For example, the water
molecule has five atomic orbitals from oxygen, Is, 2s, 2p
2p
and 2p
and a 1s orbital from each of the two hydrogens. It also has ten
electrons, eight from 0 and one each from H. By analogy with HF,
we can assume that the Is and 2s orbitals of oxygen are filled with
electron pairs and take no part in bonding. Of the three 2p orbitals,
two can be considered as interacting with one hydrogen Is orbital
each, as shown for 2p
and 2p
at the left. Each such interaction produces a bonding and an antibonding
localized MO, and when the bonding MO is filled with an electron
pair, a bond is formed between 0 and that H atom. The 2p
orbital remains unused in bonding because it has the wrong orientation
and symmetry to combine with the Is orbitals of the hydrogen atoms.
In summary, the ten electrons are added to the MO's in order of
increasing energy:
1. two to the oxygen 1s, as an electron pair in the closed inner
shell,
2. two to the oxygen
2s, as a lone pair,
3. two each to the two H--O bonding orbitals,
4. two to the oxygen 2p,
as the second lone pair.