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- The material cause of all things that exist is the coming together of atoms and void. Atoms are too small to be perceived by the senses. They are eternal and have many different shapes, and they can cluster together to create things that are perceivable. Differences in shape, arrangement, and position of atoms produce different things. By aggregation they provide bulky objects that we can perceive with our sight and other senses. We see changes in things because of the rearrangement of atoms, but atoms themselves are eternal. Words such as ‘nothing’, ‘the void’, and ‘the infinite’ describe space. Individual atoms are describable as ‘not nothing’, ‘being’, and ‘the compact’. There is no void in atoms, so they cannot be divided. I hold the same view as Leucippus regarding atoms and space: atoms are always in motion in space.



John Dalton- He proposed the Atomic Theory in 1803 which stated that (1) all matter was composed of small indivisible particles termed atoms, (2) atoms of a given element possess unique characteristics and weight, and (3) three types of atoms exist: simple (elements), compound (simple molecules), and complex (complex molecules). Dalton's theory was presented in New System of Chemical Philosophy (1808-1827). This work identified chemical elements as a specific type of atom, therefore rejecting Newton's theory of chemical affinities. Instead, Dalton inferred proportions of elements in compounds by taking ratios of the weights of reactants, setting the atomic weight of hydrogen to be identically one. Following Richter, he proposed that chemical elements combine in integral ratios. Despite the importance of the work as the first view of atoms as physically real entities and introduction of a system of chemical symbols, New System of Chemical Philosophy devoted almost as much space to the caloric theory as to atomism.

Dmitiri Mendeleev- He is best known for his work on the periodic table; arranging the 63 known elements into a Periodic Table based on atomic mass, which he published in Principles of Chemistry in 1869. His first Periodic Table was compiled on the basis of arranging the elements in ascending order of atomic weight and grouping them by similarity of properties. He predicted the existence and properties of new elements and pointed out accepted atomic weights that were in error. This organization surpassed attempts at classification by Beguyer de Chancourtois and Newlands and was published a year before the work of Lothar Meyer. Mendeleev provided for variance from strict atomic weight order, left space for new elements, and predicted three yet-to-be-discovered elements including eke-silicon and eke-boron. His table did not include any of the Noble Gases, however, which had not yet been discovered. The original table has been modified and corrected several times, notably by Mosley, but it had accommodated the discovery of isotopes, rare gases, etc. Mendeleev anticipated Andrews' concept (1869) of the critical temperature of gases. He also investigated the thermal expansion of liquids, and studied the nature and origin of petroleum. He was considered one of the greatest teachers of his time. In 1890 he resigned his professorship and in 1893 became director of the bureau of weights and measures in St. Petersburg, where he remained until his death in 1907.

J.J. Thompson- At the Cavendish Laboratory at Cambridge University, Thomson was experimenting with currents of electricity inside empty glass tubes. He was investigating a long-standing puzzle known as " cathode rays". His experiments prompted him to make a bold proposal: these mysterious rays are streams of particles much smaller than atoms, they are in fact minuscule pieces of atoms. He called these particles "corpuscles," and suggested that they might make up all of the matter in atoms. It was startling to imagine a particle residing inside the atom--most people thought that the atom was indivisible, the most fundamental unit of matter. Thomson's speculation was not unambiguously supported by his experiments. It took more experimental work by Thomson and others to sort out the confusion. The atom is now known to contain other particles as well. Yet Thomson's bold suggestion that cathode rays were material constituents of atoms turned out to be correct. The rays are made up of electrons: very small, negatively charged particles that are indeed fundamental parts of every atom. Modern ideas and technologies based on the electron, leading to television and the computer and much else, evolved through many difficult steps.

Earnest Rutherford- Rutherford, Ernest (1871-1937): Born in New Zealand, Rutherford studied under J. J. Thomson at the Cavendish Laboratory in England. His work constituted a notable landmark in the history of atomic research as he developed Bacquerel's discovery of Radioactivity into an exact and documented proof that the atoms of the heavier elements, which had been thought to be immutable, actually disintegrate (decay) into various forms of radiation.Rutherford was the first to establish the theory of the nuclear atom and to carry out a transmutation reaction (1919) (formation of hydrogen and and oxygen isotope by bombardment of nitrogen with alpha particles). Uranium emanations were shown to consist of three types of rays, alpha (helium nuclei) of low penetrating power, beta (electrons), and gamma, of exceedingly short wavelength and great energy.Ernest Rutherford also discovered the half-life of radioactive elements and applied this to studies of age determination of rocks by measuring the decay period of radium to lead-206. He also used alpha particles as atomic bullets, probed the atoms in a piece of thin (0.00006 cm) gold foil. He established that the nucleus was: very dense,very small and positively charged. He also assumed that the electrons were located outside the nucleus.

