Formal Charge Calculator
Calculate formal charge for up to 3 atoms using FC = V - L - B/2.
Checks whether the Lewis structure sum matches the molecular charge and flags the most stable arrangement.
Formal charge assigns a hypothetical charge to each atom in a Lewis structure, assuming that bonding electrons are split exactly 50/50 between the two atoms sharing them.
FC = V - L - B/2
V = valence electrons the atom would have as a free neutral atom (from the group number: H=1, C=4, N=5, O=6, F=7, Cl=7, S=6, P=5). L = lone pair electrons drawn on that atom in the Lewis structure (count all, not pairs). B = bonding electrons – every electron in a covalent bond to this atom (2 per single bond, 4 per double bond, 6 per triple bond).
Example: carbon dioxide (CO2), using the standard double-bond structure. Central carbon: V=4, L=0, B=8 (two double bonds, 4 electrons each). FC = 4 - 0 - 4 = 0. Each oxygen: V=6, L=4 (two lone pairs = 4 electrons), B=4. FC = 6 - 4 - 2 = 0. Sum = 0, matching the neutral molecule. Good.
The most stable Lewis structure minimizes formal charges overall, puts negative formal charges on the most electronegative atoms, and avoids placing the same sign of charge on adjacent atoms. When two resonance structures give different formal charge distributions, the one closer to zero on all atoms is usually dominant.
The sum of all formal charges in a molecule or ion must equal the net charge of the species. Use this as a check – if the sum does not match, the electron count in the structure is wrong.