Relation between heats of formation of alkali and pseudo-alkali halides and electronegativities of halogen ions

Abstract

The heats of formation of alkali halides (CsX, RbX, KX, NaX, LiX) and pseudo-alkali halides (NH 4X, TlX, CuX, AgX, AuX), −Δ H 0 298, are empirically expressed by the electronegativities (χ a) of the halogen ion: ▪ where a and b are empirical constants; e, r C and r A represent the charge on the electron, cation radius, and anion radius, respectively. The value of ± e 2/ r (C and/or a) corresponds to the electrostatic energy between the effective nuclear charge of the M + and/or X − ion and an electron at a distance from its nucleus equal to its ionic radius r C and/or r A. The empirical constants a and b correlate with the electronegativity of the M + ion in two different trends; one is the alkali ion series and the other is the pseudo-alkali ion series. This means that Pauling's electronegativities of alkali and pseudo-alkali ions are based on different scales. Assuming that the ( X-IP) correlations for pseudo-alkali ions constitute the same series as the ( χ-IP) correlations for alkali ions, where X is the electronegativity and IP is the first ionization potential, the electronegativities of Tl +, Ag +, Cu + and Au + are changed to 1.1, 1.3, 1.4 and 1.7, respectively, with noticeable error. The value (1.1) for the Tl + ion is consistent with the ionic character of the bonds formed by Tl + in some compounds.