358 APPENDIXand there are indications that it holds down to the dimensions of
the nucleus (10~^13 cm.). Within the atom there are other forces,
which include possible magnetic and centrifugal forces, which,
superimposed upon the force of Coulomb's law, give a condition of
stability to electrons in certain discrete positions within the atom.
If Coulomb's law alone were operative all electrons would sink into
the nucleus. The forces which prevent this and hold the plane-
tary electrons out at relatively great distances from the nucleus
are an expression of the nature of the electrons themselves, but
about this we know very little and have in the main to content
ourselves with the fact that there are certain discrete levels of
shells around the nucleus which can be occupied by electrons,
and the number of electrons in a shell cannot exceed a certain
maximum. Within the limitations of these discrete shells th(t
electrons appear to be subject to the electrostatic attraction of the
positive nucleus and the mutual repulsion of the electrons, and to
take such positions within atoms and molecules as are in conformity
with the requirements of Coulomb's law.
Electron Shells. A very theoretical study of the spectrum lines
of the light emitted by the elements, combined with a study of the
chemical properties of the elements, seems to establish the general
rule that the maximum number of electrons which can occupy the
succeeding shells* around the nucleus progresses as twice the
square of succeeding whole numbers, or as 2, 8, 18, 32
The mutual repulsion of the electrons for each other, however,
tends to prevent the larger numbers collecting in a shell, and
electrons may stay in outer shells rather than enter into an inner
one up to the maximum number. The configuration produced
by these numbers, however, seems to impart stability to a shell,
and whenever the shell is not filled to the maximum there seems
to be a strong tendency to acquire the number eight, and, next to
that, eighteen.
Stability of Inert Gases. The key to the method of building up
the electronic structure of atoms in layers was found through a
consideration of the inert gases. Their inertness points to the
great stability of their atomic structure. Since they never form
- Although spectrosoopio study indicates the existence of subgroups within
the shells here discussed, the subgrouping does not seem to exhibit itself
strongly in determining chemical properties. For the sake of simplicity it is
omitted entirely in this discussion.