19, 6 Perkins: Chlorine Dioxide and Compounds 733 
Similarly the fluorine ion, F ( : F : ), does not 
form a stable compound with an oxygen atom, F “O 
( : F : 0 : )_ because the shared pair in such a compound 
would be drawn too far away from the oxygen kernel. The 
chlorine kernel, however, has an electron binding strength 
comparable to that of the oxygen kernel, and the ion, 
cCr'<Q ( : Cl: O : ), is consequently fairly stable. 
Although fluorine does not unite with oxygen it cannot be said 
to lack a tendency to share pairs, for F—F (: F: F: ) is 
very stable. The second fluorine atom here can unite where the 
oxygen could not, because its electron binding strength matches 
that of the first fluorine atom. The present problem is not so 
much concerned with the differences between the electron bind- 
ing strengths of the different atoms as it is with the variation 
of the electron binding strength of a single atom in different 
compounds. Such a variation is only one of the many observed 
consequences of the “electron shift.® 
An obvious way to increase the electron binding strength of 
an atom is to remove one of the electrons from its shell. The 
remaining electrons will then be held more tightly than before. 
Both chemical and physical evidence is quite definite on this 
point. This is important in connection with the metals, which 
are not, however, under discussion in the present paper. 
In the case of a non-metallic, or chemically negative, atom 
there is such a tendency toward definite shell (generally octet) 
formation that the important variation in electron binding 
strength is not the one just described, but one produced by the 
° Whatever the forces tending toward a stable configuration of electrons 
in the shell of an atom, the configuration is altered by the proximity of 
a foreign charge, and a displacement of one electron tends to shift all the 
electrons in the shell. Any hypothesis of atomic structure which does not 
admit of this view, or one very similar to it, is unsatisfactory from the 
standpoint of chemical evidence. 
