362 APPENDIXThese structures possess layers of inherent stability, but the
nuclear charge is not great enough to hold the large outer layer of
18 or the next to the last of 32 against the repulsive forces.
Given the greater nuclear charge of the succeeding elements, how-
ever, and these structures become stable. (In the case of erbium
the three additional nuclear charges of lutecium are necessary
before this particular erbium kernel structure becomes stable.)
Thus the following elements derive their properties from the
tendency to revert to the stable forms of the key elements. These
are called the beta forms, or Ni/3, Pd/3, Er/3, Pt/3, and the subordi-
nate periods are based on these forms, as is apparent in the table,
in the same way that the major periods are based on the inert gases.
Non-Polar Valence. Inorganic chemists very generally ascribe
a polarity to valence even in compounds which show little or no
ionization. In the carbon compounds of organic chemistry there is
almost absolutely no evidence of polarity, and the organic chemist
has been in the habit of regarding valence merely as the number
of non-polar chemical bonds holding atoms together in molecules.
These two ideas of valence formerly led to much misunderstanding.
But this misunderstanding is obviated by an hypothesis first
suggested by G. N. Lewis in 1916 and extended by Langmuir in
1919, which allows both polar and non-polar valence to be referred
back to the same common cause. This cause is the tendency of
atoms to acquire outer layers containing stable numbers, (most
often 8, but 2 for the period ending with He and 18 in the periods
based on Ni/3, Pd/3, and Pt/3). The specific hypothesis is that in the
case of non-polar valence certain electrons may simultaneously
occupy places in the completed sheaths of two atoms. Thus in
forming the chlorine molecule from two neutral chlorine atoms
Cln 2-8-7, the two sheaths are completed if two electrons are
held jointly in both sheaths. The kernel charge of chlorine is 7.
The following formula, in which the sheath electrons are repre-
sented by dots and the symbol Cl stands for the chlorine kernel
with its 7 net positive charge, represents the arrangement in the
chlorine molecule:
: Cl: Cl :The pair of electrons held jointly constitutes the chemical bond,
for it exerts an electrostatic attraction for both kernels and thus