What Is Resonance? Delocalized Electrons Explained
Draw ozone, O₃, and you're forced into an awkward choice: one O–O single bond and one O=O double bond. Which oxygen gets the double bond? Measure the real molecule and the answer is neither — both bonds are exactly the same length. The Lewis structure isn't wrong so much as too crude, and resonance is the patch.
The short answer: resonance describes a molecule whose real electron arrangement can't be drawn as a single Lewis structure, so we draw two or more and treat the truth as an average of them. The electrons involved aren't stuck between two atoms — they're delocalised, spread over three or more.
What resonance actually is
When a molecule has more than one valid Lewis structure that differs only in where the electrons sit — the atoms stay put — the real molecule is none of them. It's a single, unchanging structure called the resonance hybrid, in which those electrons are smeared over the whole set of positions at once.
The individual drawings are called resonance structures or contributors, and they're linked by a double-headed arrow (↔), which is reserved for exactly this purpose. Don't confuse it with the equilibrium arrow ⇌, which means something completely different.
Ozone is the standard case. Structure one has the double bond on the left, structure two on the right. Neither exists. The real molecule has both O–O bonds identical at about 128 pm — comfortably between a normal O–O single bond (148 pm) and a normal O=O double bond (121 pm). Each bond has a bond order of 1.5.
The analogy that fixes the biggest misconception
Students almost universally picture the molecule flickering between structures, like a light switch flipping too fast to see. It doesn't. Resonance structures are not states the molecule visits.
A better analogy is a mule. You can describe a mule by saying "it's a bit like a horse and a bit like a donkey", but a mule doesn't spend Mondays as a horse. It's one animal, permanently, that our two available words only approximate. Resonance structures are the horse and the donkey; the hybrid is the mule.
The reason we need two drawings is a limitation of Lewis notation — a line means "two electrons between exactly these two atoms", and it simply can't express "one and a half bonds spread over three atoms".
How to spot and draw resonance
Look for a lone pair or a pi bond next to a pi bond, or next to an atom short of electrons. That adjacency is what lets electrons shift.
The rules for drawing a set of resonance structures:
- Move electrons only. Never move an atom. If the atoms have moved, you've drawn a different molecule (an isomer), not a resonance structure.
- Move pi electrons or lone pairs — never electrons in sigma bonds.
- Keep the total electron count and overall charge the same in every structure.
- Don't exceed the octet on a second-period atom (C, N, O, F).
- Use curved arrows to show where each pair went.
Not all contributors matter equally. Use formal charge to rank them: structures with charges nearest zero, with any negative charge on the most electronegative atom, and without like charges side by side, contribute most to the hybrid.
The classics
Carbonate, CO₃²⁻. Three resonance structures, the double bond taking each C–O position in turn. All three C–O bonds are equal in the real ion at about 129 pm, with a bond order of 4/3 ≈ 1.33. Nitrate, NO₃⁻, works identically.
Benzene, C₆H₆. Two Kekulé structures with alternating double bonds. The real molecule has six identical C–C bonds at 139 pm — between a single bond (154 pm) and a double bond (134 pm), bond order 1.5 — and is a perfect flat hexagon. Each carbon is sp², and the six leftover p orbitals overlap into a ring of delocalised electron density above and below the plane. That's why benzene is often drawn as a hexagon with a circle inside.
The carboxylate ion, RCOO⁻. When a carboxylic acid loses its proton, the negative charge is shared equally over both oxygens and the two C–O bonds become identical. That extra stability is precisely why carboxylic acids are far more acidic than alcohols — the anion left behind is stabilised by resonance and the alcohol's isn't.
Why resonance stabilises
Spreading electrons over a larger volume lowers their energy. A resonance hybrid is therefore more stable than any single contributing structure would be, and the gap is called the resonance (or delocalisation) energy.
For benzene it's large — roughly 150 kJ/mol, depending how it's measured. That single number explains most of benzene's chemistry: it resists the addition reactions that alkenes undergo eagerly, because addition would destroy the delocalised ring and give up all that stabilisation. Substitution, which preserves the ring, happens instead.
Worked examples
Decide whether each pair is a genuine resonance pair before reading on.
- The two Kekulé structures of benzene → yes. Only pi electrons have moved.
- CH₃–CO–CH₃ and CH₃–C(OH)=CH₂ (propanone and its enol) → no. A hydrogen atom has moved from carbon to oxygen, so these are tautomers — two separate molecules in equilibrium — not resonance contributors.
- The two structures of SO₂ (double bond left, then right) → yes. Both S–O bonds are equal in reality, about 143 pm.
- NO₂⁻ with the double bond on either oxygen → yes. Bond order 1.5 on each N–O.
- A nitro group, –NO₂ → yes. The two N–O bonds are identical and the negative charge is shared.
- Ethanol CH₃CH₂OH and dimethyl ether CH₃OCH₃ → no. Same formula, different connectivity — these are structural isomers.
Common mistakes to avoid
- Saying the molecule "flips between" structures. It doesn't. There's one structure at all times; our notation just can't draw it in a single picture.
- Moving atoms. If a hydrogen has shifted, you've drawn a tautomer or an isomer. Resonance moves electrons only.
- Using the wrong arrow. Resonance takes a double-headed arrow (↔) between structures. The equilibrium arrow (⇌) says two distinct substances are interconverting, which is exactly what is not happening.
FAQ
What is resonance in chemistry?
Resonance is used when a molecule's real electron distribution can't be shown by one Lewis structure. Several structures are drawn, differing only in electron placement, and the actual molecule is a single hybrid of them with delocalised electrons.
Do molecules really switch between resonance structures?
No. The molecule has one fixed structure at all times. The separate drawings are a limitation of Lewis notation, not stages the molecule passes through.
How do you know if a molecule has resonance?
Look for a lone pair or pi bond adjacent to another pi bond or to an electron-deficient atom. If you can shift those electrons and get a second valid structure without moving any atoms, the molecule has resonance.
Why does resonance make a molecule more stable?
Delocalising electrons over more atoms lowers their energy. The resulting hybrid is more stable than any single contributing structure — benzene's delocalisation is worth roughly 150 kJ/mol.
The takeaway
Resonance isn't a molecule changing its mind; it's our notation running out of road. When electrons are delocalised over three or more atoms, we draw several Lewis structures and read the truth as their average — equal bond lengths, fractional bond orders, and a molecule more stable than any one drawing suggests.
Foundation → What Is a Lewis Structure? Dots, Bonds, and Octets and Sigma vs Pi Bonds: What's the Difference?. Ranking the structures → [Formal Charge vs Oxidation Number] (sibling). Measuring the result → [Bond Length vs Bond Energy] (sibling).
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