WycJcoff — Crystal Structure of Magnesium Oxide. 151 



the observations for certain values of its parameters 

 u and V. Of the various ways having thirty-two mole- 

 cules in the unit cube, both (j) and (/c) are possible. All 

 three of the structures with variable parameters, however, 

 are in agreement with experiment only when u and v^ have 

 such values that the resulting arrangements approximate 

 very closely to the ' ' sodium chloride ' ' grouping. 



The nature of the forces hetiveen the atoms of magnesium oxide. 



Magnesium oxide has just been shown to have probably 

 the same structure as rock salt, (c). Some information 

 concerning the possible nature of the binding forces 

 between its atoms in the light of the existing ideas on the 

 forces of chemical combination can be obtained from its 

 analogies with sodium chloride and the other alkali 

 halides. 



Two kinds of unions between atoms can be explained 

 by the present knowledge of the structure of atoms.'^ (1) 

 The ^^electro-negative'' atoms of a compound may be 

 able to abstract electrons from the ^^ positive" atoms so 

 that the compound becomes an aggregate of charged 

 atoms held together chiefly by the electrostatic attrac- 

 tions between them. Or (2) if extremely electropositive 

 atoms are not involved in the combination, all of the atoms 

 in the compound may strive, without complete success, 

 to acquire electrons in the somewhat inexplicable, but 

 clearly real, attempt to close their clusters of eight 

 outside electrons. Electrons are thus in some way held 

 in common by two atoms — a second sort of bonding which 

 can be called a valency bonding. 



Partly because of their crystal structures the alkali 

 halides are commonly supposed to be compounds 

 exhibiting the first kind of combination.^ In sodium 

 chloride each atom, according to arrangement (c), is 

 equally distant from six atoms of the other sort and thus 

 there seems to be no connection between what is commonly 

 called the valence, in the chemical sense, of the atoms, 

 and their locations in space. 



Similarly the crystal structure of magnesium oxide 

 seems to point to the fact that the oxygen atoms have 

 been able to remove completely the two outside electrons 



' Ralph W. G. Wyckoff, J. Wash. Aead. Sci., 9, 565, 1919. 

 'J. Stark, Prinzipien der Atomdynamik, III, p. 193. 



