Resonance & Delocalization
- Resonance structures are different Lewis structures for the same molecule,
showing different electron arrangements
- The real structure is a weighted average called the resonance
hybrid — no single structure is "correct"
- Delocalized electrons: electrons spread across multiple atoms, not localized
between two atoms
- Resonance stabilizes molecules — more equivalent structures = greater stabilization
- All resonance structures must have the same arrangement of atoms (same σ-bond framework)
- Bond lengths in the hybrid are intermediate between single and double bonds
IB Exam Strategies
- Drawing: Use a double-headed arrow (↔) between resonance structures, NOT
equilibrium arrows (⇌)
- Formal charge: FC = Valence e⁻ − Lone pair e⁻ − ½(Bonding e⁻). Minimize formal
charges for best structures
- Bond order: In CO₃²⁻ with 3 equivalent resonance structures, each C–O bond
order = 4/3 ≈ 1.33
- Evidence: Equal bond lengths in X-ray crystallography prove delocalization
(e.g., all C–O bonds in CO₃²⁻ are 129 pm)
- Benzene: 6 equivalent C–C bonds of length 140 pm (between 154 pm single and 134
pm double)
Formal Charge Rules
- FC = V − L − B/2 where V = valence electrons, L = lone pair electrons, B =
bonding electrons
- The sum of all formal charges must equal the overall charge of the species
- Best structures: minimize formal charges (prefer 0 on each atom)
- Negative FC should be on more electronegative atoms
- For equivalent structures, all atoms in symmetric positions have the same formal charge