WYCKOFF: FORCES BETWEEN ATOMS IN SOLIDS 



571 



elements require twice eight additions to "close" the atomic 

 fields effectively. Then an atom of an alkali metal has one 

 outside electron, an alkaline earth two, and so on until we reach 

 the halogen with seven outside electrons and the next inert gas 

 with eight outside electrons, starting a new group. ^^ 



Tendency to form clusters of eight. — The fact that oxygen with 

 six outside electrons and chlorine with seven are negatively bl- 

 and mono-valent, respectively, points to the existence of a ten- 

 dency to form clusters of eight electrons. ^^ In other words, in 



'8 The simple elements would probably be represented as follows (if hydrogen is 

 the simplest element and helium comes next) : 



Electron Arrangement 



1 electron 



2 electrons forming a "closed cluster" of 

 two 



3 electrons: 

 inside cluster of two, 

 one outside electron 



4 electrons: 

 inside cluster of two, 

 two outside electrons 



5 electrons: 

 • inside cluster of two, 



three outside electrons 



9 electrons: 



inside cluster of two, 

 seven outside electrons 



10 electrons: 

 inside cluster of two, 

 "closed cluster" of eight 



1 1 electrons : 

 inside cluster of two, 

 inside cluster of eight 

 one outside electron 





Recently Debye (Physik. Zeits. 18: 276. 1917), Vegard (Ber. deutsch. phys. Ges. 

 19: 328. 1917), and Kroo (Physik. Zeits. 19: 297. 1918) have tried to explain 

 the K-series lines in the X-ray spectra of elements as due to an inside ring of three 

 or four electrons. If this innermost ring preserves its character when we pass to 

 the more complex atom, and if there are not one or two very light elements, as 

 yet undiscovered, it is highly probable that this inside ring contains two electrons. 

 " A. Lr. Parson, op. cit. 



