Determine the total valence electrons for the isolated atom.
Assign bonds to the atom (single = 2 electrons, double = 4, triple = 6) and count bonding electrons around the atom.
Count nonbonding (lone pair) electrons on the atom.
Use: formal charge = (valence electrons) − (nonbonding electrons) − (1/2 × bonding electrons).
If formal charge is positive, the atom has fewer electrons than its neutral valence count; if negative, it has more.
For a molecule/ion, ensure the sum of formal charges equals the overall charge.
Prefer the structure with minimal total absolute formal charges and with negative formal charges on more electronegative atoms.