MolDraw › Chemistry tools › Resonance Structure Generator
Resonance Structure Generator
In short: Draw a molecule or ion, or paste its SMILES. You get every resonance structure, the curly arrows that turn one into the next, and which contributors matter most.
Works for carboxylates, carbonate, nitrate, nitro groups, enolates, phenoxide, amides, allyl cations, anions and radicals, benzene and conjugated carbonyls.
CC(=O)[O-], has two equivalent resonance structures: a lone pair on O⁻ forms a C=O bond while the old C=O π bond moves onto the other oxygen. Each oxygen carries half the negative charge.Loading editor…
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How to use the resonance generator
- Draw the molecule or ion with its formal charges, or paste a SMILES such as
CC(=O)[O-]. - The top row shows every resonance structure joined by double-headed arrows, labelled major, minor or negligible.
- Below, each structure is shown next to the one it comes from, with the curly arrows that move the electrons.
- The "charge in the hybrid" line shows where the charge really sits, averaged over the contributors.
Rules for drawing resonance structures
- Only electrons move. Atoms stay in place; you move lone pairs and π electrons, never σ bonds.
- Curly arrows start at electrons (a lone pair or a π bond) and end where the pair goes (a bond or an atom).
- Keep octets: C, N and O never have more than 8 electrons. N, O and halogens must keep a full octet; carbon may be left with 6 (a carbocation).
- The total charge stays the same in every structure.
Which structure contributes most?
- Most atoms with a full octet.
- Fewest formal charges (least charge separation).
- Negative charge on the more electronegative atom (O > N > C); positive charge on the less electronegative atom.
- No like charges on neighbouring atoms.
- Equivalent structures contribute equally and give extra stabilisation.
The four common patterns
- Lone pair next to a π bond (enolate, amide, phenoxide): the lone pair forms a new π bond and the old π bond moves out to the far atom.
- Lone pair next to a positive charge: the lone pair forms a π bond to the cation.
- π bond next to a positive charge (allyl cation): the π bond shifts toward the cation.
- π bond between atoms of different electronegativity (C=O): the π electrons move onto oxygen, giving C⁺–O⁻.
Worked examples
Carbonate ion
Three equivalent structures. Each C–O bond has bond order 4/3 and each O carries −2/3 charge.
All three C–O bonds are the same length.
Amide
The N lone pair donates into C=O: C(=O)–NH₂ ↔ C(–O⁻)=NH₂⁺. This minor form explains the planar amide and restricted C–N rotation.
The neutral form is major.
Phenoxide
The negative charge spreads to the ortho and para carbons, which is why phenoxide reacts with electrophiles there (Reimer–Tiemann, Kolbe).
O⁻ forms are major; carbanion forms are minor.
Common mistakes
- Moving a hydrogen or an atom. That is a tautomer, not a resonance structure.
- Giving carbon five bonds, or nitrogen four bonds without a positive charge.
- Starting an arrow from a positive charge. Arrows always start at electrons.
- Thinking the molecule flips between the structures. It is one hybrid all the time.
The generator applies the textbook electron moves on the structure you enter and keeps every result that obeys the octet rules. Very large conjugated systems are limited to the first 16 structures.
Draw it properly. Open the structure in the free MolDraw editor to add wedges, arrows and labels, then export a PNG or SVG for notes and slides.
Open full MolDraw editorFrequently asked questions
What are resonance structures?
Two or more Lewis structures of the same molecule that differ only in where the π electrons and lone pairs are. The real molecule is a hybrid of them.
How do you draw resonance structures?
Find a lone pair or π bond next to a π bond, a positive charge or an empty orbital. Move the electrons with curly arrows, keep every atom in place, keep the octet rule and keep the total charge.
How do I know which resonance structure is major?
The major contributor has the most full octets, the fewest formal charges, and any negative charge on the most electronegative atom.
Does benzene have resonance?
Yes. Its two Kekulé structures are equivalent, so all six C–C bonds are the same length (139 pm), between a single and a double bond.
What is the difference between resonance and tautomerism?
In resonance only electrons move. In tautomerism an atom, usually hydrogen, moves, giving two different compounds in equilibrium (for example keto and enol forms).
Is the resonance structure generator free?
Yes. It runs in your browser with no sign-up.
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