Relations between the heats of formation of MX 3 halides and the electronegativities of the halogen ions

Abstract

The heats of formation of MX 3 halides (M  B, Al, Sc, Y, La, In and Ga), -Δ H o 298, may be expressed empirically in terms of the electronegativities χ a of the halogen ions ▪ where a and b are empirical constants and the factor 3, e and r C represent the valence of the cation, the electronic charge and the cation radius, respectively. The value of 3 e 2/ r C corresponds to the electrostatic energy arising between the effective nuclear charge of the M 3+ ion (3 e) and an electron at a distance from the nucleus equal to the ionic radius r C. The empirical constants a and b correlate with the electronegativity of the M 3+ ion. Although its physical meaning is not clear, this empirical equation is useful in predicting the values of electronegativity and/or ionic radius from the heat of formation, and vice versa. The heats of formation of YBr 3 (solid) and BI 3 (gaseous) are estimated as −200 and 6 kcal (th) mol −1, respectively. Ga is more electronegative than In. This result is consistent with that obtained from a structure refinement carried out for NaGaSi 2O 6-pyroxene. The electronegativity of Ga 3+ should therefore be revised to 1.9 on Pauling's scale.