19, 6 Perkins: Addition to the Double Bond, I 649 
boundary, composed of sections, each section being defined in 
reference to the pair of electrons which it contains, The exact 
shape of such a shell boundary cannot be determined without 
more knowledge of the forces involved than is available at 
present, but the boundary of an ordinary four-pair shell is 
probably roughly tetrahedral with very rounded corners. 
Loosely or firmly held electrons—An electron whose position 
of equilibrium is well within a shell boundary requires a rela- 
tively large external force to move it even a small distance, and 
—s 
> 
| 
Repulsion A tlraction 
<< 
Be 
Listance from ricleus C 
tee 
Fic. 8. Symmetrically weak union; loose electron. 
will be spoken of as firmly held. A shared electron near the 
boundaries of both shells requires a much smaller force to move ~ 
it the same distance, and will be spoken of as loosely held (see 
fig. 3). Such an electron is easily removed from either shell, 
both because it is near the boundary and because its constraint 
is slight. 
The distribution of electrons in a shell—tThe final solution of 
this problem has not yet been reached by physicists. As a con- 
venient working hypothesis shell electrons will be considered to 
be ring-shaped magnetons,": which have sufficient magnetic at- 
"Parson, A. L., Smithsonian Misc. Coll. 65 (1915) No. 11. 
