360 APPENDIXwill take up the electron lost by the cesium atom and become the
chlorine ion, Cli 7 2-8-8.
Major Periods. The major periods of the periodic classification
embrace all the elements between two succeeding inert gases.
The first period comprises but two elements, hydrogen and
helium.
The next two periods comprise eight elements each.
The next two periods comprise eighteen elements each.
The next period comprises thirty-two elements.
Only a fragment of the next period is known. Obviously nuclei
of elements of a higher atomic number than 92 are unable to exist.
In fact, all the known elements of this period, and RaF (or polon-
ium) in the preceding period, are radioactive.
In the eight element periods the chemical properties of the
elements are dependent on the tendency for the atoms to gain or
lose electrons to acquire the structure of the atoms of the inert
gases at either end.
The electrostatic strain of acquiring a large charge (either posi-
tive or negative) is great and distinctly polar valences of more than
3 are very unusual. Yet sulphur in H2SO4 is considered to have
a positive valence of 6 due to its tendency to revert to the structure
of neon; and chlorine in HCIO4 is considered to have a positive
valence of 7 due to the same cause. Yet these high valences are
not altogether of a polar nature, although they are obviously
dependent on the distance of the element beyond the preceding
inert gas.
In the longer periods (of 18 and 32 elements) the elements near
the beginning and end obviously derive their properties from their
proximity to the inert gases.
In the middle parts of the longer periods the electrons to be
gained or lost to acquire the structure of the nearest inert gas
would impart too great an electrostatic strain. The usually
encountered positive valences of two or three of such elements is
attributed to a shifting of electrons into the kernel layers, thus
leaving but two or three electrons in the valence layer (or sheath)
of the neutral atom. Take, for example, the period beginning with
XeM: The elements Cs 6 6, Ba 6 e, La 57 display the expected valences
of 1, 2, and 3, respectively. Cerium, Ce 5 8, sometimes displays a
valence of 4 which is derived from the structure of xenon, Ce 68
2-8-18-18-8-4, but cerium shows an even stronger inclination to