TH E ATOM 87 



formulated by Rutherford, and calling in the quan- 

 tum theory (Planck), advanced the concept of the 

 atom as a dynamic system in which the negative 

 electrons revolve about the nucleus as the planets do 

 about the sun, but in quantised orbits. Others pic- 

 tured a cubical atom, with the valence electrons 

 fixed in certain equilibrium positions. However, 

 though the cubical atom seemed to harmonize sin- 

 gularly well with the facts of organic chemistry, the 

 idea that the atom possibly may be a system with 

 static valence electrons, has been definitely and com- 

 pletely abandoned. 



The Bohr theory, that at first described only 

 circular orbits of the one orbital electron of hydro- 

 gen and ionized helium, was greatly expanded by 

 Arnold Sommerfeld, who describes elliptic orbits as 

 well as circular orbits and applies the theory to 

 all the elements. The Bohr-Sommerfeld atom has 

 been found to succeed to a remarkable degree in 

 the interpretation of spectra and the chemical prop- 

 erties of the atoms. 



Every physicist today holds the atom to be a 

 dynamic system, and the modified Bohr atom is 

 the accepted theory of atomic structure. Millikan 

 says of it: "For the present at least it is truth, 

 and no other theory of atomic structure need be 

 considered until it has shown itself to approach it 



