FAQ
No. The molecule/ion exists as a single resonance hybrid.
The different resonance structures are bookkeeping drawings for electron placement, not separate species in dynamic equilibrium.
Equivalent resonance forms have the same connectivity and the same pattern of charges/bonds, just in different positions on equivalent atoms.
If moving the electrons produces a symmetry-related structure, they are usually equivalent.
Because delocalisation spreads $\pi$ bonding over multiple positions.
Measured bond lengths often reflect an average: neither purely single nor purely double, but intermediate.
Yes. A lone pair adjacent to a $\pi$ bond or electron-deficient site can be part of resonance.
Resonance commonly shifts a lone pair into bonding while a $\pi$ bond shifts elsewhere.
Delocalisation distributes electron density more widely, often lowering electron–electron repulsion and reducing charge concentration.
This typically lowers the potential energy compared with a model that localises charge or $\pi$ electrons on one bond/atom.
Practice Questions
(2 marks) For the nitrate ion, , state how many equivalent resonance structures can be drawn and describe one structural feature that all the resonance structures have in common.
Correct number of equivalent resonance structures: 3 (1 mark)
Common feature stated, e.g. same atom connectivity with N central and three O attached / overall charge conserved / one and two in each form (any one) (1 mark)
(5 marks) Ozone, , can be represented using resonance. Explain how resonance refines the Lewis model for and state two qualitative predictions that improve when resonance is included.
Identifies that more than one equivalent Lewis structure is possible for (1 mark)
States that resonance forms differ only in electron placement (not atom positions) and are connected by (1 mark)
Explains that the actual structure is a resonance hybrid with delocalised electron density (1 mark)
Prediction 1: the two O–O bonds have equal/intermediate bond order so similar/intermediate bond lengths (1 mark)
Prediction 2: charge is delocalised (e.g. partial charges spread over terminal/central O rather than fixed on one atom) OR more accurate qualitative stability due to delocalisation (1 mark)
