10 The Philippine Journal of Science 1921 
atom in Table 1, one finds a graded weakness in going either to 
the right or upward. Another observed fact is that the electro- 
negative tendency of an atom varies according to the atoms com- 
bined with it, in a manner which shows that electrons in a shell 
are held by forces of an elastic nature, and that they shift their 
positions of equilibrium under the influence of outside electro- 
static forces. 
Due to these natural and acquired differences in electro- 
negative powers, it is only in such symmetrical cases as 
Cl — Cl, H 3 C — CH 3 that a shared pair of electrons is shared 
equally by two atoms. If A is more electronegative than B, 
in general the shared pair will be held more closely by A than 
by B, in such a manner that A will become negatively charged 
as compared to B, thus: A~ — B + . Such a union has long been 
called a polar union, and is generally represented by an arrow 
indicating a partial electron transfer: A<— B. There is no 
difficulty in introducing the arrow into the proposed system 
in cases where it is desired to point out polarity, as H— >C1, 
H-»O^H. As the complete polarity of the salt-forming union 
has already been well represented by the sign ~ + , the arrow 
will be used only to denote polarity in direct unions. 
The distinction and relation between a borrowing union and 
a polar union should be clearly understood. The borrowing 
union sign A 20 B indicates that both electrons of the shared 
pair belonged exclusively to the lending atom, B, before the 
union took place. The polar union sign A<— B has in the past 
signified nothing as to the origin of the shared pair but only 
that after union there is an electrostatic dipole A^-B+. 
There are then two cases of polar union, depending on 
whether the union is normal or borrowing. From the defi- 
nition it is evident that the borrowing union A 30 B is essentially 
a polar union, because the borrowing atom — A — acquires an 
interest in two electrons with which it originally had no 
connection. Therefore if — A — was originally neutral, it 
becomes a negative pole. In fact, A so B can be written A - — + B. 
Nevertheless, A shows no tendency to leave B in the form of 
a negative ion, and the shared pair is almost always held more 
closely by B than by A ; so the distinct symbol A do B should 
be retained, and the symbol A<^B used only to represent a 
normal polar union; that is, one in which each atom supplies 
one electron of the shared pair, and in which after union the 
shared pair is more closely associated with A than with B, 
so that an electrostatic dipole is formed. The union A^B, 
