590 
CHEMISTRY: G. N. LEWIS 
spects be regarded as analogous, namely, CI — CI, HO — OH, H2N — NH2, 
H3C — CH3. The chlorine molecule acquiring rotational energy due to 
temperature has a certain tendency, through centrifugal force, to break 
at the bond of union into monatomic molecules. The molecule of bro- 
mine with its heavier atoms has, especially in view of modern quantum 
theory, a higher moment of momentum and is therefore dissociated to a 
higher degree, while iodine with its still heavier atoms is to a large de- 
gree dissociated at easily attainable temperatures. No one has sug- 
gested that this dissociation of iodine vapor is due to steric hindrance. 
The volume occupied by an atom is at best a most indefinite concep- 
tion, but the atomic volume commonly attributed to iodine is not 
greater than that attributed to bromine or chlorine. It is doubtless the 
mass of the iodine atom which is chiefly responsible for its greater 
dissociation. 
Since diatomic iodine dissociates to give nul-valent iodine we may 
expect to find a properly substituted hydrogen peroxide dissociate to 
give a compound of univalent oxygen, as the substituted hydrazines and 
ethanes have been found to give compounds of bivalent nitrogen and 
trivalent carbon. Both in hydrazine and in ethane it is the substitu- 
tion of heavy groups which causes appreciable dissociation. Thus 
pentaphenylethane does not dissociate sufficiently to show the proper- 
ties of a free radical below 180°, where it absorbs oxygen readily.^ 
Hexaphenylethane is at room temperature dissociated to the extent of 
several per cent. The successive substitution of phenyl by biphenyP 
increases the dissociation regularly. Now it is possible, but not ob- 
vious, that biphenyl occupies more space immediately about the methyl 
carbon than phenyl does. It certainly seems better to assume that by 
increasing the mass of the group and thus increasing the centrifugal 
force at the central bond, it produces a greater dissociation. 
Another interesting illustration of the effect of the mass of the sub- 
stituent upon dissociation is afforded by the interesting substance ob- 
tained by Wieland in which one oxygen of NO2 is replaced by two 
phenyl groups. While NO2 is largely associated even at ordinary tem- 
peratures (C6H5)2NO remains monomolecular even at the lowest tem- 
perature obtainable with solid CO2 and ether. 
The relation between nul-valent chlorine, univalent oxygen, biva- 
lent nitrogen and trivalent carbon is more readily visualized if we 
express the equations of dissociation in the s3rmbols of my previous 
paper: 
