Resonance and Formal Charge

Resonance

• When more than one valid Lewis structure can be drawn for a molecule or ion. The actual structure is a hybrid (average) of all resonance forms

• Example: O₃, NO₃⁻, CO₃²⁻

Resonance hybrid

• The true structure of a molecule with resonance — it’s not flipping between forms; it’s a single, stable structure that is an average of all forms

• Bonds are identical in length and strength

• Represented with dashed lines or double-headed arrows (↔)

Why resonance occurs

Because electrons (especially in double bonds and lone pairs) can be delocalized, spread out over multiple atoms, leading to greater stability

Rules for drawing resonance structures

• Only electrons move (lone pairs or pi bonds)

• Atoms do NOT move

• All structures must be valid Lewis structures

• The overall charge stays the same

Pi bond delocalization

• In resonance, π (pi) electrons are shared across three or more atoms instead of being fixed between two

• Lowers potential energy → increases stability

Formal charge

• A bookkeeping tool to help determine the best Lewis structure

• FC = (# valence e⁻) – (# lone pair e⁻) – ½(# bonding e⁻)

How to calculate formal charge

• For each atom:

• Count its valence electrons (from periodic table)

• Count lone pair electrons on it

• Count half the bonding electrons around it

• Then plug into formula

Best lewis structure

• The structure with:

• Formal charges closest to zero

• Negative charge on the most electronegative atom

• Minimal separation of charge

Formal charge vs. actual charge

• Formal charge is a calculation, not real charge

• Actual charge (like in ions) is real and measurable

• Sum of all formal charges = total charge of molecule/ion

Resonance stabilization energy

• The extra stability gained from electron delocalization.

• Molecules with resonance have lower energy than any single resonance form suggests

• Makes them less reactive

When structures are equivalent

• If all resonance forms are identical (like in benzene or carbonate), they contribute equally to the hybrid

• If not, the one with lowest formal charges contributes more

Arrow notation for resonance

• Use double-headed arrow (↔) between resonance structures

• ❌ Never use equilibrium arrow (⇌), resonance is not equilibrium!

Moving atoms is a mistake

• You cannot move atoms, only electrons!

• Wrong: Turning O₃ into a ring

• Right: Move double bond and lone pairs only

How resonance affects bond length

• In molecules with resonance, bond lengths are intermediate between single and double

• Example: In benzene, all C–C bonds are 139 pm (between 154 and 134 pm)

How resonance affects reactivity

• More resonance = more stable = less reactive

• Benzene is less reactive than alkenes due to resonance

• Carboxylate ions are stable because charge is delocalized

Why resonance is important

• It explains:

• Why some bonds are stronger/weaker than expected

• Why certain ions are stable (like acetate)

• Why molecules don’t react as predicted

• Without resonance, chemistry wouldn’t make sense!