VB Treatment of Heteronuclear Diatomic Molecules

Consider the valence-bond ground state wave function of HF. We expect a single bond to be formed by the pairing of the hydrogen Is electron and the unpaired fluorine 2pa electron. The Heitler-London function corresponding to this pairing is [Eq. (13.119)] 

The optimum values of Ci and C2 are found by the variation method. This leads to the usual secular equatiou. We have ionic-covaleut resonance, involving the structures H—F and H+F. The true molecular structure is iutermediate between the covalent and ionic structures. A term C31 EhI h correspoudiug to the ionic structure H F+ could also be included in the wave function, but this should contribute only slightly for HF. For molecules that are less ionic, both ionic structures might well be included. 

Cesium has the lowest ionization energy, 3.9 eV. Chlorine has the highest electron affinity, 3.6 eV. Thus, even for CsCl and CsF, the separated ground-state neutral atoms are more stable than the separated ground-state ions. There are, however, cases of excited states of diatomic molecules that dissociate to ions. 

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