formal charge

Meaning: Formal charge is the charge an atom would carry in a Lewis structure if each bonding pair were split equally between its two atoms. It is a bookkeeping tool for comparing structures, not a measured charge. The formal charges in a species add up to its overall charge. The preferred structure usually has formal charges closest to zero, and any negative formal charge sits on the more electronegative atom.

Equation: FC = V − N − B/2, where V is the valence electrons of the neutral atom, N is the nonbonding electrons on the atom in the structure, and B is the bonding electrons around it (B/2 equals the number of bonds). All three are electron counts, and FC is in units of the elementary charge, 1.60 × 10⁻¹⁹ C.

On the MCAT: You are shown a Lewis structure or resonance forms, for example of NO₃⁻, CO₃²⁻ or SO₄²⁻, and asked for the formal charge on one atom or which structure is most stable. Useful patterns: N with four bonds is +1, O with one bond and three lone pairs is −1, and O with two bonds and two lone pairs is 0.

Common confusion: Formal charge is not oxidation state. Formal charge splits bonding electrons equally, while oxidation state gives them all to the more electronegative atom.