Resonance
Sometimes one Lewis structure cannot show a molecule correctly. Several equal structures, called resonance forms, differ only in where electrons are placed. The real molecule is a blend (hybrid) of them.
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Analogy
A mule is a mix of a horse and a donkey, not one then the other. The real molecule is the mix.
Example
Three equal resonance forms.Each has one C=O and two C–O⁻.Real bond order = 4/3Each O carries −⅔ overall.
In practice
All three C–O bonds in carbonate (CO₃²⁻) are identical in length, between a single and a double bond.
Practice
Flashcards
- What is resonance?Several valid Lewis structures for one molecule. The real structure is a hybrid of all of them.
Interview questions
- Draw the Lewis structure of the nitrate ion and explain resonance and formal charge.
- Total valence electrons: 24
- Three equivalent resonance forms
- Formal charges: N +1, one O −1 in each form
- Real bond order 4/3
Questions people ask
Does a molecule flip between resonance forms?
No. The molecule does not switch back and forth. It has a single fixed structure that is an average of all the resonance forms. The drawings are only our way of describing electrons that are spread over several atoms, called delocalised electrons.
How does resonance affect stability?
Spreading electrons over more atoms lowers the energy, so the molecule is more stable than any single drawing suggests. This extra stabilisation is called resonance energy. Benzene is a famous example and is much more stable than a single ring with three isolated double bonds.