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).
What it unlocks
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.
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
Common mistake
Practice