Valence bond modelBonds result from
overlap of
s and
p valence
orbitals on
adjacent atoms in
a
σ fashion (
end-on - bond density around bond axis)
s with s, s with p, or p with p if both p orbitals lie on the bond axis
or a
π fashion (
side-on - bond density above and below bond axis)
p with p if both p orbitals are perpendicular to bond axis
Bonding electrons are
localized around the bonded atoms.
The model was modified to include use of
hybrid orbitals (mixtures of
s and
p atomic orbitals) to rationalize composition and geometry of compounds of second-row elements (such as CH
4 and BF
3). Hybrid orbitals
overlap in a
σ fashion with other hybrid orbitals (or
s or
p orbitals) on adjacent atoms.
Molecular orbital theoryAccording to this more complete theory of bonding, electrons occupy molecular
orbitals delocalized over the entire molecule. Because this is more complicated, we will only consider diatomic molecules.