he showed that the cathode rays are deflected by an electric field. the first known particle that is smaller than the atom. Using better vacuum pumps avoided this problem. Wilhelm Wein deflects canal rays with magnetic and electrical fields. Thomson's experiment with canal rays revealed at least two kinds of neon atoms. in his study of positive canal rays (in the late 1980’s and early 1900’s). which are individual particles are embedded in this positive electricity. a year before electrons are discovered and 15 years before it is known that electron are constituents of atoms. The amount of positive electricity is enough to counterbalance the . Thomson’s work can be divided into three components: 1) he improved J. 1902 Lord Kelvin suggests that the positive electricity in an atom is spread in a diffuse. they must therefore contain positive charges as well. never released more than one. he measures the charge of the electron and thus completes his discovery of the electron.split. 1897 Walter Kaufmann determines the ratio of the charge to mass for cathode rays in April. he concludes that they are positively charged particles with charge-to-mass ratios at least a thousand times greater than Thomson’s particles. but Kaufmann fails to consider that the rays might be subatomic particles. one with an atomic mass of 20. he also recognizes ionization to be a splitting of atoms and that particles emitted by the photoelectric effect have the same charge to mass ratio as cathode rays. but some slip through the holes where they can be studied. The deflection is related to both charge and mass of a particle. Perrin’s method of collecting charge inside the vacuum tube. he. It was quickly seen that an analysis of the scattering of alpha particles by a thin piece of metal could serve to test the model for accuracy. Geiger came to me and said. it would be in a region of average zero electric charge and could not be deflected. Geiger found that the most probable scattering angle is 0. The same would apply if it went too far from the center. Arthur Compton uses the scattering of electrons and high energy radiation in the 1920s and Robert Hofstadter uses it in the 1950s to study the fine structure of the nuclei and nucleons. so I said. with a great deal of energy. Hans Geiger recalled many years later how he found out Rutherford's theory: . Later he uses gold foil. 1903-1904 Thomson investigates this model more closely and makes a series of calculations concerning its stability. Many years later Rutherford recalls the events: One day. making the atom neutral in charge. but about 1 alpha particle in 8. 1908 Geiger reports that the number of scattered particles decreases rapidly with increased scattering angle and that no alpha particles were observed to be scattered by more than a few degrees. This result is not in accord with Thomson’s model of 1904. As the alpha particle approached the center of the atom. this is the key discovery that eventually leads to the concept of atomic number. presents his theory of the atom. J. so Geiger's measurements would help to remove its influence in calculations. “Don't you think that young Marsden. however. to the Manchester Literary and Philsophical society on March 7. “We have been able to get some of the alpha particles coming backwards…” It was quite the most incredible event that has ever happened to me in my life. Thomson’s “On the Structure of the Atom” proposes the “plum-pudding model” of the atom in which the electrons are embedded in a sphere of diffused positive charge. Thomson is awarded the Nobel Prize. determines that some alpha particles bounce back from a thin gold foil. in which a positive nucleus is surrounded by a ring of thousands of electrons. and you could show that if the scattering was due to the accumulated effect of a number of small scatterings the chance of an alpha particle being scattered backwards was very small. Electrons spherical cloud of positive charge (+8 in amount) 1904 “Kinetics of a System of Particles Illustrating the Line and Band Spectrum and the Phenomena of Radioactivity” by Hantaro Nagaoka includes his “Saturnian model” of the atom. Lenard suggests that the positive and negative charges are grouped in pairs. Alpha particle scattering had been discovered in 1903 by Rutherford and had initially been seen as a problem plaguing his research. It comes to be known as the Thomson model. consisting of a charged. ought to begin a small research?” Now I had thought that too. (The series of publications by Thomson’s group expounding on the model extended over the period 1903-1904) 1906 Ernest Rutherford studies scattering of alpha particles as they pass through mica.J. Then I remember two or three days later Geiger coming to me in great excitement and saying. it arose out of a need to satisfy mechanical and electrical stability. that no experiment gave rise to this model. whom I am training in radioactive methods. It was almost as incredible as if you fired a 15-inch shell at a piece of tissue paper and it came back and hit you. 1909 Ernest Marsden. Please note also that the sphere of positive charges are not protons.000 is scattered through an angle greater than 90 degrees. under the direction of Geiger and Rutherford. The electrons are seen as hard objects embedded in this cloud of positive charge. dense nucleus. This model was rejected by Thomson when it was shown to be unstable. 