IV. THE NUMBER OF ELECTRONS IN AN ATOM
If it be considered as fairly conclusively established by the experiments just described that an atom consists of a heavy but very minute positively charged nucleus which holds light negative electrons in some sort of a configuration about it, then the number of negative electrons outside the nucleus must be such as to have a total charge equal to the free positive charge of the nucleus, since otherwise the atom could not be neutral.
But the positive charge on the nucleus has been approximately determined as follows: With the aid of the knowledge, already obtained through the determination of
, of the exact number of atoms in a given weight of a given substance, Sir Ernest Rutherford[141] first computed the chance that a single helium atom in being shot with a known speed through a sheet of gold foil containing a known number of atoms per unit of area of the sheet would suffer a deflection through a given angle. This computation can easily be made in terms of the known kinetic energy and charge of the
-particle, the known number of atoms in the gold foil, and the unknown charge on the nucleus of the gold atom (see [Appendix F]). Geiger and Marsden[142] then actually counted in Rutherford’s laboratory, by means of the scintillations produced on a zinc-sulphide screen, what fraction of, say, a thousand
-particles, which were shot normally into the gold foil, were deflected through a given angle, and from this observed number and Rutherford’s theory they obtained the number of free positive charges on the nucleus of the gold atom.
Repeating the experiment and the computations with foils made from a considerable number of other metals, they found that in every case the number of free positive charges on the atoms of different substances was approximately equal to half its atomic weight. This means that the aluminum atom, for example, has a nucleus containing about thirteen free positive charges and that the nucleus of the atom of gold contains in the neighborhood of a hundred. This result was in excellent agreement with the conclusion reached independently by Barkla[143] from experiments of a wholly different kind, namely, experiments on the scattering of X-rays. These indicated that the number of scattering centers in an atom—that is, its number of free negative electrons—was equal to about half the atomic weight. But this number must, of course, equal the number of free positive electrons in the nucleus.