Valence Bond and Molecular Orbital Theory: What Each Explains
Two descriptions of the same bonding, each succeeding where the other fails, and knowing which to reach for is the practical skill.
The two pictures
| Valence bond | Molecular orbital | |
|---|---|---|
| Electrons are | Localised in bonds between two atoms | Delocalised over the whole molecule |
| Bonds formed by | Overlap of atomic or hybrid orbitals | Combination of atomic orbitals into molecular ones |
| Geometry explained by | Hybridisation | Symmetry of the combinations |
| Delocalisation handled by | Resonance | Naturally, without extra apparatus |
| Magnetism | Often wrong | Correct |
Where valence bond theory succeeds
It gives a natural and intuitive account of molecular shape. Mixing atomic orbitals into hybrids of the right number and orientation predicts geometry directly, and for most main-group molecules the prediction is correct and easily obtained.
It also connects well to the way chemists draw structures, since a line between two atoms corresponds directly to a shared pair in an overlapping orbital.
Where it fails
Other failures follow the same pattern — situations where electrons are not neatly paired in localised bonds:
- Odd-electron species such as nitric oxide, where a localised structure cannot be drawn sensibly but a molecular orbital diagram gives a bond order of two and a half.
- Fractional bond orders in ions, which come out directly from counting bonding and antibonding electrons.
- Electronic spectra, which require defined energy levels for the whole molecule.
- Delocalised systems, which valence bond theory can only describe by invoking resonance as an additional device.
Resonance versus delocalisation
Both theories describe the same physical reality, but they express it differently. Valence bond theory draws several localised structures and says the molecule is a hybrid of them. Molecular orbital theory simply constructs orbitals spread over the relevant atoms, so no hybrid is needed.
The molecular orbital account is more economical, but the resonance description remains useful because it connects to conventional structural drawings. Stating that they are alternative descriptions rather than competing claims is the accurate position.
Which to use
| Question about | Use |
|---|---|
| Molecular shape and bond angles | Valence bond with hybridisation, or VSEPR |
| Magnetic behaviour | Molecular orbital |
| Bond order in ions or odd-electron species | Molecular orbital |
| Comparing bond lengths in a series | Molecular orbital, through bond order |
| Reaction mechanism drawing | Valence bond, since it matches structural notation |
| Electronic spectra | Molecular orbital |
In practice chemists use both, choosing whichever answers the question at hand. Presenting them as complementary rather than as one being correct is what an examiner is looking for.
Frequently asked questions
Why does valence bond theory get oxygen wrong?
Because it forces electrons into localised pairs, and the correct description has two unpaired electrons in degenerate antibonding orbitals spread over both atoms.
Is hybridisation a real physical process?
No. It is a mathematical construction that produces orbitals of the right shape and orientation to match observed geometry. Atoms do not hybridise before bonding.
Can molecular orbital theory explain shape?
Yes, through symmetry arguments and correlation diagrams, but the analysis is considerably more involved than hybridisation for simple molecules, which is why the valence bond account remains in use.
Which theory is correct?
Both are approximations to the same underlying quantum mechanics, and each becomes exact in the limit of a complete treatment. They are complementary rather than competing.
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