2.6 Resonance and Formal Charge

Resonance structures

When a molecule's actual bonding can't be shown by a single Lewis structure, chemists draw resonance structures — different valid arrangements of the same atoms with electrons (usually pi bonds/lone pairs) in different positions. The real molecule is a resonance hybrid, an average of all structures, with bond lengths/orders intermediate between single and double bonds.

Formal charge

Formal charge (FC) = valence electrons − (nonbonding electrons + bonding electrons/2). It helps identify the most reasonable Lewis structure. The best structure minimizes formal charges, keeps negative FC on the most electronegative atoms, and avoids unnecessary charge separation.

Choosing the best structure

Among possible resonance structures, the most important (lowest energy) ones have formal charges closest to zero. If charges can't be zero, negative FC should sit on the more electronegative atom. Structures with large charge separation or positive charge on electronegative atoms contribute less to the real hybrid.

Key terms

Resonance structures

Multiple valid Lewis structures for the same molecule differing only in electron (usually pi bond/lone pair) placement.

Resonance hybrid

The true structure of a molecule, an average of all contributing resonance structures with intermediate bond properties.

Formal charge formula

FC = valence electrons − (nonbonding electrons + bonding electrons/2).

Criteria for best Lewis structure

Minimize formal charges overall; place any negative formal charge on the most electronegative atom.

Effect of resonance on bond length

Bonds involved in resonance become equal in length, intermediate between single and double bond lengths (e.g., ozone's O–O bonds).

Try an AP-style question

What is the formal charge on the central O in ozone, O₃ (with one double and one single bond)?

1. −1
2. +2
3. +1
4. 0