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.