MOLECULAR ARCHITECTURE 



PART II 



By R. T. COLGATE and E. H RODD 



The Crystalline Structure of Binary Compounds 



In the first part of this article ^ it was pointed out that with 

 very few exceptions the elements crystallise in forms belonging 

 either to the holohedral cubic or hexagonal systems ; it was 

 shown that these two forms correspond to the cubic and 

 hexagonal modes of packing equal spheres closely. The crystal 

 structures of the elements correspond in their symmetry to 

 such close-packed assemblages of equal spheres. 



We have now to consider the crystal structure of compounds 

 of a very simple type — the binary compounds of the type 

 X y, where X and Y are two atoms generally of equal 

 valency. From the conclusion drawn by Barlow and Pope that 

 atoms of the same valency appropriate approximately equal 

 domains or spheres of influence when present in the same 

 compound, it follows that in order to represent the crystal 

 structure of such a binary compound by the same method, 

 two kinds of spheres will have to be used not exactly but 

 nearly equal in individual volume and otherwise distinguish- 

 able the one from the other. A symmetrical distribution of 

 the two kinds closely packed to form an assemblage of spheres 

 of such a nature as will faithfully represent the distribution 

 of the atoms in a crystal of the compound must, it is evident, 

 present nearly the same relative dimensions as would be the 

 case if the spheres arranged were all equal ; the symmetry of 

 the arrangement, however, will be a function not merely of the 

 kind of close-packing but also of the nature of the distribution 

 of the two sorts of spheres. Two new factors are thus intro- 

 duced, (a) the presence of some sort of pattern in place of the 

 complete uniformity of an assemblage composed of but one 

 kind of sphere, (Z>) a slight departure from the simplicity of 



' Part I. appeared in Science Progress, January 191 1, vol. v. pp. 345-70. 



693 



