Resonance, Resonance Energy & Formal Charge

CompleteFundamentals of Chemistry

Resonance describes a molecule that cannot be drawn with one Lewis structure — it is a hybrid of several contributing structures.

Full notes

Definition: Resonance is the concept in which a molecule is represented by two or more Lewis structures (canonical forms) that differ only in the position of electrons, and the real molecule is a hybrid of all of them.

1. The basic idea

For ozone, benzene, and the carboxylate ion, no single Lewis structure is correct. O₃ is written ↔ as two structures with the double bond in two positions. The actual molecule has equal bonds — a resonance hybrid that is more stable than any single contributing structure.

2. Resonance energy

The difference between the energy of the most stable canonical structure and the actual (hybrid) molecule is called the resonance energy. Larger resonance energy = greater stability. For benzene it is about 150 kJ/mol, which is why benzene does not behave like a typical cyclohexatriene.

3. Rules for writing canonical forms

Only electron positions change — atoms do not move.
The number of unpaired electrons stays constant.
All canonical structures must be valid Lewis structures.
More stable structure = more π bonds + more complete octets + charge on electronegative atoms & separation minimized.
Equivalent structures (equal energy) give maximum stabilization (e.g., carbonate, carboxylate).

4. Formal charge

Formal charge = (valence electrons of atom) − (lone-pair electrons) − ½(bonding electrons). For the best Lewis structure we choose the one with formal charges closest to zero on all atoms, and negative formal charge on the most electronegative element.

Example: Benzene (C₆H₆): two Kekulé structures contribute equally; observed C–C bond length (1.39 Å) is between single (1.54 Å) and double (1.34 Å) — evidence of the hybrid.

Note: Exam point: write the 'resonance hybrid is more stable' rule and give ozone or carbonate as the classic example.

Exam notes

Resonance = delocalization (π electrons move, atoms fixed). Hybrid more stable than individual contributors. Resonance energy = measure of that extra stability.

Formal charge = V.E. − non-bonding − ½ bonding
Best structure: charges closest to zero
Examples: O₃, CO₃²⁻, NO₃⁻, benzene, carboxylate ion

Important points

Resonance vs tautomerism: resonance is electron movement; tautomerism involves atom movement (proton) — equilibrium.
Resonance explains the extra stability (acidity of phenol & carboxylic acids).

Common mistakes

Moving atoms or creating/removing unpaired electrons while writing canonical forms.

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