Binding Energy and Dipole Moment of Alkali Halide Molecules

A simple theory of alkali halide gas molecules in the spirit of Born-Mayer lattice theory is presented. The molecule is considered to be constituted of ions, each of which is polarized by the electrostatic field of the other. This deformation polarization has two important consequences: (1) the net dipole moment of the molecule becomes appreciably lower than that given by the product of the electric charge e, and the internuclear separation a; (2) the repulsion constants (determined from empirical data) become different from the corresponding constants for the crystal. The theory yields satisfactory results for the binding energy, vibration frequency, and dipole moment, μ, in all instances where necessary data are available. In cases where the internuclear distances are not known experimentally, they are calculated from the theory using experimental binding energies. It is possible to assign to the ions Goldschmidt-like radii, the sums of which reproduce reasonably well both the calculated and the observed internuclear distances. Finally, item (1) above explains why Pauling's criterion for the fraction of ionic character, f=μ/ea, is not a very good measure of ionicity for these completely ionic substances.