2.6 Resonance & Formal Charge

01Resonance

When more than one valid Lewis structure exists, the real molecule is an average (hybrid). In NO₃⁻ all three N–O bonds are identical, with bond order 1⅓.

Formal charge picks the best structure: FC = valence − lone-pair electrons − ½ bonding electrons. Best = charges closest to zero, negative charge on the most electronegative atom.

02Notebook box

FC = V − N − B/2. Bond order with resonance = total bonds ÷ number of positions.

Worked example: O₃: bonds are 1.5 order, equal length, between single and double.

Key takeaways

✦Resonance = averaged bonds.
✦Electrons don't flip back and forth.
✦Minimize formal charges.

Watch outResonance structures are not real molecules switching — the hybrid is the only real one.

Quick check

Did it stick?

1.Bond order of each N–O in NO₃⁻?

2.Formal charge of O with 3 lone pairs and 1 bond?

3.In O₃ the two bonds are…