CHEM 121 glossary

Formal charge

The charge an atom would have if every bonding pair were shared exactly equally: $\text{FC} = V - N - \tfrac{1}{2}B$ (valence electrons, minus lone-pair electrons, minus half the bonding electrons). The formal charges add up to the species' overall charge, and the best Lewis structure keeps them as small as possible.

\ce{NO3-}

: N is

+1

, each single-bonded O is

-1

; the sum is

-1

Example

In $\ce{H3O+}$ , O has 3 bonds and 1 lone pair: $\text{FC} = 6 - 2 - \tfrac{1}{2}(6) = +1$ , which matches the ion's charge.