On the Electron Affinities of Halogens and the Strength of Ionic Interactions

Abstract

A new interpretation is forwarded for the enthalpies of ionic interactions AH,\B-From observed trends in AH\R values for alkalisalt molecules, it is concluded that the electron affinities of halogens are in need of revision.For an interpretation of bond strengths in simple diatomic heteronuclear molecules several empirical formalisms have been proposed. Pauling's well known bond energy equation 1 , whereby the bond energy is split up into ionic and covalent contributions, has led to the concept of electronegativity. The procedure is, however, largely dependent on the value ascribed to the purely hypothetical covalent bonding term, which seems highly problematic.If the so-called ionic-covalent resonance formalism is to be retained, it would be more illustrating to compare the actual energy of a partly ionic bond with the energy of a completely and permanently ionic bond, which, in the usual terminology, is supposed to take part in the resonance-system. Thus, in the case of heteronuclear AB bonds, the energies of the completely ionic bonds A + B~ and/or A~B + should be known.A first step in achieving this, consists in considering the enthalpies of ionic interactions A + + B~ AB which have recently been shown to fit into the E -C formalism as it was first advanced for the interactions of donor-acceptor interactions 2 . In general, the following reactions and enthalpies AHXQ can be considered for AB bonds:A + + AB with AHA^-1EA-EAB + EAB (1)A~ + B + -> AB withif /Fx and EAx represent the energy of, respectively, the first and second valence electron of X -and if Fab is the bond energy. At first sight, these AH AB values are even more complicated than FAB values allone, on which the Pauling procedure was originally applied.Reprint requests to Dr. G. van Hooydonk, Rijksuniversiteit te Gent, Centrale Bibliotheek, Rozier 9, B-9000 Gent, Belgie.However, the valence electron energy of a pair of ions A + + B~ for instance at infinite internuclear separation (r=oc), is 1E B + EA B . Now, this is exactly the energy of a permanently 100% ionic bond at equilibrium internuclear separation (r = T\B) also. In fact, as soon as the ionization energy of a given element A is zero, no electron withdrawal force will be present at the nucleus of A, when this element is involved in bond formation with B. IEA equal to zero means Z A (the effective nuclear charge of A) equal to zero and/or rA (the covalent radius of A) infinite, whence, in first approximation, i. e. neglecting polarization, rAB ~ R A + R B = 00 ? AN D COR " respondingly e~jrAB = 0! Therefore, a simple example of a completely and permanently ionic bond is the negative ion B _ , which results from "bonding" between a neutral element B and a free electron 3 . This has quite important consequences for the interpretation of the AH AB values (1) and (2) and the role of FAB therein.Indeed, AH AB values are now seen to represent simply the difference in energy between partly and completely ionic bonds. Although it is only formally valid to speak of bonds in the extreme case of X",...