H.G.J. Moseley- Henry Moseley's research career lasted only forty months before tragically ending with his death on a Gallipoli battlefield in World War I. But in his classic study of the x-ray spectra of elements, he established the truly scientific basis of the Periodic Table by arranging chemical elements in the order of their atomic numbers. During this time period the first coherent theory of the structure of the atom was just then being developed by Rutherford and his research group, which, besides Moseley, included Niels Bohr, Hans Geiger, Kasimir Fajans, and others. The nature of x rays was also receiving new interest because of the discovery by the German physicist Max von Laue in 1912 that they were diffracted by their passage through crystals and therefore possessed a wave nature. Succeeding experiments by William L. Bragg the same year showed that similar results could be obtained by the reflection of x rays from the face of a crystal. Moseley persuaded Rutherford to allow him and a colleague, C. S. Darwin, to further study the nature of x rays. Their work demonstrated that the spectral line of platinum, which they were using as the anticathode in their x-ray tube, was characteristic of that element alone. Moseley returned to Oxford, and despite the experimental deficiencies of his laboratories, measured the x-ray spectral lines of nearly all the elements from aluminum to gold. The results of his study showed a clear and simple progression of the elements that was based on the number of protons in the atomic nucleus, rather than the order based on atomic weights that was then the basis of the Periodic Table.

Niels Bohr- Bohr began to work on the problem of the atom's structure. Ernest Rutherford had recently suggested the atom had a miniature, dense nucleus surrounded by a cloud of nearly weightless electrons. There were a few problems with the model, however. For example, according to classical physics, the electrons orbiting the nucleus should lose energy until they spiral down into the center, collapsing the atom. Bohr proposed adding to the model the new idea of quanta put forth by Max Planck in 1901. That way, electrons existed at set levels of energy, that is, at fixed distances from the nucleus. If the atom absorbed energy, the electron jumped to a level further from the nucleus; if it radiated energy, it fell to a level closer to the nucleus. His model was a huge leap forward in making theory fit the experimental evidence that other physicists had found over the years. A few inaccuracies remained to be ironed out by others over the next few years, but his essential idea was proved correct. He received the Nobel Prize for this work in 1922, and it's what he's most famous for. But he was only 37 at the time, and he didn't stop there. Among other things, he put forth the theory of the nucleus as a liquid drop, and the idea of "complementarity" -- that things may have a dual nature (as the electron is both particle and wave) but we can only experience one aspect at a time. After Hitler took power in Germany, Bohr was deeply concerned for his colleagues there, and offered a place for many escaping Jewish scientists to live and work. He later donated his gold Nobel medal to the Finnish war effort. In 1939 Bohr visited the United States with the news from Lise Meitner (who had escaped German-occupied Austria) that German scientists were working on splitting the atom. This spurred the United States to launch the Manhattan Project to develop the atomic bomb. Shortly after Bohr's return home, the German army occupied Denmark. Three years later Bohr's family fled to Sweden in a fishing boat. Then Bohr and his son Aage left Sweden traveling in the empty bomb rack of a British military plane. They ultimately went to the United States, where both joined the government's team of physicists working on atomic bomb at Los Alamos. Bohr had qualms about the consequences of the bomb. He angered Winston Churchill by wanting to share information with the Soviet Union and supporting postwar arms control. Bohr went on to organize the Atoms for Peace Conference in Geneva in 1955.

Schrodinger- In 1920 he took up an academic position as assistant to Msx Wein, followed by positions at Stuttgart (extraordinary professor), Breslau (ordinary professor), and at the University of Zurich (replacing von Laue) where he settled for six years. In later years Schrödinger looked back to his Zurich period with great pleasure - it was here that he enjoyed so much the contact and friendship of many of his colleagues, among whom were Hermann Weyl and Peter Debye. It was also his most fruitful period, being actively engaged in a variety of subjects of theoretical physics. His papers at that time dealt with specific heats of solids, with problems of thermodynamics (he was greatly interested in Boltzmann's probability theory) and of atomic spectra; in addition, he indulged in physiological studies of colour (as a result of his contacts with Kohlrausch and Exner, and of Helmholtz's lectures). His great discovery, Schrödinger's wave equation, was made at the end of this epoch-during the first half of 1926.It came as a result of his dissatisfaction with the quantum condition in Bohr's orbit theory and his belief that atomic spectra should really be determined by some kind of eigenvalue problem. Shrodinger viewed electrons as continuous clouds and introduced "wave mechanics" as a mathematical model of the atom.For this work he shared with Dirac the Nobel Prize for 1933.