96 WHAT IS LIFE 



the nucleus, and successively the L-shell, the M-shell, 

 and other shells. However, it is fully understood, 

 these "shells" are not actual barriers in space in 

 the atom, but mere "energy-levels," since the or- 

 bits of some of the outer electrons penetrate the 

 orbits of inner electrons. Indeed the complexity of 

 interpenetrating orbits is very great in some atoms 

 because of the large number of electrons in each 

 class of orbits and because the orbits occupy all 

 three dimensions of space. As J. D. Main Smith points 

 out: "Only by an extreme license in the use of words 

 can each class of orbit be regarded as constituting an 

 energy level or shell in an atom, for the orbital inter- 

 penetration makes a precise conception of levels im- 

 possible except in the case of truly circular orbits. "^^ 

 Such interpenetration of orbits affects the stability of 

 the atom. Atoms vary greatly in respect to stability. 

 The spectral lines of the atoms, checked by the 

 periodic properties of the atoms, are being inter- 

 preted as necessitating the various orbits with various 

 numbers of electrons in an orbit, and the orbits at 

 various distances and of various forms. Further, the 

 spectral lines are interpreted in terms of wave- 

 lengths and in terms of a quantum analysis. It is a 

 postulate of the Bohr theory that an atom can radi- 

 ate or absorb energy only when an electron is 



'* Chemistry and Atomic Strvcture, 170. 



