Lindsey Kunze

Examine the history of the development of the atom. We have already discussed the major players in the development of the atom. Pick out at least 8 major players who helped aid in the development of the concept of the atom. Then explain each experiment in a concise explanation. Then explain how each one aided in the development of the next or how their concept assisted in the further development of the current quantum model of the atom.

Democritus: He opposed the atomic theory, but in doing so he summarized its principal doctrines. Thus he attributed to Leucippus the ideas that the atoms are "infinite in number and imperceptible because of the minuteness of their size. They move about in empty space (for there is empty space) and by joining together they produce perceptible objects, which are destroyed when the atoms separate. according to Democritus, the atom was the irreducibly minimal quantity of matter. The concept of the infinite divisibility of matter was flatly contradicted by the atomic theory, since within the interior of the atom there could be no physical parts or unoccupied space. Every atom was exactly like every other atom as a piece of corporeal stuff. But the atoms differed in shape, and since their contours showed an infinite variety and could be oriented in any direction and arranged in any order, the atoms could enter into countless combinations. In their solid interior there was no motion, while they themselves could move about in empty space. Thus, for the atomic theory, the physical universe had two basic ingredients: impenetrable atoms and penetrable space. For Democritus, space was infinite in extent, and the atoms were infinite in number. By their very nature the atoms were endowed with a motion that was eternal and not initiated by any outside force. Since the atoms were not created at any time in the past and would never disintegrate at any time in the future, the total quantity of matter in the universe remained constant: this fundamental principle of Democritus's atomic theory implies the conservation of matter, the sum total of which in the universe neither increases nor diminishes. Though Democritus's conception of the atom has been modified in several essential respects in modern times, his atomic theory remains the foundation of modern science. According to Democritus, the atom was the irreducibly minimal quantity of matter. The concept of the infinite divisibility of matter was flatly contradicted by the atomic theory, since within the interior of the atom there could be no physical parts or unoccupied space. Every atom was exactly like every other atom as a piece of corporeal stuff. But the atoms differed in shape, and since their contours showed an infinite variety and could be oriented in any direction and arranged in any order, the atoms could enter into countless combinations. In their solid interior there was no motion, while they themselves could move about in empty space. Thus, for the atomic theory, the physical universe had two basic ingredients: impenetrable atoms and penetrable space. For Democritus, space was infinite in extent, and the atoms were infinite in number. By their very nature the atoms were endowed with a motion that was eternal and not initiated by any outside force. Since the atoms were not created at any time in the past and would never disintegrate at any time in the future, the total quantity of matter in the universe remained constant: this fundamental principle of Democritus's atomic theory implies the conservation of matter, the sum total of which in the universe neither increases nor diminishes. Though Democritus's conception of the atom has been modified in several essential respects in modern times, his atomic theory remains the foundation of modern science.

John Dalton: Dalton's theory was different in that it had the weight of careful chemical measurements behind it. It wasn't just a philosophical statement that there are atoms because there must be atoms. His atomic theory, stated that elements consisted of tiny particles called atoms. He said that the reason an element is pure is because all atoms of an element were identical and that in particular they had the same mass. He also said that the reason elements differed from one another was that atoms of each element were different from one another; in particular, they had different masses. He also said that compounds consisted of atoms of different elements combined together. Compounds are pure substances (remember they cannot be separated into elements by phase changes) because the atoms of different elements are bonded to one another somehow, perhaps by hooks, and are not easily separated from one another. Compounds have constant composition because they contain a fixed ratio of atoms and each atom has its own characteristic weight, thus fixing the weight ratio of one element to the other. In addition he said that chemical reactions involved the rearrangement of combinations of those atoms.

