572 wyckoff: forces between atoms in solids 



addition to the force holding the normal number of electrons 

 to the nucleus, there is a force which causes the atoms to tend 

 to add electrons beyond the number equal to the positive charge 

 on the nucleus. 



The force responsible for this tendency to form eights may 

 be mainly an electrostatic attraction between positive and 

 negative charges. This has been mentioned by Langmuir:'-'' 

 "According to ordinary potential theory, electrons uniformly 

 distributed throughout a spherical shell should exert no forces 

 on electrons inside the shell, but should repel those outside the 

 shell as though the electrons in the shell were concentrated at 

 the center. On the other hand, an electron in the spherical 

 shell itself is repelled by the others in the shell as if one-half 

 of the other electrons were removed altogether, while the sec- 

 ond half were concentrated at the center. Thus, let us consider 

 a carbon atom (A^ = 6) which has taken up 4 extra electrons 

 and has completed its octet. An electron in the outside shell 

 is thus attracted by the nucleus which has 6 positive charges, 

 is repelled by the two electrons in the first shell as though they 

 were concentrated at the center, and is repelled by the 8 elec- 

 trons in the outside shell as if 4 of them were concentrated at 

 the center. The repulsion of the electrons is thus only just 

 able to neutralize the attraction by the nucleus, notwithstanding 

 the fact that the whole atom has an excess of 4 negative charges." 

 In the case of an oxygen atom (A^ = 8) which has acquired two 

 electrons to complete its cluster of eight electrons, an electron 

 in this outer cluster is attracted by the positive charge of eight 

 units and is repelled by the equivalent of a negative charge of 

 six units (by the two inner electrons and by the outer eight as 

 if four were concentrated at the center). There is thus a strong 

 extra attraction holding these additional electrons to the neutral 

 oxygen atom. Fluorine would hold a single extra electron still 

 more strongly.'-^ 



2° Op. cit., p. 909. 



21 This discussion considered only the case where the electrons were distributed 

 uniformly throughout a shell. A similar state of affairs will exist if the inside 

 electrons are in revolution. 



