Formal Charge
The hypothetical charge on an atom in a Lewis structure assuming all bonding electrons are shared equally.
Formula / equation
FC = V − L − B/2
Unit
Dimensionless
In depth
Calculated from valence electrons minus lone-pair electrons minus half the bonding electrons, formal charge identifies the most plausible Lewis structure: the best one keeps formal charges closest to zero and places any negative charge on the most electronegative atom. It is a bookkeeping device, not a real charge.
Examples in the real world
In O=C=O each atom has zero formal charge; in the ammonium ion nitrogen carries +1.