Dmitri Mendeleev: Mendeleev's original work covered a wide range, from questions in applied chemistry to the most general problems of chemical and physical theory. His name is best known for his work on the Periodic Law. Various chemists had traced numerical sequences among the atomic weights of some of the elements and noted connections between them and the properties of the different substances; but it was left to him to give a full expression to the generalization, and to treat it not merely as a system of classifying the elements according to certain observed facts, but as a "law of nature" which could be relied upon to predict new facts and to disclose errors in what were supposed to be old facts. Thus in 1871 he was led by certain gaps in his tables to assert the existence of three new elements so far unknown to the chemist, and to assign them definite properties. These three he called ekaboron, ekaaluminium, and ekasilicon; and his prophecy was completely vindicated within fifteen years by the discovery of gallium in 1871, scandium in 1879, and germanium in 1886. Again, in several cases he ventured to question the correctness of the "accepted atomic weights", on the ground that they did not correspond with the Periodic Law, and here also he was justified by subsequent investigation. In 1902, in an attempt at a chemical conception of the ether, he put forward the hypothesis that there are in existence two elements of smaller atomic weight than hydrogen, and that the lighter of these is a chemically inert, exceedingly mobile, all-penetrating and all-pervading gas, which constitutes the "aether." Mendeleev also devoted much study to the nature of such "indefinite" compounds as solutions, which he looked upon as homogeneous liquid systems of unstable dissociating compounds of the solvent with the substance dissolved, holding the opinion that they are merely an instance of ordinary definite or atomic compounds, subject to John Dalton's laws. In another department of physical chemistry he investigated the expansion of liquids with heat, and devised a formula for its expression similar to Gay-Lussac's law of the uniformity of the expansion of gases, while so far back as 1861 he anticipated T. Andrews's conception of the critical temperature of gases by defining the absolute boiling-point of a substance as the temperature at which cohesion and heat of vaporization become equal to zero and the liquid changes to vapor, irrespective of the pressure and volume. Mendeleev wrote largely on chemical topics, his most widely known book probably being The Principles of Chemistry, which was written in 1868-70, and has gone through many subsequent editions in various languages. For his work on the Periodic Law he was awarded in 1882, at the same time as L. Meyer, the Davy medal of the Royal Society, and in 1905 he received its Copley medal. He died at St. Petersburg on the 2nd of February 1907.

Pierre and Marie Curie: Pierre and Marie Curie are best known for their pioneering work in the study of radioactivity, which led to their discovery in 1898 of the elements radium and polonium. He discovered the phenomenon of piezoelectricity, whereby changes in the volume of certain crystals excite small electric potentials. Along with work on crystal symmetry, Pierre Curie studied the magnetic properties of materials and constructed a torsion balance with a tolerance of 0.01 mg. He discovered that the magnetic susceptibility of paramagnetic materials is inversely proportional to the absolute temperature (Weiss-Curie's law) and that there exists a critical temperature above which the magnetic properties disappear (Curie temperature). Marie measured the strength of the radiation emitted from uranium compounds and found it proportional to the uranium content, constant over a long period of time, and uninfluenced by external conditions. She detected a similar immutable radiation in the compounds of thorium. While checking these results, she made the unexpected discovery that uranium pitchblende and the mineral chalcolite emitted about four times as much radiation as could be expected from their uranium content. In 1898 she therefore drew the revolutionary conclusion that pitchblende contains a small amount of an unknown radiating element.

