CHEMISTRY: I, LANGMUIR 
255 
It seems that this simple equation is a practically complete statement of a 
theory of valence that applies with very few exceptions to all compounds formed 
from the first twenty elements. With some modifications it applies also to 
most compounds of other elements. In the case of organic compounds it is 
found that each pair of electrons held in common between atoms corresponds 
exactly to the valence bond used in the ordinary theory of valence. It is 
therefore proposed to define valence as the number of pairs of electrons which 
a given atom shares with others. In view of the fact known that valence is 
very often used to express something quite different, it is recommended that 
the word covalence be used to denote valence defined as above. 
Equation (1) expresses the fact that the number of covalence bonds in a 
molecule must be related to the number of available electrons in the molecule. 
A simple mathematical analysis^ shows that all structural formulas written 
according to the ordinary valence theory in which the valence for each element 
is taken equal to 8-E, will satisfy Equation (1). Thus the Octet Theory 
requires no modification in any formula written with the following valencies; 
carbon-four, nitrogen- three; oxygen- two; chlorine-one and hydrogen-one. In 
some cases, however, the Octet Theory suggests that other formulas besides 
those usually adopted may be possible. Whenever the old theory of valence 
has assumed valencies other than those mentioned above, such as five for ni- 
trogen or phosphorus; four or six for sulphur; three, five or seven for chlorine, 
etc., the Octet Theory gives quite different structural formulas from those 
usually assumed. This is readily seen when it is considered that the covalency 
of an element according to the Octet Theory can never exceed four, since there 
are only four pairs of electrons in an octet. 
A careful examination of the cases showing a discrepancy between the old 
and new theories furnishes the strongest kind of evidence in support of the 
Octet Theory. The non-existence of such compounds as H4S, HeS, SCle, 
NCI5, NH5, etc., is in full accord with the theory as is also the existence of 
SO2, SO3, N2O5, HNO3, NH4CI, etc. In these latter cases, however, the formu- 
las written are different from those usually adopted. For example, the co- 
valence of sulphur is three in SO2, four in SO3; that of nitrogen is four in 
N2O5, HNO3, and NH4CI. These covalencies are, however, not assumed as in 
the ordinary valence theory, but are i/emec? from Equation (1), which is the 
same equation as that which applies to all ordinary organic compounds. In a 
similar way it is found that the Octet Theory fully explains the structures of 
such compounds as N2O, N2O3, N2O4, HN3, N(CH3)4C1, H3PO3, H3PO4, HCIO, 
HCIO2, HCIO3, HCIO4, H2O2, and even so-called complex compounds such as 
B(CH3)3, NH3, K2PtCl2.2NH3, KBF4, Na2S5, etc. 
From this viewpoint a very large number of compounds previously consid- 
ered by Werner are found to be typical primary valence compounds not essen- 
tially different in their structures from organic compounds. It is especially 
significant that the structure of these compounds is found from Equation (1), 
without any additional assumptions. Thus we are lead to a single theory of 
