576 WYCKOFF: FORCES BETWEEN ATOMS IN SOLIDS 



in very completely the fields about the atom. With sodium, 

 possessing a single cluster of eight electrons, the fields are more 

 condensed. Where clusters of twice-eight electrons are effec- 

 tive (potassium, rubidium, caesium) the outside fields become of 

 little importance. This suggests an explanation for the marked 

 difference in properties exhibited by the transition elements of 

 a group. As the atom gets larger the detachable electrons be- 

 come farther removed from the nucleus and consequently less 

 tightly bound. Elements of the sub-group have a single cluster 

 of eight so that, although they resemble the alkalies in having 

 a single outside electron, the forces about the atom which tend 

 to enmesh and hold the electron are very much greater. 



The atoms of the other groups will be similarly constituted. 

 Those of the second group possess two outside electrons, the 

 third three, and so on, up through the eighth group. 



For the eighth group the theory is not so simple. In this 

 group there is a single cluster of eight electrons and no outside 

 electrons. The atom is quite complex, however, and a single 

 cluster of eight electrons is unable to close the fields completely. 

 As a result the atom possesses considerable reactivity. The 

 facts of chemistry show that, perhaps on account of the vigor 

 of these forces, two and sometimes three of these electrons can 

 be detached. The existence of three similar elements in this 

 group is conveniently explained by the assumption that at this 

 point, when the two additional electrons are added, rearrange- 

 ments of the internal rings are more stable than the addition 

 of an outside electron. ^^ 



The character of the rare-earth elements could be explained 

 by an assumption like the last one, namely, that when this point 

 in the periodic series is reached, the stable arrangements, for a 

 number of successive increases in the nuclear charge, result 



2* The system is seen to be tending towards a more stable condition with each 

 readjustment. The outside fields become more and more drawn in and the elec- 

 trons become less loosely bound. This is shown by the transition in properties 

 from iron, Fe" (less stable) and Fe"' (more stable), through cobalt, Co'" (less 

 stable) and Co" (more stable), to nickel, Ni" only. 



The assumption made by Langmuir that ten electrons are necessary to form 

 the first half of the larger closed groups is about as satisfactory. In the absence 

 of any real information upon the subject either can be used. 



