In this chapter we extend the discussion to polyatomic
molecules (i.e. those containing three or more atoms).
Within the valence bond model, treatment of a molecule
XYn (n 2) raises the question of compatibility (or not)
between the positions of the Y atoms and the directionalities
of the atomic orbitals on the central atom X. Although
an s atomic orbital is spherically symmetric, other atomic
orbitals possess directional properties (see Section 1.6).
Consider H2O: Figure 4.1 illustrates that, if the atoms of
the H2O molecule lie in (for example) the yz plane, the
directionalities of the 2py and 2pz atomic orbital of
oxygen are not compatible with the directionalities of the
two OH bonds. Although we could define the z axis to
coincide with one OH bond, the y axis could not (at the
same time) coincide with the other OH bond. Hence,
there is a problem in trying to derive a localized bonding
scheme in terms of an atomic orbital basis set (see Section
1.13). In the next section we describe a bonding model
within valence bond (VB) theory that overcomes this
problem. After we have considered how VB theory views
the bonding in a range of XYn species, we move on to the
problems of applying molecular orbital theory to polyatomic
species.