Relations between heats of formation of MX 2 halides and electronegativities of halogen ions

Abstract

The heats of formation of MX 2 halides (M being Ba, Sr, Ca, Mg, Be, Mn Fe, Co, Ni, Cu, Zn, Cd and Hg), - Δ H 0 298, are expressed empirically by electronegativities (ϵ A) of halogen ion ▪ where a and b are empirical constants; factor 2, e, r C and r A represent the valence number of the cation, the charge on the electron, cationic radius, and anionic radius, respectively. The value of 2 e 2/ r C corresponds to the electrostatic energy between the effective nuclear charge of the M 2+ ion (2 e) and an electron at a distance from its nucleus equal to its ionic radius r C. The empirical constants a and b correlate with the electronegativity of the M 2+ ion as three different trends; Ba, Sr, Ca, Mg, Be series, Mn, Fe, Co, Ni, Cu series and Zn, Cd, Hg series. Although physical meaning is not clear, this empirical equation is useful to predict the values of electronegativity and/or ionic radius from the heat of formation, and vice versa. The electronegativity of Cd 2+ in halides is found to be 1.3 in Pauling's scale. This value is consistent with that obtained from the structure refinement of Cd 3Al 2Si 3O 12 garnet.