Ernest Rutherford: In 1899 Ernest Rutherford studied the absorption of radioactivity by thin sheets of metal foil and found two components: alpha (a) radiation, which is absorbed by a few thousandths of a centimeter of metal foil, and beta (b) radiation, which can pass through 100 times as much foil before it was absorbed. Shortly thereafter, a third form of radiation, named gamma (g) rays, was discovered that can penetrate as much as several centimeters of lead. The three kinds of radiation also differ in the way they are affected by electric and magnetic fields. Rutherford began his graduate work by studying the effect of x-rays on various materials. Shortly after the discovery of radioactivity, he turned to the study of the -particles emitted by uranium metal and its compounds. Before he could study the effect of -particles on matter, Rutherford had to develop a way of counting individual -particles. He found that a screen coated with zinc sulfide emitted a flash of light each time it was hit by an -particle. Rutherford and his assistant, Hans Geiger, would sit in the dark until his eyes became sensitive enough. They would then try to count the flashes of light given off by the ZnS screen. (It is not surprising that Geiger was motivated to develop the electronic radioactivity counter that carries his name.) Rutherford found that a narrow beam of -particles was broadened when it passed through a thin film of mica or metal. He therefore had Geiger measure the angle through which these -particles were scattered by a thin piece of metal foil. Because it is unusually ductile, gold can be made into a foil that is only 0.00004 cm thick. When this foil was bombarded with -particles, Geiger found that the scattering was small, on the order of one degree.
These results were consistent with Rutherford's expectations. He knew that the -particle had a considerable mass and moved quite rapidly. He therefore anticipated that virtually all of the -particles would be able to penetrate the metal foil, although they would be scattered slightly by collisions with the atoms through which they passed. In other words, Rutherford expected the -particles to pass through the metal foil the way a rifle bullet would penetrate a bag of sand.One day, Geiger suggested that a research project should be given to Ernest Marsden, who was working in Rutherford's laboratory. Rutherford responded, "Why not let him see whether any -particles can be scattered through a large angle?" When this experiment was done, Marsden found that a small fraction (perhaps 1 in 20,000) of the -particles were scattered through angles larger than 90o (see Figure 6.7a). Many years later, reflecting on his reaction to these results, Rutherford said: "It was quite the most incredible event that has ever happened to me in my life. 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." Rutherford concluded that there was only one way to explain these results. He assumed that the positive charge and the mass of an atom are concentrated in a small fraction of the total volume and then derived mathematical equations for the scattering that would occur. These equations predicted that the number of -particles scattered through a given angle should be proportional to the thickness of the foil and the square of the charge on the nucleus, and inversely proportional to the velocity with which the -particles moved raised to the fourth power. In a series of experiments, Geiger and Marsden verified each of these predictions.When he published the results of these experiments in 1911, Rutherford proposed a model for the structure of the atom that is still accepted today. He concluded that all of the positive charge and essentially all of the mass of the atom is concentrated in an infinitesimally small fraction of the total volume of the atom, which he called the nucleus (from the Latin for little nut). Most of the -particles were able to pass through the gold foil without encountering anything large enough to significantly deflect their path. A small fraction of the particles came close to the nucleus of a gold atom as they passed through the foil. When this happened, the force of repulsion between the positively charged -particle and the nucleus deflected the -particle by a small angle. Occasionally, an -particle traveled along a path that would eventually lead to a direct collision with the nucleus of one of the 2000 or so atoms it had to pass through. When this happened, repulsion between the nucleus and the -particle deflected the -particle through an angle of 90o or more. By carefully measuring the fraction of the -particles deflected through large angles, Rutherford was able to estimate the size of the nucleus. According to his calculations, the radius of the nucleus is at least 10,000 times smaller than the radius of the atom. The vast majority of the volume of an atom is therefore empty space.
Naming the Proton (Ernest Rutherford)
Shortly after the World War I, in 1920, Rutherford proposed the name proton for the positively charged particles in the nucleus of an atom.
Proposing the Neutron (Ernest Rutherfor)
At the same time that Rutherford proposed the name proton for the positively charged particle in the nucleus of an atom, he proposed that the nucleus also contained a neutral particle, eventually named the neutron. It was not until 1932, however, that James Chadwickwas able to prove that these neutral particles exist.


J.J. Thomson: He discovered the electron in a series of experiments designed to study the nature of electric discharge in a high-vacuum cathode-ray tube, an area being investigated by numerous scientists at the time. Thomson interpreted the deflection of the rays by electrically charged plates and magnets as evidence of "bodies much smaller than atoms" that he calculated as having a very large value for the charge-to-mass ratio. Later he estimated the value of the charge itself. In 1904 Thomson suggested a model of the atom as a sphere of positive matter in which electrons are positioned by electrostatic forces. His efforts to estimate the number of electrons in an atom from measurements of the scattering of light, X, beta, and gamma rays initiated the research trajectory along which his student Ernest Rutherford moved. Thomson's last important experimental program focused on determining the nature of positively charged particles. Here his techniques led to the development of the mass spectrograph. His assistant, Francis Aston, developed Thomson's instrument further and with the improved version was able to discover isotopes—atoms of the same element with different atomic weights—in a large number of nonradioactive elements.

