WYCKOFF: FORCES BETWEEN ATOMS IN SOLIDS 58 1 



are gradually becoming more opened up and hence more readily 

 broken up by other atoms. 



As the temperature is lowered, if the pressure is made suffi- 

 ciently high, that is, if as many molecules are furnished within 

 a definite space as could possibly be needed, the tendency of 

 the molecules to stick together will approach in value the tearing- 

 apart tendency of the heat motion. The critical temperature 

 is the point where these opposing tendencies are equal. At no 

 higher temperature can the gas be liquefied; at all lower tem- 

 peratures the substance can be condensed. As the temperature 

 of the liquid is lowered the motion of the molecule becomes less 

 and less till at a certain point the residual forces of attraction 

 (stray fields) become able to hold the molecules in definite posi- 

 tions. This happens at the melting point. According to this 

 view solid chlorine consists of molecules held together by stray 

 fields. Valency forces are involved in holding the atoms together 

 in the molecule. ^^ 



Methane is a substance of the same type. A molecule of 

 methane may be represented thus : 



© ■ 



®"©."(H) 



\1 

 ® 



It has a normal dielectric constant and presumably does not 

 possess any doublets of large moment.- This indicates that 

 the tendencies of either carbon or hydrogen to acquire electrons 



33 It is possible that with the close proximity of other atoms, both the ingoing 

 and the outgoing unions will not remain directed towards the same atom. In 

 that case each atom would be linked by primary valence with two atoms, each of 

 which would be linked with other atoms and so on throughout the mass. With 

 this state of affairs the molecule as ordinarily understood would lose its identity. 

 The smallest unit, aside from the atoms themselves, becomes the entire mass. 

 This is a possible structure for solid chlorine. There is at present no evidence in 

 its favor. If this second view is correct the vapor subliming from the solid ought 

 possibly to contain an appreciable number of monatomic molecules. 



