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Resonance

When one Lewis structure isn't enough, and what the real delocalized hybrid means.

Why it matters

Some molecules simply cannot be captured by a single . Resonance is how chemistry admits that and describes the real, averaged molecule, which explains why bonds that look like they should differ turn out identical.

When you use it

Whenever more than one equally good Lewis structure is possible, usually equal bonds that one drawing cannot show (spot it when a double bond could sit in several equivalent spots).

Where it leads

Delocalization, between whole numbers, and the extra stability that makes ions like carbonate and nitrate so common.

What resonance is

Resonance structures are valid Lewis structures for the same molecule that differ only in where the electrons sit. The atoms never move, only a double bond and a lone pair trade places. Nitrate below has three equivalent contributors: the double bond can go to any of the three oxygens.

−NOOOFormal charge on N: +1. A formal charge is what an atom's charge would be if every bond were split exactly down the middle, so it shows where charge sits in the drawing. Formal charge = valence electrons − nonbonding electrons − half the bonding electrons. Click for this atom's full working.+Formal charge on O: −1. A formal charge is what an atom's charge would be if every bond were split exactly down the middle, so it shows where charge sits in the drawing. Formal charge = valence electrons − nonbonding electrons − half the bonding electrons. Click for this atom's full working.−Formal charge on O: −1. A formal charge is what an atom's charge would be if every bond were split exactly down the middle, so it shows where charge sits in the drawing. Formal charge = valence electrons − nonbonding electrons − half the bonding electrons. Click for this atom's full working.−

Where to expect it

  • Ions with identical outer atoms: NO₃⁻, CO₃²⁻, SO₄²⁻.
  • Conjugated π systems: ozone (O₃) and benzene (C₆H₆).
  • Any structure where one double bond could sit in more than one equivalent spot.

Memory trick

Resonance uses a double-headed arrow between the drawings, not the two half arrows of a chemical equilibrium. That arrow does not mean the molecule flips back and forth; it means "average these into one real structure."

Common mistake

The molecule does not flip between its resonance structures. It is a single delocalized hybrid at every instant, so every N−O bond in nitrate is identical, about one and a third bonds strong, all the time.

Practice

Nitrate, NO3−, has three resonance structures: in each one, a single N−O bond is a double (order 2) and the other two are single (order 1). What is the average N−O bond order in the real ion?
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