1907 Hans Geiger begins a program of research on the scattering of alpha particles as they pass through thin metal foils.negative. produced by scattering of X-ray beams. Scattering is a classic technique that is still used in science today. small. since we knew that the alpha particle was a very fast massive particle. Barkla discovers that each element has a characteristic X-ray. Note.87 degrees. 1911 Rutherford. Charles G. “Why not let him see if any alpha particles can be scattered through a large angle?” I may tell you in confidence that I did not believe that there would be. (Replication of the Rutherford experiment in a world of antimatter would yield the same results found in our world of matter.60217733(49) x 10-19 coulomb. Geiger and Marsden start a program of carefully measuring the fraction of alpha particles scattered by various angles. Moseley comfirms Bohr’s prediction and associates this order with an increase in the nuclear charge that increases in whole number multiples of the electron charge. Thus they establish that Rutherford’s picture of an atomic nucleus surrounded by electrons is correct. an uncharged particle that is part of the nucleus. He says. electrons in this case) give off energy and should spiral into the nucleus. “. 1915 Moseley is killed in battle.” He finally used the velocities of alpha particle emission by radioactive elements to eliminate the negative charge.592 x 10-19 coulomb. deduces the place of elements in the periodic table from their X-ray spectra and formulates his law of numbers of electrons. forming a kind of nuclear doublet.” 1914 Robert A.” Only an abstract of that early March talk survives. Such an atom would have . The 49 refers to the plus/minus error in the last two digits (the 33).) One problem of the model had to do with the orbiting electrons. independent of Bohr.“…one day. on a par with the electron. obviously in the best of spirits. that the atom (10-8 cm in diameter) was 99. the alpha particle deflections observed would have been the same if the nucleus had been either positively of negatively charged. This is less than 1% from the currently accepted value of 1. Bohr publishes a formula which gives the length of the radiation (usually X-rays) given off when an electron enters one of the innermost orbits of the atom in terms of the electrical charge on the nucleus. In 1913 they report that their experimental results are in good agreement with Rutherford’s calculations. (The only difference being the nuclear charge is positive and the charge on the electron negative. “The main deductions from the theory are independent of whether the central charge is supposed to be positive or negative. the alpha particles mostly went through empty space. from continued scattering experiments. but once in a while one came close to the nucleus (Rutheford's "charge concentration") and was deflected strongly (more than 90°). way out of scale (as in way. Interestingly. an element has the same number of electrons as its atomic number.e. the elements had been put in order (with a few exceptions) by increasing atomic mass. James Clerk Maxwell had shown that moving electric charges (i.) 1913 Rutherford. that the nuclear charge is exactly equal to the “atomic number” used to order the elements in the periodic table.. In the Rutherford Model. He sees it as a combination of an electron and a proton.99% empty space. Rutherford also concludes that nuclear charge is about half the atomic weight. In fact. 1920 Rutherford proposes the name “proton” for the fundamental particle which makes up the hydrogen nucleus. Hence. He also proposes the existence of the neutron. Rutherford wrote. Henry Gwyn-Jeffries Moseley. Rutherford. He also identifies atomic number (up to this time simply the numerical order of the elements in the periodic table) with the nuclear charge. The word proton had been used from about 1908 as a general term for a building block from which all elements are built. notes that atomic mass and the number of electrons in an atom are independent.. 1913 Antonius van der Broeck. Charles Barkla reaches the same conclusion from x-ray scattering experiments. The Thomson Model has no such possibility because the charges were equally spread throughout the atom. it may be possible for an electron to combine much more closely with the H nucleus. He also suggests. They do not. He recognizes this particle as being truly elementary. way too big) in the Rutherford Model. concludes that “the hydrogen atom has the simplest possible structure of a nucleus with one unit charge. The nucleus was so tiny (10-12 cm in diameter). how to explain this. 1912 Neils Bohr visits Rutherford and seizes on the problem of explaining the behavior of the electrons in their orbits around the nucleus. but later in the year he published a long article explaining his ideas. In the decay products are nuclei of hydrogen (what we now call protons). Millikan measures the charge on the electron directly. came into my room and told me that he now knew what the atom looked like and how to explain the large deflections of alpha particles. Before this time. 1919 Rutherford carries out the first artifical nuclear reaction. His work shows that the electric charge always comes in integer multiples of 1. The Rutherford Model Rutherford's Model of a thin gold foil Thomson's Model might be might look like this: pictured this way The nucleus dots are way. the beam bends.’ ” Other people had measured the e/m ratio or suggested that the cathode rays were composed of particles. see quarks). neon and these metal electrodes: gold.) The two magnetic fields interact and since the lines of force are curved.H Freeman: New York. (This point is developed in physics. A moving electrical charge creates a magnetic field. displacement of beam c.1957 Robert Hofstadter uses the scattering of electrons by the nucleus to study the detailed structure of the nucleus. In other words. but Thomson was the first to say that the cathode ray was a building block of the atom. He could not determine e or m independently with a cathode ray.. J. 227 in the text). Why? Electricity and magnetism are two forms of the same force. As he was to recall much later. matter derived from different sources such as hydrogen.. Today it is called deuterium.e. a term representing one negative electron and one hydrogen nucleus. he determined the charge-to-mass ratio (symbol e/m). oxygen. this matter being the substance from which the chemical elements are built up. Cathode ray bends toward positive electrical pole Why? Ray is negative in charge. electric field strength d. chlorine. he also proposes the existence of a hydrogen isotope with mass two. etc. When he solved his equations. not chemistry. And study continues to this day. 6. I was even told long afterwards by a distinguished physicist who had been present at my [1897] lecture at the Royal Institution that he thought I had been ‘pulling their legs. The best determination of e was done by Robert A. 7. What did Thomson measure? a. Millikan in 1914 in the oil-drop experiment (p. magnetic field strength e. traveled at less than the speed of light 4. The value was ALWAYS the same. nitrogen. 2. i.J. silver. Evidence that cathode rays were particles. suppose you measured the e/m ratio found in every possible combination of these gases: hydrogen. deflected by electric or magnetic fields b. protons. but he was proved right and for his courage he is . Thomson Discovers the Electron 1. velocity of beam b.” 1932 James Chadwick discovers the neutron. W. This was reaching quite far beyond what he had actually discovered. 3. a state in which the subdivision of matter is carried very much farther than in the ordinary gaseous state: a state in which all matter–that is. 1950 . ”At first there were very few who believed in the existence of these bodies smaller than atoms. 1921 William Draper Harkins introduces the term neutron: “. Harold Urey discovers deuterium. they had a charge. not electromagnetic radiation a. He also proposed the name proton for the nucleus of the hydrogen atom. not a combination (although. platnium. Partons come to be known as quarks.” Today we see it as a particle in and of itself. Thomson determined that the e/m ratio was the same for every combination of gas present in the tube and metal used as cathode and anode.–is of one and the same kind.” He had leaped to the conclusion that the particles in the cathode ray (which we now call electrons) were a fundamental part of all matter. amount of time spent in fields 5. A clear and lucid explanation of the equations Thomson used and his reasoning is found in The Discovery of Subatomic Particles (1983). It was a risky thing. “…we have in the cathode rays matter in a new state. copper. Why do We Remember Thomson as the Discoverer of the Electron? To put it into his own words. Steven Weinberg.very novel properties. had a detectable mass c. Interestingly. Cathode ray bends under magnet. 1964 Richard Feynman and George Zweig independently propose the parton model for the structure of protons and neutrons. and neutrons. 2. The e/m ratio for hydrogen was known from electrolysis experiments dating to the 1830's. nitrogen. The Production of Canal Rays a. The positively charged gas molecules are attracted to the negative cathode. Some slip through the holes in the cathode (the canals) and are studied. 