734 The Philippine Journal of Science 1921 
partial removal of two or more electrons from a shell. If two 
or more electrons are pulled away from certain other electrons 
in the same shell, these other electrons are less repelled by them 
and are consequently bound closer to the nucleus than before. 
For example, the electrons in the shellof Cl! ( : Cl : ) are 
assumed to be held by a balance of forces near certain equili- 
brium positions (which perhaps are the corners of a cube). 
The difficulty *° which another atom would have in removing 
simultaneously two of these electrons, may be considered as a 
measure of the electron binding strength of unicovalent chlo- 
rine. Similarly the difficulty of removing two of the unshared — 
chlorine electrons in Cl <O ( : Cl : O : ) is a meas- 
ure of the electron binding strength of dicovalent chlorine. By el- 
ementary physical considerations, and much more convincingly 
by chemical evidence, we are led to believe that the shared pair in 
Cl<O (: Cl: 0: ) is farther from the chlorine 
kernel than the electrons in the shell of Cl ( : Cl : ), 
and that consequently the remaining (unshared) pairsin Cl «O 
are held more firmly to the chlorine kernel than are the shell 
electrons in CI. In general the effect of sharing a pair of elec- 
trons which were originally unshared is to draw this pair away 
from the kernel, and consequently allow all the other electrons 
to approach more closely to the kernel. That is, excluding double 
or triple bonds, the electron binding strength of an atom increases 
as its covalence is increased. With multiple bonds the reason- 
ing is the same, but it is evident that the change from a single 
to a double bond moves a pair of electrons closer to, rather than 
farther from, the original bond, consequently weakening it. 
* A physicist would probably prefer to speak of the potential energy 
of these electrons. This, or some other definition of “difficulty” would be 
feasible if we knew the mechanism by which another atom removes, OF 
partially removes, the two electrons. At present it seems more satisfactory 
for a chemist to leave the physical definition somewhat open, and to judge 
of the “difficulty” directly from chemical] data. : 
