Group 18 elements in the solid state

The group 18 elements are the noble gases (see Chapter 17), and Table 5.1 lists selected physical data for these elements. Each element (with the exception of helium, see footnote in Table 5.1) solidifies only at low temperatures. The enthalpy changes accompanying the fusion processes are very small, consistent with the fact that only weak van der Waals forces operate between the atoms in the solid state. In the crystalline solid, ccp structures are adopted by each of solid Ne, Ar, Kr and Xe. 

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 17.4). 

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 (301 K) and Ga (303 K) mean that in some hot cUmates, these metals are liquids. Table 6.2 shows that most metals crystallize with ccp, hep or bcc lattices. 

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About 80% of the elements are solid metals in their standard states at 298 K. Of the metallic elements, only mercury is a liquid at 298 K and at 1 atmosphere pressure (caesium melts at 302 K, gallium at 302.9 K). The non-metallic elements exist as either discrete small molecules, in the solid (S8), liquid (Br2) or gaseous (H2) states, or as extended atomic arrays in the solid state (C as graphite or diamond). The elements of Group 18 are monatomic gases at 298 K. 

Due to the inert-pair effect. Group 13 elements sometimes produce low-valent monomeric metallocenes as well as high-valent oligomeric metallocene clusters and polymeric metallocene chains. Half-sandwich metallocenes of Group 13 elements can be found in the gas phase, but exist as clusters and polymeric forms in the solid state. Schleyer calculated the CpE, E = Be, B, C, and N, molecules at the SCF level and found the shortest C-C ring bond length in CpB (18). The... 

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