9. Molecular Orbitals and      Molecular Structure   Previous PageNext Page
     Bonds between different kinds of atoms

This trend is illustrated at the right for H and HF, and in more exaggerated form for LiF. If the bonding MO has more of a contribution from F because of its electronegativity, then filling this orbital with an electron pair means giving the electrons more to F than to the other atom. The bonding pair is unequally shared, and the bond has a partial ionic character. The larger the energy spread between orbitals of the original atoms, the more the bonding orbital resembles the AO from the more electronegative atom, and the more ionic the bond will be. H-H is totally covalent, with equal sharing. H-F is partially ionic, with a partial displacement of the bonding electron pair toward F. In LiF the energy gap is so large that the bonding MO is almost indistinguishable from the 2p orbital of fluorine. The bonding electron pair is transferred almost completely to the F atom as a lone pair. Molecular orbital theory views covalent and ionic bonds as two extremes of a continuum of bond types, a matter of degree rather than kind. What determines the covalent or ionic character of a bond is the difference in energy between the atomic orbitals being combined, which in turn reflects the relative electronegativities of the two atoms.

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