H2 and F2 in the solid state

The liquefaction of gaseous H2 occurs at 20.4 and solidification at 14.0 K. However, even in the solid state, H2 molecules have sufficient energy to rotate about a fixed lattice point and consequently the space occupied by each diatomic can be represented by a sphere. In the solid state, these spheres adopt an hep arrangement. 

Difluorine solidifies at 53 K, and on cooling to 45 K, a phase change occurs to give a distorted close-packed structure. This description is applicable because, like H2, each F2 molecule rotates freely about a fixed lattice-point. (The second phase above 45 K has a more complicated structure.) 

The application of the packing-of-spheres model to the crystalline structures of H2 and F2 is only valid because they contain freely rotating molecules. Other diatomics such as the heavier halogens do not behave in this manner (see Section 16.4). 

With the exception of Hg, all metals are solid at 298 K the statement solid at room temperature is ambiguous because the low melting points of Cs (301K) and Ga (303 K) mean that in some countries, these metals are liquids. Table 5.2 shows that most metals crystallize with ccp, hep or bcc lattices. However, many metals are polymorphic and exhibit more than one structure depending upon the conditions of temperature and/or pressure we return to this later. 

On the basis of the hard sphere model, close-packing represents the most efficient use of space with a common packing efficiency of 74%. The bcc structure is not much less efficient in packing terms, for although there are only eight nearest neighbours, each at a distance x (compared 

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