4 The Philippine Journal of Science 1921 
KERNEL 0 
The completed inner shells of an atom (that is, all the shells 
except the outermost) together with its nucleus, constitute 
the kernel. This is bound together by such strong electrostatic 
and magnetic forces that it is never disrupted in ordinary chem- 
ical reactions, but acts as an unchangeable unit except under 
extremely penetrating forces, such as those of X-rays. 
SHELL 
For brevity the outer shell is called simply the shell. Unless 
this is complete, as in the helium group, the neutral atom exerts 
forces, both magnetic and electrostatic, effective at a considerable 
distance. It is upon these forces that all chemical action depends. 
This is, roughly, the theory of atomic structure which seems 
to meet with general acceptance among chemists at the present 
time. 
CHEMICAL UNION 
Two types of chemical union have been distinguished ; namely, 
primary valence unions and secondary valence unions. The 
present paper deals chiefly with the primary type, which involves 
the main uniting forces of atoms. This has been subdivided into 
two kinds, which may be called salt-forming unions and direct 
unions. 
Salt-forming unions are caused by the fact that an atom with 
a nearly complete shell has a strong tendency toward completing 
its shell, and is able to appropriate from another atom with a less 
complete shell one or more electrons. Both atoms thus become 
charged and attract each other. This type of union has often 
been represented as follows: 
Na 4 - - Cl 
Direct unions have been a much more puzzling problem, but 
Lewis 6 7 has advanced the explanation, later developed by Lang- 
muir, that they are brought about by the sharing of one or more 
pairs of electrons by two atomic shells. For such a union to take 
place it is necessary that both atoms have a tendency to complete 
their shells. Since, in such a case, neither can detach an electron 
from the other, they compromise by sharing 2, 4, or 6 (but not 1, 
6 Lewis, G. N., loc. eit.; cf. Bohr. N., Phil. Mag. 26 (1913) 1476; Parson, 
A. L., Smithsonian Misc. Coll. 65, No. 11; Milliken, R. A., Science 45 
(1917) 321; Silberstein, L., Phil. Mag. 39 (1920) 46. 
7 Loc. cit. ; cf. Parson’s “group of fourteen,” op. cit., p. 29. 
