STRUCTURE OF CRYSTALS — WYCKOFF. 



219 



seemed probable that the atoms in crystals of potassium chloride and 

 related compounds are electrically charged. Upon this basis, then, 

 the alkali halides appear as aggregates of equal numbers of positively 

 and negatively charged atoms. When the atoms are doubly charged, 

 as is presumably the case with magnesium or calcium oxide, for in- 

 stance, the distance between the atoms is much less than between those 

 of the alkali halides. Calcium carbonate and the divalent carbonates 

 isomorphous with it (MnC0 3 , FeC0 3 , etc.) have the same structures 

 if the carbonate groups are substituted for the electronegative atom. 33 

 The large C0 3 group may be thought of as squeezing out the sides 

 of the cube till it has the form of an obtuse rhombohedron. Simi- 

 larly caesium dichloroiodide (CsICl 2 ) has the same arrangement as 



.^fc^l x*? 



Fig. 25. — The pyrite arrangement. The black sulphur atoms are so spaced along the 

 diagonals of these little cubes that each iron atom has six and each sulphur atom four 

 equally near atoms. 



caesium chloride if the group of atoms IC1 2 is substituted for chlorine. 

 In this instance the IC1 2 atoms are all strung along the body 

 diagonal so that it may be imagined as distorted from a cube to an 

 acute rhombohedron. 34 



Other simple compounds. — A number of other simple compounds 

 have been studied with a considerable degree of care. If carbon 

 atoms are placed at the positions occupied by both zinc and sulphur 

 atoms in zinc blende, we obtain the arrangement of the atoms in the 

 diamond. 35 (Fig. 24.) In pyrites each sulphur atom is surrounded 

 by four equally distant atoms and each iron atom by six atoms. 38 

 (Fig. 25.) In cuprous oxide each copper atom is equally near to two 



33 W. L. Bragg, Proc. Roy. Soc. A 89, 468, 1914. W. H. Bragg, Phil. Trans., A 215, 253, 

 1915. Ralph W. G. Wyckoff, Am. J. Sci., 50, 317, 1920. 

 84 Ralph W. G. Wyckoff, J. Am. Chem. Soc, 42, 1100, 1920. 



35 W. L. Bragg, Proc. Roy. Soc, A 89, 277, 1913. W. II. and W. L. Btag&, ibid., 468. 

 38 W. L. Bragg, op. clt. P. P. Ewald u. W. Friedrich, Ann. d. Phys. 44. 1183, 1914. 



