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Resonance Structures Explained

GENERAL CHEMISTRY / DELOCALIZATION

Resonance: One Structure, Multiple Contributors

Represent electron delocalization without moving atomic nuclei or treating contributors as separate molecules.

Illustrated chemistry concepts for Resonance: One Structure, Multiple Contributors

Learning objectives

  • Identify equivalent resonance contributors of common ions.
  • Keep the atom connectivity and net charge unchanged.
  • Explain why a resonance hybrid is not a rapid mixture of isolated contributors.

1. What resonance means

A Lewis structure may not represent a delocalized electron distribution with a single drawing. We use two or more resonance contributors to describe one underlying species. The atoms stay in the same positions; electrons are redistributed.

Worked example

NO₂⁻: O=N–O⁻ ↔ ⁻O–N=O (each contributor includes one lone pair on N).

2. Move electrons, not atoms

Move π electrons or lone pairs while keeping the same σ-bond connectivity. Verify all changes obey the relevant valence and electron budget. An arrow between resonance contributors is ↔, not a reaction equilibrium arrow.

Worked example

CO₃²⁻ has three equivalent major octet-satisfying contributors.

3. Maintain charge bookkeeping

The sum of formal charges must remain equal to the original total charge. In a nitrate contributor, N is +1, two singly bonded oxygens are −1, and the double-bonded oxygen is 0; total −1.

Worked example

NO₃⁻: +1 + (−1) + (−1) + 0 = −1.

4. Understand equivalent contributors

Equivalent contributors can place the formal double bond at different symmetry-related oxygen atoms, but the experimental molecule does not switch between different localized species. Equivalent resonance forms contribute equally in the basic Lewis model.

Worked example

NO₃⁻ has three equivalent common contributors.

5. Resonance does not predict every fact

Resonance is one model for electronic delocalization. It does not mean atoms migrate, and it should not be used to claim every bond in a larger organic structure is identical.

Worked example

O₃ is often drawn with two equivalent major resonance contributors.

Common mistakes

  • Moving a nucleus rather than electrons.
  • Using a reaction arrow (→) to mean resonance.
  • Treating resonance contributors as separate molecules in rapid equilibrium.

Check your understanding

How many equivalent common resonance contributors are drawn for carbonate, CO₃²⁻?

Show explanation

Three, with the C=O double bond placed on a different oxygen in each drawing.