Henry Mosley: British physicist who first established the atomic numbers of the elements by studying their X-ray spectra. This led to a complete classification of the elements, and also provided an experimental basis for an understanding of the structure of the atom. he made discoveries which were of fundamental importance to the development of both physics and chemistry. When he joined Rutherford's group in 1910, Rutherford was researching the phenomena associated with natural radioactivity. Moseley at first helped Rutherford in this work, but when reports of the diffraction of X-rays by Max von Laue (1879-1960) reached him in 1912, Moseley persuaded Rutherford to allow him to study X-ray spectra. He received instruction in X-ray diffraction from Lawrence Bragg (1890-1971) and in 1913 Moseley introduced X-ray spectroscopy to determine the X-ray spectra of the elements. In a series of brilliant investigations, Moseley allowed the X-rays produced from various substances used as a target in an X-ray tube to be diffracted by a crystal of potassium ferrocyanide. The glancing angles were measured accurately and the position of the diffracted beams determined to obtain the wavelengths and frequencies of the X-rays emitted. Moseley examined metals from aluminum to gold and he found that their X-ray spectra were similar but with a deviation that changed regularly through the series. He found that a graph of the square root of the frequency of each radiation against the number representing the element's position in the periodic table gave a straight line. He called this number the atomic number of the element, which has since been shown to be the positive charge on the nucleus and thus the number of protons in the nucleus. Since the atom is electrically neutral, the atomic number is also the number of electrons surrounding the nucleus. It was as a direct result of this work that atomic numbers were placed on a sound experimental foundation. Moseley found that when the elements are arranged in the periodic table according to their atomic numbers, all irregularity caused in the older system of grouping elements by their atomic weight disappeared. Now that the elements were numbered, the rare earth elements could be sorted out, a process that Moseley began at Oxford towards the end of his life. The numbering system also enabled Moseley to predict that several more elements would be discovered, namely those with atomic numbers of 43, 61, 72, 75, 87 and 91. These were all found in due course. Although the number of elements which Moseley was able to examine was limited, the equation relating the square root of the frequency to the atomic number has been found to hold in all cases. The equation is known as Moseley's Law. It has enabled scientists to identify a total of 105 elements in a continuous series of atomic numbers. Any further elements that might be produced by nuclear reactions can only have greater atomic numbers. In 1913 and 1914, the young physicist published his findings in two remarkable papers in the Philosophical Magazine and entitled them The High-Frequency Spectra of the Elements. Moseley's discovery told how many electrons were present in any element, and tied in nicely with the quantum theory of the hydrogen atom which was published in 1913 by Niels Bohr (1885-1962). Moseley's fundamental discovery was a milestone in our knowledge of the constitution of the atom, and we are left to ponder on what this great brain might have discovered had he not been so tragically killed at so young an age.

James Chadwick: James Chadwick proved the existence of neutrons, the elementary particle without any electrical charge and a fundamental building block of the atom's nucleus. One of the most important scientific discoveries of the twentieth century, it effectively solved the jigsaw puzzle of the atom, and earned Chadwick the 1935 Nobel Prize for Physics. Following Chadwick's breakthrough, he and other scientists began experimenting on all types of materials with neutrons, leading to the discovery of nuclear fission when uranium is bombarded with neutrons, and the eventual development of nuclear weapons and nuclear power production. Chadwick proofs the existence of the neutron in 1932 what accelerated the research in nuclear physics immensely. Chadwick's own research focused on radioactivity. In 1919 Rutherford developed a theory on the atom's nucleus and theorized on the existence of a neuron, a non charged particle within the atom's nucleus. But they and other researchers were finding that the neutron did not seem to be the only particle in the nucleus. As they studied atomic disintegration, they kept seeing that the atomic number (number of protons in the nucleus, equivalent to the positive charge of the atom) was less than the atomic mass (average mass of the atom). For example, a helium atom has an atomic mass of 4, but an atomic number (or positive charge) of 2. Since electrons have almost no mass, it seemed that something besides the protons in the nucleus were adding to the mass. One leading explanation was that there were electrons and additional protons in the nucleus as well -- the protons still contributed their mass but their positive charge was canceled out by the negatively charged electrons. So in the helium example, there would be four protons and two electrons in the nucleus to yield a mass of 4 but a charge of only 2. Rutherford also put out the idea that there could be a particle with mass but no charge. He called it a neutron, and imagined it as a paired proton and electron. There was no evidence for any of these ideas. Chadwick kept the problem in the back of his mind while working on other things. Experiments in Europe caught his eye, especially those of Frederic and Irene Joliot-Curie. They used a different method for tracking particle radiation. Chadwick repeated their experiments but with the goal of looking for a neutral particle -- one with the same mass as a proton, but with zero charge. His experiments were successful. He was able to determine that the neutron did exist and that its mass was about 0.1 percent more than the proton's. He published his findings with characteristic modesty in a first paper entitled "Possible Existence of Neutron." In 1935 he received the Nobel Prize for his discovery. His findings were quickly accepted and Werner Heisenberg then showed that the neutron could not be a proton-electron pairing, but had to be its own unique particle -- the third piece of the atom to be found. This new idea dramatically changed the picture of the atom and accelerated discoveries in atomic physics. Physicists soon found that the neutron made an ideal "bullet" for bombarding other nuclei. Unlike charged particles, it was not repelled by similarly-charged particles and could smash right into the nucleus. Before long, neutron bombardment was applied to the uranium atom, splitting its nucleus and releasing the huge amounts of energy predicted by Einstein's equation E = mc2.