The last point refers to electrons in bonding orbitals. For electrons in antibonding
orbitals, the opposite is true: the greater contribution come from the less electronegative
atom. This distinction is illustrated in FIG. 4.33.
To find the ground-state electron configurations of heteronuclear diatomic
molecules, we use the same approach as for homonuclear diatomic molecules, but
first we must modify the energy-level diagrams. For example, consider the HF molecule.
The -bond in this molecule consists of an electron pair in a -orbital built
from all the atomic orbitals with the appropriate symmetry: these are the F2pz- and
F2s-orbitals and the H1s-orbital. Because the electronegativity of fluorine is 3.98
and that of hydrogen is 2.20, we can expect the bonding -orbital to be mainly composed
of fluorine orbitals and the antibonding *-orbital to be mainly hydrogen in
character. These expectations are confirmed by calculation. Because the two electrons
in the bonding orbital are more likely to be found in the fluorine orbitals than
in the H1s-orbital, there is a partial negative charge on the F atom and a partial
positive charge on the H atom.
The molecular orbital energy-level diagrams of heteronuclear diatomic molecules
are much harder to predict qualitatively, and we have to calculate each one
explicitly because the atomic orbitals contribute differently to each one. Moreover,
the diagrams have to show how all atomic orbitals of the appropriate symmetry
contribute to a given molecular orbital. This requirement means that, as for HF, we
have to allow for -orbitals to be formed by combining both s- and pz-orbitals on
the atoms. When both atoms are in Period 2, each contributes an s- and a pz-orbital,
for four atomic orbitals in total, and we can expect four -orbitals. The same is true
of -orbitals: there are two sets of suitable atomic orbitals on each atom (the pxand
py-orbitals), so we can expect four -orbitals in all. FIGURE 4.34 shows the
calculated scheme typically found for CO and NO. We can use this diagram to state
the electron configuration by using the same procedure as for homonuclear
diatomic molecules.