When electrons spread over more atoms than one Lewis structure can show, we draw a set of pictures and take the truth to be their blend.
The lesson in three molecules: a charge spread over two oxygens, a ring of six shared electrons, and a conjugated chain. Drawn live.
1. Resonance Structures Are One Molecule Drawn Several Ways
There is exactly one molecule that never changes; resonance forms are just our notation's inability to place electrons "in between" atoms — as in acetate, whose two C–O bonds are really identical.
Two drawings, one ion. The real acetate is the average — a hybrid with two equal C–O bonds.
2. Curved Arrows Move Electron Pairs — Atoms Stay Put
Each arrow starts at an electron pair (lone pair or π bond) and moves it — only π electrons and lone pairs move, never σ bonds or atoms, as the allyl cation shows.
Allyl cation: the double bond and the positive charge trade ends. Both carbons carry half the charge.
3. The Best Contributor Gives Every Atom a Full Octet
The major contributor gives every second-row atom a complete octet — as when the amide nitrogen's lone pair feeds the carbonyl, locking the C–N bond flat.
Both contributors keep full octets, so both count — locking the amide flat.
4. Put Negative Charge on the More Electronegative Atom
When both satisfy the octet, break the tie by placing negative charge on the more electronegative atom (and positive on the less) with the fewest formal charges — the nitro group's two equivalent forms show it.
Nitro group: two equivalent contributors put the negative charge on oxygen both times.
5. More Delocalization Means More Stability
Resonance lowers energy, so the more good contributors a species has the more stable it is — benzene's two equivalent Kekulé forms give it the extra calm we call aromaticity.
Benzene's two equivalent Kekulé structures average into one ring of six shared electrons.
6. Putting It to Work: The Enolate
In the enolate, charge delocalizes from carbon onto the more electronegative oxygen (the major form) — which is why it reacts through carbon yet owes its stability to oxygen.
Enolate: the oxygen contributor dominates, but the carbon contributor explains the reactivity.
7. Summary
One molecule, alternate electron placements · move between them with curved arrows pushing pairs, atoms fixed · rank by full octets, then more bonds, then fewer/better-placed charges · equivalent contributors count equally · more delocalization = lower energy.
Quiz yourself
Tap a question to reveal the answer — free, no login.
No — never. Only electron pairs (lone pairs and π bonds) move. If you had to move an atom to get your second drawing, you drew a different molecule (an isomer), not a resonance form.
The one with the negative charge on the more electronegative atom. Oxygen holds negative charge more comfortably than nitrogen or carbon, so that contributor looks most like the real molecule.
Because the negative charge in acetate is delocalized over two equivalent oxygens by resonance. Spreading the charge lowers the energy of the conjugate base, making it stable and easy to form.
It always starts at an electron pair — a lone pair or a π bond — and it moves exactly two electrons (one pair). Single-electron shifts use fishhook arrows and are only for radicals.
Draw this on the whiteboard
Open the OChem Board whiteboard — benzene rings, wedge/dash bonds, and a clickable periodic table built in. No account needed.