Heteropolar One-Electron Bond

Factors which are characteristic for covalent binding between different atoms are analyzed through an investigation of the one-electron heteronuclear bond. The examination of these model systems shows that the origin of binding can be understood by a partitioning of the binding energy in terms of five contributions with well defined conceptual meanings: Charge transfer from one atom to another, quasiclassical Coulombic interactions between the atoms, interference interactions between the atoms, atomic orbital contraction, and equalization of atomic orbital sizes. A stable bond is found to exist only if the nuclear charges differ by less than about 20%; a metastable bond exists if the difference in the nuclear charges lies approximately between 20%-40%; a purely repulsive binding energy curve results if the nuclear charges differ by more than about 40%. A variational analysis shows the reason for this behavior to be the high energetic cost of electronic charge transfer required to establish electron sharing. It is also confirmed that the interference energy is adversely affected by bond polarity. Finally, it is observed that, in these heteronuclear bonds, there exists a pronounced and explainable tendency for the atomic orbitals of the two parent atoms to become more similar in size upon molecule formation, while both contract. The basic origin of covalent binding is found to have the same explanation as in the homonuclear case.