Formal ChargeFoundation

FC = V − N − B/2, atom by atom — interactive Chemistry simulation for IIT-JEE.

Concept

Formal charge audits a Lewis structure: give each atom its lone-pair electrons plus HALF of every bond, and compare with its free-atom valence count. It exposes hidden charges — ozone's central O is +1 even though O₃ is neutral — and picks the best resonance structure (minimal charges, negative on the most electronegative atom).

Key formula

FC = V - N_{lone} - \tfrac{1}{2}N_{bonding}, \qquad \sum FC = \text{overall charge}

Derivation

Bookkeeping with perfectly shared bonds (contrast oxidation number, which gives bonds entirely to the more electronegative atom).

Ozone: central O has 2 lone electrons + 6 bonding → FC = 6 − 2 − 3 = +1; the single-bonded O is 6 − 6 − 1 = −1. Sum 0 ✓. That +/− pair explains ozone's dipole and reactivity.

Scenarios to explore

Formal Charge — FC = V − N − B/2 audited atom by atom — ozone to nitrate.

Real-world applications

Choosing dominant resonance structures (CO₂'s O=C=O beats O≡C–O⁻).
Predicting reactive sites — nucleophiles attack + centres.
Understanding why CO binds metals via CARBON (C bears −1).

JEE exam tips

Best structure: fewest nonzero FCs; negative FC on the most electronegative atom.
Isoelectronic tricks: CO, N₂, CN⁻, NO⁺ share the same FC pattern.
FC and oxidation number bracket the 'true' charge — reality sits between.

Common mistakes

Counting bonding electrons fully instead of half.
Confusing formal charge with oxidation number (different splits of the same bonds).
Forgetting the sum must equal the ion's charge — free error check.

Exam traps to avoid

CO: FC gives C = −1, O = +1 — backwards from electronegativity intuition, and exactly why CO is a carbon-donor ligand.
All three N–O bonds in NO₃⁻ are identical (resonance) despite unequal FCs in any single drawing.