In chemistry, a formal charge (FC) on an atom in a molecule is defined as
FC = number of valence electrons of the atom - number of Lone pair electrons on this atom - half the total number of electrons participating in covalent bonds with this atom
When determining the correct Lewis structure (or predominant resonance structure) for a molecule, the structure is chosen such that the formal charge on each of the atoms is minimized.
Examples:
FC = 4 - 0 - 8/2 = 0
FC = 5 - 2 - 6/2 = 0
- double bonded oxygen in NO2-
FC = 6 - 4 - 4/2 = 0
- single bonded oxygen in NO2-
FC = 6 - 6 - 2/2 = -1