Formal Charge: Definition and Calculation

Formal charge = valence electrons - non-bonding electrons - half the bonding electrons. An equivalent phrasing is valence minus lone pair electrons minus the number of bonds, because each bond contributes one electron to the atom under this scheme.

Formal charge asks how many electrons an atom is assigned under equal sharing. Assign it every nonbonding electron on that atom and one electron from each shared pair. Subtract that assigned count from the neutral atom's valence contribution V. Thus FC = V − N_nonbonding − B_electrons/2. If using bond lines instead, count a single as one, a double as two and a triple as three; counting merely the number of neighboring atoms gives the wrong shortcut for multiple bonds.

For the oxygen in water: valence 6, non-bonding 4, bonding 4, so 6 - 4 - 2 = 0. For each hydrogen: 1 - 0 - 1 = 0. Neutral molecules drawn well usually give zero everywhere, which is a fast sanity check.

The notation N in the formula means the number of nonbonding electrons, not the number of lone pairs and not the element nitrogen. Three lone pairs mean six nonbonding electrons. For hydroxide O with three lone pairs and one bond, FC(O) = 6 − 6 − 1 = −1. For hydronium O with one lone pair and three bonds, FC(O) = 6 − 2 − 3 = +1. The reference valence contribution of oxygen stays six in both structures.

Formal charge and oxidation number are bookkeeping conventions; partial charge describes a partition of the actual electron density and is model dependent. A partial charge is not obtained by averaging those two integers, and it need not lie numerically between them in every species. The fact that BF₄⁻ formally assigns −1 to boron does not mean boron physically attracts the B–F bonding electrons more strongly than fluorine.

The total is +1 + 0 − 1 − 1 = −1, matching nitrate. This sum works because every electron in the structure has been assigned once across all atoms. If the sum disagrees with the bracket charge, check the original budget, all lone pairs and each multiple-bond count. A correct sum is necessary but does not prove the drawing is the best contributor; several valid drawings can have the same net charge and different charge separation.