9. Molecular Orbitals and      Molecular Structure   Previous PageNext Page
     Localised Molecular Orbitals

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.

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