Structures Containing Nonequivalent Spheres

13 Some of these substances have low-temperature modifications in which the molecules do not rotate. In the high-temperature forms the rotation of the molecules is not completely free, but is somewhat hindered in bo me cases it may be described as rapid change among alternative orientations. 

M The structures can accordingly be described by the sequence c h c h , indicating alternation of cubic and hexagonal closest packing. The three other structures have the similar sequences h c c h c c f h h c h k c , and hkechhcr. 

The structures reported for the metals, as determined by x-ray diffraction, are listed in Table 11-2. 

Closest-packed Structures.—If the stability of a metal crystal were determined by the number of bonds formed at a minimum interatomic distance, with no contribution of longer bonds, the structures with closest packing would be the most stable for the metallic elements. These structures, which have been described in the preceding section, involve contact between each atom and the 12 nearest neighbor. (The next interatomic distances are 41 percent larger and presumably have little significance.) 

It is noteworthy that 46 of the 58 metallic elements listed in Table 

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