PAPERS PRESENTED AT GENERAL SESSIONS 53 



atoms, we find evidence that the outer shells are no long- 

 er perfectly definite arrangements in number, but that 

 they are more or less variable in their makeup. Electrons 

 may apj^arently shift from one shell to another. But the 

 geometry of the atom can hardly explain why the elec- 

 trons do not fall into the powerful positive nucleus. That 

 they do not is a fact. T\liy they do not is a problem for 

 the jihysicist rather than the chemist. 



The importance of symmetry in the stability of elec- 

 tron arrangement should be emphasized. We have al- 

 ready mentioned Lewis' observation that so compara- 

 tively few of the many thousands of compounds contain 

 a total number of electrons (measured by the sum of 

 their atomic numbers) which is odd. Those which do 

 are relatively unstable and almost without exception 

 highly colored. Apparently when the number of elec- 

 trons is odd, instead of each electron's having a definite 

 position, on-e or more electrons are able to oscillate be- 

 tween two or more positions, thus absorbing light of some 

 wave length in the visible spectrum and causing the 

 phenomena of color. It has been noted that the complex 

 atoms have a variable structure in their electron shells. 

 These atoms are all colored. 



To repeat, the ^most important question is, why the 

 shells of electrons do not collapse into the positive 

 nucleus. 



The physicist has offered an explanation, and a natural 

 enough one. He says that the electrons are rotating 

 around the positive nucleus in orbits just as the planets 

 rotate around the sun, and that they do not fall into the 

 nucleus for the same reason that the planets do not fall 

 into the sun. The chemist has a very grave objection to 

 this d^'namic atom of the physicist. The chemist has 

 reason to believe that the atoms are combined in mole- 

 cules of extraordinaiy complexity and rigidity of struc- 

 ture. These molecules retain their structure unchanged 

 throughout the ordinaiy processes of fusion, evaporation 

 and chemical reaction. The chemist believes that the 

 atoms are held together in these molecules by electrons 

 which occupy definite positions between the different 



