Sublimation Heats of Elements and Compounds

The sublimation (A fs) heats of elements at normal thermodynamic conditions are listed in Table 9.3. AHs vary periodically (Fig. 9.5), similarly to the atomization energies. The AHJAH ratio for metals is almost constant (0.035 on average), for semiconductor elements with continuous covalent-bond network it is about three times higher, while for molecular structures it is close to 1. Crystalline inorganic compounds with continuous bond network have approximately the same AHmlAHs ratios as metals. Values of AHs for oxides halides, and chalcogenides are presented in Tables 9.4-9.6. 

The coordination numbers of atoms in crystalline compounds exceed their valences, resulting in lower electron density per bond and therefore higher bond polarity. Hence the dependence of AHg on the atontic charges, as has been shown firstly by Fajans [71] (see also [72]) and later by Urusov [73]. This dependence can be elucidated by comparing the sublimation heats of polar compounds with those of the corresponding elements, where the bonds are perfectly covalent. Alkali halides provide the most suitable example, where a crystal molecule transition changes only 

The sublimation heats of molecular solids, due to weak vdW interactions, are much lower [83,84]. As arule, they increase with the size of the molecule, e.g. AHs = 9.20 kJ/mol for methane, 20.09 for ethane, 27.43 for propane, 49.68 for benzene, 72.63 for naphthalene, 73.42 kJ/mol for ferrocene. For elemental substances, the sublimation heat increases with an increase of polarizability, viz. for N2, Ar and I2, A7fs = 6.88, 7.73 and 62.44 kJ/mol, respectively. Polar molecules, although having lower polarizability, show much higher AHs due to ion-dipole interactions or hydrogen bonding, cf. AHs = 20.08 kJ/mol for HCl, 24.15 for N2O, 26.19 for CO2, 29.22 for NH3 and 47.35 for H2O. 

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