WYCKOFF: FORCES BETWEEN ATOMS IN SOLIDS 575 



size of the atom may be very small compared with this distance. 

 Atoms in a solid would then be held apart by some repulsive 

 force, the intensity of which must decrease rapidly from the 

 origin. Concerning its possible nature we have no inkling. 

 Presumably the heat vibrations would aid this force. But the 

 diameter of the atom, meaning the diameter of the outermost 

 ring of electrons, may be comparable with the distance apart of the 

 atoms. The tightly drawn-in fields about the closed groups of 

 electrons make the atom, except for a few valency electrons, 

 behave like an elastic solid, so that two atoms can interpenetrate 

 only to an extent involving these outside electrons. This effect, 

 together with the thermal agitation, is quite sufficient to explain 

 the ordinary phenomena observed. The assumption of a fur- 

 ther repulsive force, essential to the other theory, does not 

 seem necessary. The view that the atoms are held apart simply 

 as the result of their own impenetrability and their thermal 

 vibration will therefore be used. 



The influences of which we are aware that affect the combina- 

 tion of atoms one with another are then : 



1. The electrostatic attraction between positively charged 

 nuclei and negatively charged electrons. 



2. The large condensation and drawing in of the fields of force 

 which accompany the formation of certain clusters of electrons 

 (two, eight or twice eight). The result of (i) and (2) is an ap- 

 parent tendency to form clusters of electrons. 



3 . The thermal agitation of the atoms (and probably in certain 

 cases groups of atoms acting together) which acts as a force of 

 repulsion. 



4. The magnetic fields (our ideas of which are at present of 

 an indefinite nature). 



Exterior structure of the elements. — An alkali metal results 

 when the total number of electrons needed to form the "closed 

 clusters," including the outermost "closed cluster," is i less 

 than that required to neutralize the nuclear charge, i. e., there 

 is one outside or "left-over" electron. The closed cluster in 

 the case of lithium contains fewer electrons than eight, pre- 

 sumably two. So few electrons can hardly be expected to draw 



