Atomic and Covalent Refractions

The application of refractions to the study of structures is based on comparing the experimental values with those calculated on various structural assumptions, of which the most important is additivity (Landolt, 1862) in the first approximation (within ca 10 %), the refraction of a compound is the sum of constant increments of different atoms, ions and bonds. Refractions of some isolated atoms can be measured by the deviation of an atomic beam in an inhomogeneous electric field or by spectroscopic methods. In other cases electronic polarizabilities of free atoms were calculated by ab initio methods. All available experimental and the best of the computed refractions of free atoms are presented in Table 11.5. These values can be used to calculate the energy of van der Waals interactions, magnetic susceptibility, or to establish correlations with atomic and molecular-physical properties. The formation of covalent bonds changes the refractions of isolated atoms and their values transform into the covalent refractions, which are different for isolated molecules and for crystals. Direct measurements of RI of A2 molecules or elemental solids give the most accurate information on the covalent refractions, in other cases the latter have to be calculated from molecular refractions by the additive method. 

Experimental refractions of elements of Groups 14-17 are close to the additive values of atomic increments derived by Eisenlohr [131, 132], Vogel et al. [133, 134] 

In inorganic crystals atoms have higher and, as follows from Table 11.3 and the polarizabilities of clusters (Table 11.6), their refractions must be lower than in the molecular state. Because the Lorentz-Lorenz function approaches 1 when n oo, and metals have very high RIs (at X = 10 (.im, Cu has n = 29.7, Ag 9.9, Au 8.2, Hg 14.0, V 12.8, Nb 16.0, Cr 21.2, etc [147]), we assumed that R = V foi solid metals [148], These refractions of metals, Rm (Table 11.5, lower lines) in some cases are close to the additive values [13,14]. Rm cannot be applied directly to calculate molar refractions of crystalline inorganic compounds because of the differences in Nc, but can be used [149] to calculate refractions of metals for such Nc as they have in the structures of their compounds, using the formula 

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