258 
CHEMISTRY: I. LANGMUIR 
There is one property however, which is in marked contrast to those given 
above. The freezing-point of N2O is - 102° while that of CO2 is -56°. This 
fact may be taken as an indication that the freezing-point is a property which 
is abnormally sensitive to even slight differences in structure. The evidences 
seem to indicate that the molecule of CO2 is slightly more symmetrical, and 
has a slightly weaker external held of force than that of N2O. Such differences 
could easily be produced by the difference in the charges on the kernels. 
There are many other examples of compounds having similarly formed 
molecules. It will be convenient to call these isosteric compounds or isosteres. 
These may be defined as compounds whose molecules have the same number 
and arrangement of electrons. 
Another example of a pair of isosteres is that of HN3 and HCNO. The 
similarity of properties should be most marked in the salts of these acids. The 
available data on solubilities and crystalline form seem to show that the salts 
of these two acids are very closely similar in physical properties. 
This relationship of compounds may be carried much further. Thus, ac- 
cording to the Octet Theory, we should regard CH4 as an isostere of the NH4 
ion. The electric charge on the ion prevents a direct comparison of physical 
properties. Other examples are: 
The carbonate and nitrate ions: 
O 
€ = O and 
O 
The orthophosphate, sulphate and perchlorate ions: 
0 0 o 
0-P-O, 0-S-O and 0-Cl-O 
O 
O 
O 
Sulfur trioxide and the raetaphosphate ion: 
>S = O and >P = O 
0^ 
The hydrofluoric acid molecule and the hydroxyl ipn: 
H - F and O - H 
The sulfite and the chlorate ions: 
O 
O 
0-S-O and 0-Cl-O 
We may attribute the differences in physical properties in all these cases to 
the effect of the differences in the electric charges. 
