STRUCTURE OF CRYSTALS — WYCKOFF. 



217 



metry, and (2) the more important cubic face-centered arrangement, 

 which is nearly as close a grouping. (Fig. 22.) Aluminum, 27 silver, 

 thorium, lead, copper, calcium, indium, palladium, and iridium are 

 among the elements which have been shown to have this cubic close- 

 packed grouping. Zinc, 28 cadmium, ruthenium, and magnesium, 

 on the other hand, approach to the hexagonal closest packing. 

 Several elements, however, such as iron, 29 chromium, titanium, 

 sodium, tantalum, and tungsten, crystallize in the less dense body- 

 centered grouping. (Fig. 23.) Nickel 30 appears to be sometimes 

 body-centered and sometimes face-centered. In attempting to de- 

 duce from these 

 structures any in- 

 formation con- 

 cerning either the 

 bonding forces be- 

 tween the atoms 

 or the structures 

 of the atoms 

 themselves it must 

 be borne in mind 

 that there is al- 

 ways the possi- 

 bility in a metal 

 of one or more 

 electrons f unction- 

 i n g as " silent 

 (so far as X rays 

 are concerned) 

 partners" in the 

 arrangement. For 

 this reason the 

 metals, though the 

 simplest of compounds, are of all simple compounds perhaps the 

 most dangerous as bases for further deductions. 



Compounds of the type AB. — Several compounds of the type AB, 

 where A is a metal atom and B is an electronegative atom, have been 

 studied. All of the alkali halides have the " sodium chloride ar- 

 rangement " (fig. 22) wherein each atom has six atoms of the oppo- 

 site kind equally near to it. The divalent metal oxides, magnesium 



Fig. 22. — The sodium chloride arrangement. The black 

 circles represent the positions of the atoms of one kind ; 

 the white circles those of the second kind of atoms. The 

 alkali halides and the oxides of the divalent metals seem 

 to have this atomic arrangement. Either the black atoms 

 or the white ones have the grouping of the closest cubic 

 packing of spheres. 



27 A. W. Hull, Phys. Rev., 10, 661, 1917 ; Science, 52, 227, 1920. P. Scherrer, Phys. Z., 

 19, 23, 1918. H. Bohlin, Ann. d. Phys., 61, 421, 1920. W. L. Bragg, Phil. Mag. (6), 28, 

 355, 1914. L. Vegard, Phil. Mag. (6), 31, 83, 1916. 



28 A. W. Hull, op. cit. 



* A. W. Hull, op. cit. P. Debye, Phys. Z., 18, 483, 1917. 

 30 A. W. Hull, Phys. Rev., 10, 661, 1917. H. Bohlin, op. cit. 



