Chapter 10 · Chemical Bonding
10.6Resonance: Equivalent Lewis Structures for the Same Molecule
10 min · two checks
Predict
Why does ozone need two Lewis structures?
The idea
- Explain resonance without claiming the bonds flip back and forth.
- Recognize when a structure requires it.
Some molecules cannot be drawn as one Lewis structure that matches the real bond lengths. Ozone, O₃, and the carbonate ion are the teaching cases. You draw two or more valid structures, called resonance structures, connected by a double-headed arrow. The actual molecule is a blend: the extra bonding is shared. It is not oscillating between the drawings. If it were, you would measure one short bond and one long bond. In ozone both bonds are the same length, between a single and a double.
Only move electrons between drawings. Do not move atoms. A structure in which the octets are worse, or the formal charges are more extreme, contributes less, but a first course mainly needs the idea of the blend. When a problem asks for “the” Lewis structure of carbonate, showing all three, or drawing dashed partial bonds, is the honest answer.
Keep these
- Resonance is a blend, not a flip-flop.
- Use it when one drawing would make equivalent bonds look different.
- Atoms stay put; only the electron drawing changes.