AP Chemistry · Unit 2 Molecular & Ionic Compound Structure
Learning Objectives
Resonance: When a single Lewis structure cannot accurately represent a molecule, multiple valid structures (resonance forms) are drawn. The TRUE structure is the weighted average (hybrid) of all forms.
Formal Charge: FC = (Valence e⁻) − (Lone pair e⁻) − (½ × Bonding e⁻). The best Lewis structure minimizes formal charges on all atoms.
Electron Delocalization: Resonance doesn't mean electrons "flip" between arrangements. They are simultaneously spread (delocalized) across all possible positions.
Bond Order: Resonance gives fractional bond orders. In CO₃²⁻, each C–O bond is 4/3 (between single and double), so all three bonds are identical.
Tags
ResonanceLewisFormal ChargeDelocalized
FORMAL CHARGE FORMULA
FC = V − L − ½B
Select Molecule
RESONANCE HYBRID PROPERTIES
Resonance Forms:3
Bond Order:4/3 ≈ 1.33
Equivalent Bonds:Yes (all 3)
Best FC Sum:|ΣFC| = 2
Carbonate CO₃²⁻: The double bond can be drawn to any of the 3 oxygens, giving 3 equivalent resonance structures. The hybrid has all 3 C–O bonds identical at bond order 1.33, with the extra electrons delocalized evenly. Each oxygen carries formal charge −2/3.