(^14) THE ELEMENTS.
subordinate periods which are spaced from the elements Ni, Pd,
Er, Pt, in the same way that the two short periods are spaced
from He and Ne. The number of electrons in each of these
elements is such as might give the following structures:
Ni 28 2-8-18
Pd 46 2-8-18-18
Er 68 2-8-18-32-8
Pt 78 2-8-18-32-18
These 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 last of 32 against the repulsive forces. Given the
greater nuclear charge of the following atoms and these structures
become stable and consequently the succeeding elements revert
to those forms which are known as the beta forms or Ni/3, Pd/3,
Er/3 and Pt/3, respectively, and the subordinate periods are based
on these beta atoms in the same way that the major periods are
based on the inert gases.
THE ELEMENTS.
OCCUBBENCE. The relative amounts of the more common elements which
occur in the earth's surface, including the oceans and the atmosphere, are
shown by the following table compiled from statistics by F. W. Clarke:
Oxygen
Silicon
Aluminum
Iron
Calcium
Sodium
Potassium
49.78
26.08
7.34
4.11
3.19
2.33
2.28
Magnesium
Hydrogen
Titanium
Chlorine
Carbon
Phosphorus
Sulphur2.24
0.95
0.37
0.21
0.19
0.11
0.11
Barium
Manganese
Strontium
Nitrogen
Fluorine
Other elements..0.09
0.07
0.03
0.02
0.02
0.48
It is noteworthy that certain well-known elements, which are important
from a purely chemical as well as from a technical standpoint, are less abund-
ant than others that have been studied less and are regarded as "rare";
such as, for example, titanium, which is widely distributed but usually occurs
only in small amounts.
The elements, with the exception of those contained in the atmosphere,
sulphur, and the so-called "noble" metals, are not as a rule found in a free
or uncombined state. The most important natural compounds are the oxides
and sulphides (e.g., of silicon, iron, zinc, lead, antimony, mercury), the halides
(e.g., of sodium, potassium, magnesium), and the salts containing oxygen
(silicates, sulphates and carbonates).
EXTRACTION. The free elements may be separated from one another,
and from impurities, by distillation. In this way the constituents of the