THE STRUCTURE OF ATOMS 25 



ionizing potential, the chemical inertness, etc., of helium 

 prove that the arrangement of the electrons in the helium 

 atom is more stable than that in any other atom. Since 

 this atom contains two electrons we must conclude that a 

 pair of electrons in the presence of a nucleus represents a 

 very stable group. It is reasonable that with elements 

 of higher atomic number there should be an even greater 

 tendency for this stable pair of electrons to form about 

 the nucleus. There are two sets of facts which furnish 

 conclusive experimental evidence that this stable pair ex- 

 ists in all atoms above helium. 



In the first place, the properties of lithium, beryllium, 

 etc., show that in these elements also the first two electrons 

 are held firmly while the remainder are easily detached. 

 Thus, lithium readily forms a univalent positive ion by 

 the detachment of one of the three electrons in its neutral 

 atom. The divalence and other properties of beryllium 

 prove that there is little or no tendency for a second stable 

 pair of electrons to surround the first pair. 



In the second place, the absence of irregularities in the 

 observed K and L series of the X-ray spectra of the vari- 

 ous elements proves that there are no sudden changes in 

 the number of electrons in the innermost layers of elec- 

 trons about the nucleus. From these two sets of facts, 

 as well as from other evidence, we may take it as a fun- 

 damental principle that the arrangement of the inner elec- 

 trons undergoes no change as we pass from elements of 

 smaller to those of higher atomic number. 



The properties of neon indicate that its atoms are more 

 stable than those of any other element except helium. 

 Since the atomic number is 10, and the first 2 electrons 

 form a stable pair about the nucleus as in the helium atom, 

 it follows directly that the other eight electrons arrange 

 themselves in a second layer or shell possessing a very 

 high stability. If these 8 electrons revolved about the 

 nucleus in a single circular orbit or ring, as would be sug- 

 gested by Bohr's theory, there is no apparent reason why 