1.J.) 6. direction of cathode ray cathode (negative) anode (postive) b. It never showed both e/m ratios at the same time and it never switched values. with hydrogen being positive and the electron negative. but not at the same time. he noticed that neon gave two spots (very close to each other) at the same time. The location and design of the cathode and anode determine if cathode rays or canal rays are studied. This makes the gas molecules positive in charge.put anode and cathode in solution of chemicals. d. Electrons are removed from the gas molecules by the impact. Cathode rays are produced. Thomson determined the e/m ratio of the canal rays. A critical assumption (see #8 below) here was that hydrogen and the electron both had the same absolute amount of electrical charge. J. positive molecules move to cathode cathode (negative) anode (positive) e. This work culminated in about 1902. It was always the same value at the low setting and always some other value at . He found that it varied depending on the type of gas that was present in the tube. The largest value obtained in these experiments was the e/m ratio for hydrogen. In other words. Cathode rays (made of electrons) hit residual gas molecules between cathode and anode. Thomson Discovers the Proton Remember that J. it had been established by Faraday that a given amount of current (e) would produce a given mass of chemical (m). Both types of rays are produced in the two types of tube. (Electrolysis . each had an e/m value different from the others. 4. 5. In the case of neon. There were also subtle variations within each value that led to the concept of isotopes. W. chlorine. This was consistent with the values from electrolysis. Note that the cathode and anode are placed close together. In 1898. when hydrogen.76 x 108. Wein determined the e/m ratio for hydrogen in canal rays to be about 104. A chemical reaction takes place. cathode anode the arrow shows the direction of the canal ray. In other words. 3. or neon was used.remembered as the discoverer of the electron. Also. Turn on current. c. (This is expressed in the modern way. which travel cathode to anode. For example. I think Thomson discussed it as the m/e ratio. the same sample of neon was producing two slightly different e/m ratios.) This meant that the electron must be much lighter than the hydrogen atom. canal rays differed from cathode rays in another important respect.J. At low voltage it had one ratio and at high voltage it had another ratio. Note that Thomson knew the e/m ratio for the electron to be 1. Thomson announced the discovery of the electron in 1897. Hydrogen had the largest e/m ratio when compared to other elements. From 1830's. because it was the lightest element. It was about 105. as Thomson's equipment and technique became better. helium had two possible e/m ratios. 7. such as water. The Proton Carl E. The discharges being interrupted . A contemporary of Faraday. Modern texts. The roots of this discovery can be traced back to the early works of Michael Faraday (2). Since essentially no uncombined hydrogen nuclei exist in electron donor solvents (1).. the gaseous medium is polarized anterior to [prior to] each discharge. It seems unlikely that Grove could have had a correct vision of the mechanism of proton production. This was explained as follows. 10 October 1985. an experimenter par excellence. Thus. with one negative charge. Moore and Bruno Jaselskis Loyola University. and polarized not merely physically. We offer an excerpt from Grove's paper which we believe is the first mention of the hydrogen cation in the gas phase (5). Faraday had introduced the terms anion and cation in 1834. Vol. the experiments above detailed appear to me to furnish a previously deficient link in the chain of analogy connecting dielectric induction with electrolysis.4) on continuous electrical discharges through rarefied gases. 859-860 The interesting anectodotal material describing the series of events which led to the recognition of the proton as a fundamental particle of nature seems worthy of a more prominent place in the teaching of physics and chemistry than currently given it. Grove reasoned that anions and cations are formed when an electric discharge proceeds through a tube containing a mixture of rarefied hydrogen and oxygen. As a consequence of these experiments. give only a few of the bare bones that litter the trail of this fundamental particle. the oxygen or anion being determined to [appearing at] the positive . And it had one positive charge (eventually called the proton). Grove was familiar with this theory and had. As may be gathered from my opening remarks. IL 60637 Published in the Journal of Chemical Education. which he described in the Philosophical Transactions of the Royal Society in 1852 (5). carried out a series of experiments on electrical discharges through gases. 228 of Heath text) times lighter than the proton. it is not surprising that the proton was first perceived in the plasmas of the gas phase reactions which occur at low pressures in electrical discharges. It NEVER released more than one at any given voltage and therefore had ONLY one e/m ratio. At low voltage one electron was released. pg. Hydrogen was the exception. This was explained as hydrogen having one electron.the higher setting. pointed out an inadequacy in the theory when it was applied to his gas battery. At higher voltage. in fact. Today the electron is known to be 1837 (p. No. 9. 62. . two electrons were released. at best. as is generally admitted. Chicago. This was the popular model until the advent of Arrhenius theory. he applied the von Grotthuss Model of electrical conduction by electrolytes. Chicago. 