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OF THE NATIONAL RESEARCH COUNCIL Vol. 2, Part 6 JULY, 1921 Number 14 A GENERAL SURVEY OF THE PRESENT STATUS OF THE ATOMIC STRUCTURE PROBLEM Report of the Committee on Atomic Structure of the *This committee of the Division of Physical Sciences of the National Research Coun cil consists of the following members: F. A. Saunders, Chairman, P. W. Bridgman, Irving Langmuir, G. N. Lewis, R. A. Millikan, Leigh Page, D. L. Webster. The late Professor H. A. Bumstead was chairman of the committee as originally constituted and much of the work preliminary to the preparation of the report was done under his leadership. PART I THE PRESENT CONCEPTION OF ATOMIC STUCTURE BY DAVID L. Webster PROBLEMS And MethodS A hundred years ago, when Dalton first used the idea of atoms, there was no question of atomic structure. The very name "atom" meant "indivisible," and an atom was supposed to be the last stage in the division of matter. At that time there was no way of knowing how small an atom was. Since then, it has been found to be about ten thousand times as small as the smallest particle whose structure we can see at all, even with a microscope. Nevertheless it appears that the atom has a structure, and not only that, but that along with the problem of atomic structure we now have the problem of atomic dynamics as well. These problems date, practically, from the discovery of the electron. Maxwell,' in 1869, had explained the dispersion of light in a prism by the forced vibrations of minute electrified particles in the material of the prism. And in 1897, Sir J. J. Thomson2 proved that cathode rays were indeed just such electrified particles as Maxwell had spoken of, and that they were smaller and lighter than any atoms. Then it appeared, from the study of cathode rays, that these minute particles were all exactly alike, no matter what sort of material they were extracted from, and that the mass of each one of them was only about one two thousandth part of that of a hydrogen atom. So they were given a new name, "corpuscles," or "electrons," and they were assumed to be the most important units of which all atoms are built up,-not the only units, however. For the electrons are always charged negatively, and an atom is often as a whole electrostatically neutral, or even positive. So there must be positive electricity in it also. But the only positively charged particles that have ever been isolated, as electrons have, are positively charged ions, and they may be identified by their much greater masses as practically whole atoms. So the electrons, or negative corpuscles, being so light and mobile, are more in evidence in most phenomena than the positive electricity. Thus the first problem of atomic structure is how many of these electrons there are in any atom, and the next is what sort of positions they occupy. 1J. C. Maxwell, Math. Tripos Exam., 1869. |