WYCKOFF: FORCES BETWEEN ATOMS IN SOLIDS 569 



atom model meets his fancy (always remembering that his par- 

 ticular type of atom is only a convenience).^^ 



Arrangement of outside electrons. — -Although we are unable at 

 the present time to determine the general arrangement of the 

 electrons within the atom, the facts of chemistry furnish consider- 



'^ It has been found convenient to picture an atom, the inner electrons of which 

 are in rapid revolution, the outer electrons of which might be held in positions of 

 equilibrium between the atoms in a molecule. 



Langmuir (see notes, p. 566) has suggested an ingenious arrangement for the 

 electrons in a stationary model. The examples of the application of his model, 

 however, to chemical compounds are those which would be equally well satisfied 

 by any type of atom of the kind described above. As already stated, chemical 

 facts, simply because they involve only the outside electrons, cannot give direct 

 information concerning the arrangement of the inner electrons. The stationary- 

 and revolving-electron atoms might differ from one another in the nature of the 

 electrical fields surrounding the atom. The electrical fields about the revolving- 

 electron atom would be expected to be quite uniform while those about the other 

 type might be clustered in patches. In the Stark atom (Prinz. d. Atomdynamik, 

 III) the "positive electrification" was grouped in patches and electrons took up 

 equilibrium positions about these patches. It is not evident whether a definite 

 choice between the two atom types can ever be made upon these grounds. 



Langmuir has lu-ged that the existence of charcoals and similar porous sub- 

 stances having the form of solids of large apparent volume, where each atom of 

 carbon (taking charcoal to be specific) is surrounded by fewer than four carbon 

 atoms, is a proof that the electrons are stationary within the atom. This does not 

 necessarily follow. In a structiue of this sort the valence bonds which were linked 

 up with other atoms in the formation of wood are, in the charcoal, partly or com- 

 pletely saturated by holding adsorbed gas (as will be seen later, adsorbed gas is 

 probably held to charcoal and similar substances by primary valence bonds). As 

 indicated by Bohr (Phil. Mag. (6) 26: 857. 1916) and discussed by Kossel {op. 

 cit.) the four outside electrons in a revolving-electron atom in the case of carbon 

 would be expected to place themselves, if possible, at the corners of a tetrahedron. 

 As a consequence of these facts the large apparent volume of charcoal is not a 

 proof of the inherently directed natiue of the valence bonds of carbon. Many 

 sulfides of large apparent volume are known which, when heated to a certain tem- 

 perature, will suffer a rather sudden change in shape (and apparent volume), i. e., 

 they crumble. This quite possibly is due to the loss of adsorbed gas. 



These objections are lu^ged, not as proofs that the electrons are not stationary, 

 but simply as showing that the evidence in favor of their stationary nature is not 

 conclusive. The experiments of Hull (cited by Langmuir, op. cit. p. 869) are 

 inconclusive. As S. Nishikawa has pointed out, the effects which led Hull to 

 believe that electrons occupy definite positions in the crystal lattice are in some 

 cases similar to those effects which would be expected to result from the thermal 

 agitation of the atoms in the crystal. The best way to determine whether or not 

 the effect is real would be to make the X-ray studies at a point where the specific 

 heat is very small. The author hopes to make such a study in the near future. 