8. William Robert Grove. but one should not rule out the tantalizing possibility that his visions were decades ahead of his time. but chemically. who appears to have done the first experiments (3. . The only satisfactory rationale which I can present to my own mind of these phenomena is the following. To explain the results of his experiments. Grove was thinking in terms of hydrogen as a cation as early as 1852. IL 60626 Alfred von Smolinski University of Illinois at Chicago. terminal or anode. 1886 (8). We call attention to two papers by Goldstein in which he described the use of perforated cathodes. Research in this area of discharge tube phenomena was greatly accelerated by the development of the high-voltage transformer by Ruhmkorff (5) and the large-scale capacitor by Despretz (5). One can only wonder if the great intellect of von Helmholtz made a direct input into this research.8) and Wilhelm Wien (9). . it was Eugen Goldstein some decades later who gave these rays the name Kathodenstrahlen (cathode rays). after a year (1869-1870) at Breslau joined von Helmholtz at Berlin. and a similar layer of hydrogen or of electropositive molecules in contact with the cathode. and the hydrogen or cation being determined to [appearing atl the negative terminal or cathode. His talents as a scientist brought about the invention of the first fuel cell. thus making it into a formidable organization of scientists. With the death of Gustav Magnus the chair in physics at Berlin was vacated. law and natural philosophy. Plücker in Germany. in other words. The second is the oft quoted paper on Kanalstrahlen (canal rays). Helmholtz assumed the chair with the avowed purpose of bringing order into what he characterized as a pathless wilderness of competing theories and mathematical formulas. In fact. at the instant preceding discharge there would then be a molecule or superficial layer of oxygen or of electro-negative molecules in contact with the anode. While his name is not directly associated with the proton. who first noted that objects in the path of cathode rays cast shadows (2). and it was W. The first of these was published in 1876 (7) and the second 10 years later. Hittorf. William Robert Grove. At this stage in the development of the understanding of gaseous discharge phenomena. the electrodes in gas would be polarized as the electrodes in liquid are. and both were sponsored by von Helmholtz. von Helmholtz's name appears on both the 1876 and 1886 papers of Goldstein. Herman Ludwig Ferdinand von Helmholtz. but another contemporary and great experimenter in discharge-tube phenomena. However. This man. His legal talents and training allowed him via shrewd committee service to exercise a powerful behindthe. Both appeared in the Monthly Report of the Royal Prussian Academy of Science at Berlin. is credited with the discovery of cathode rays. who did the definitive studies on this fundamental particle. a student of Plücker. J. A contemporary of Faraday and Grove and the vice president of the Royal Society. who is credited with the discovery of canal rays. Eugen Goldstein. he sponsored both Eugen Goldstein (7. a dominant intellect entered physics from the field of physiology. in his Bakerian Lecture of 1858 reported deflections of the electrical discharges in rarefied gases by both magnetic and electrostatic means. where he received the doctorate in 1881. placed Grove in a position to play pivotal roles in both the experimental and structural sides of the science of his time. Gassiot. Thus he seems to have cast a large and perhaps benevolent shadow over this development. has received only limited attention in the literature. who held a professorship in the London Institution. John P. had recently completed his great treatises on physiology and was looking for new fields to conquer.scenes influence in the restructuring of the Royal Society. and the construction of one of the most popular voltaic cells of his time. His interesting mix of interests. which was based on hydrogen and oxygen electrodes. In addition. It remained for Wilhelm Wien. Thus. at first. Finally he noted that electrostatic deflection served as a good means of identifying the canal rays. The relatively weak magnetic fields that he employed did not give a discernible deflection of these light rays. and the first e/m measurements of the proton. it appears that to Wien must go the credit for the following: recognition that the canal rays produced in electrical discharges in low pressure hydrogen gas are positively charged particles. His provisional name became the accepted name. noted that these strange rays changed their color from gas to gas (8). a canal ray. but after publishing a third paper in the monthly reports of the Berlin Academy he allowed Wiedemann to republish all three papers (10). Wiedemann followed Röntgen's three papers with a fourth: namely.000 volts and determined e/m ratios for the proton where e is the charge and m is the mass of the particle. It is worth noting