19,1 Perkins: The Octet Theory of Valence 7 
between the ions is due to electrostatic attraction. This is the 
salt-forming union, and will be represented by a dotted line be- 
tween the ions. 
3. The normal direct union. — When two atoms are held to- 
gether due to the fact that the shell-completing forces of both 
atoms act on a pair of electrons which is shared between them, 
the union may be said to be direct. Such a union is to be repre- 
sented by the usual line for a “satisfied bond” if it is normal; 
that is, if one electron of the shared pair has been supplied by 
each atom. In this case it is evident that each shell involved has 
filled one electron vacancy by the process of sharing and has thus 
“satisfied” one “bond.” 
Examples: H — H, H — 0 — H. 
4. The borroiving direct union. — A direct union in which one 
atom supplies both electrons of the shared pair may be called 
a borrowing union. In this case the borrowing atom fills two 
vacancies in its shell (that is, satisfies two bonds), and the 
lending atom neither gains nor loses electrons. A convenient 
way of representing such a union, whereby — A — satisfies two 
bonds, and B none, is A x> B. The sign ro has here neither its 
mathematical nor its astronomical significance, but has consider- 
able graphic value in representing that two “valence bonds” of A 
and none of B, are satisfied. 
Cl\ 
Examples: H — Cl** 0 Cl Per 0 
Cl/ 
5. Double and triple bonds. — Each symbol for a normal union, 
— , or for a borrowing union, represents one shared pair of 
electrons. If two or three pairs are shared by two atoms each 
part of the total union is represented by the appropriate symbol. 
Examples: N N = 0, H - C C - H. 
GRADATIONS IN THE ELECTROPOSITIVE TENDENCY 
An atom which shows a tendency to become positively charged 
is called an electropositive atom (in the chemical sense). No 
atom actually repels one of its electrons to a very long distance, 
so this electropositive tendency is really only a comparative 
weakness which some atoms have in holding their electrons 
against any outside detaching force. In electropositive atoms 
the forces binding shell electrons seem to be calculable as ordi- 
nary electrostatic forces. 12 Therefore the smaller the kernel, 
12 Langmuir, I., Journ. Am. Chem. Soc. 41 (1919) 877. 