that Goldstein's work attracted little attention in the circle of German physicists (10) largely because they considered his research too descriptive. a very perceptive editor. The best source regarding this choice of a name seems to us to be found in a footnote by E. not hydrogen alone. but even with a weak horseshoe magnet that the cathode rays could be deflected and the canal rays were not noticeably deflected (9). These interesting observations of Goldstein lay buried in the monthly reports of the Berlin Academy for nearly 12 years. while experimenting with a number of gases. to recognize that canal rays were positively charged particles. Georg Wiedemann. He also specifically stated that the positive electricity carried by the canal rays was an identifying characteristic of the rays. His results agree rather well with results obtained by later investigators. However. However. In the 64th volume of the Annalen. who authored his papers as Willy Wien. refused permission. At that time the best opinion in the German circle on the origin of gaseous tube discharge phenomena was based on an electromagnetic concept (10). Rutherford that is appended to a paper by O. From his measurements on discharges through hydrogen he said that one is easily led to the opinion that canal rays were the hydrogen ions themselves. Röntgen. He noted that one could not visually distinguish them from weak cathode rays. They appeared in the Monthly Reports of the Würzburger Physics and Medicine Society. He suggested calling them canal rays until such time that someone selected a suitable name. Goldstein's work seemed to point to an explanation based on the presence of particulate matter. Masson . During the latter part of this 12-year period. for the canal rays were deflected to the negative pole of the electrostatic device. recognition that these rays contain hydrogen ions. It is not clear just how the term proton (from the Greek protos. Goldstein. the name canal ray came about from a general phenomenon. a publication of limited circulation. Wien designed deflection equipment using potentials up to 30. the identical magnetic fields strongly deflected the cathode rays. His conclusion was that for the canal rays observed he was dealing with a phenomenon which he could not explain. the 1866 paper of Goldstein. Röntgen discovered Röntgen Strahlen (X-rays) and published his first two communications. first) became associated with the positively charged hydrogen atom. Goldstein observed that in a tube fitted with a perforated cathode containing a rarefied gas a sheaf of light rays (canal rays) came through each perforation in a direction opposite the path of the cathode rays (electrons). We are principally interested in the proton. sought the permission of Röntgen to republish these two seminal papers in his widely distributed and prestigious Annalen. W. J. Mag. (6) Arrhenius.. (5) Grove. 465 (1936). 561 (1907). W." Ann. this interesting footnote does not give a definitive answer as to whom the choice of the term should be attributed. "Über die Elekricitätsleitung der Gase." Berlin Akd. (10) Ruckhardt. 1543.R.(11).. Green and Co.. Thomson followed up on Wien's measurements with a paper entitled "On Rays of Positive Electricity" (12). 1952. E." 3rd. The reader is referred to J. "Conduction of Electricity Through Gases. mathematical treatment.. 1... "Zur Entdeclcung der Kanalstrahlen vor fünfzig Jahren. CXXXVI. ed. and Thomson. Inc. However. Trans. Monatsber..J.. Cambridge University Press. (9) Wien. "VII. Dutton and Co. 244 (1902). J. II." Naturwissenschaften. Chem. "Text-Book of Electrochemistry. 30. 1914. Literature Cited (1) Bell. van Nostrand Company. 1928. E.J. (2) Hittorf.. New York. On Rays of Positive Electricity." Longmans. 576 of Everyman's Library. . 1950. Michael.P." D. G. 1902. (4) Faraday.. In May 1907. (3) Faraday.. On the Electro-Chemical Polarity of Gases. 142.." Phil. and additional points of history in the recognition of the proton. Phys... E. Svante. Inc. Michael. "Source Book on Atomic Energy. New York.J. Thomson's historic treatise "Conduction of Electricity Through Gases" (13) for further information on the instrumentation. .. Eugen. (7) Goldstein. Untersuchungen über die electrische Entladung in verdünnten Gasen.. 691 (1886). "Über eine noch nicht untersuchte Strahlungsform an der Kathodeinducirter Entladungen. Inc. (1869)." Ann. "Experimental Researches No. (8) Goldstein. W. "Experimental Researches In Electrochemstry. (I) 87 (1852)."JohnWiley and Sons. "2. "Vorläufige Mittheilungen über electrishe Entladungen in Verdünnten Gasen. der Physik 8.J. 279 (1876). (13) Thomson. Monatsber. J.. (12) Thomson." Berlin Akd. New York. New York. In these experiments and e/m measurements he used improved apparatus and greater experimental sophistication and observed both the proton and what appears to be the hydrogen molecule cation [H2+]. Soc. Roy. Samuel." Phil.P. experimental method. 13.R.." No. "XLVII. 1838.'s 1542."Acids andBases—Their Quantitative Behaviour. (11) Glasstone. and 1544...P.
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