β‘ Formal Charge Calculator
Charges for Lewis structure analysis.
How to Use This Calculator
Enter the atom's valence electron count, its nonbonding (lone pair) electrons, and its number of bonds. The calculator returns the formal charge.
The Formula
Each bond = 2 bonding e-, half = 1 per atom
Count dots for nonbonding electrons and lines for bonds. A double bond counts as 2 bonds. Sum of all formal charges equals the ion's overall charge, which is a good self-check.
Worked Example
Oxygen in water: valence 6, nonbonding 4, bonds 2. FC = 6 - (4 + 2) = 0. Neutral, as expected. Oxygen in hydronium with 3 bonds: FC = 6 - (2 + 3) = +1.
Formal Charge vs Real Charge
Formal charge is a bookkeeping fiction that assumes perfect electron sharing, but real bonds are polar. In carbon monoxide, the formal charges are -1 on carbon and +1 on oxygen, yet oxygen is more electronegative and actually carries the partial negative charge. Formal charge picks the best Lewis structure; it does not describe the real electron distribution. Electrostatic potential maps show the truth.
Resonance and Formal Charge
When several valid Lewis structures exist, the real molecule is a hybrid of all of them, weighted toward the structures with the lowest formal charges. Ozone's two resonance forms each put +1 on the central oxygen and -1 on one end; the hybrid spreads -0.5 over each terminal oxygen. Benzene's six identical bonds come from averaging two Kekule structures.
Expanded Octets
Third-period elements and below can hold more than eight electrons using d orbitals. SF6 puts twelve electrons around sulfur with all formal charges at zero, which is why it is so stable and inert. This is impossible for second-period elements like nitrogen and oxygen, which is why NO3- must carry formal charges.
Frequently Asked Questions
What is formal charge?
The charge an atom would have if bonding electrons were shared equally: formal charge = valence electrons - (nonbonding electrons + half the bonding electrons). It is a bookkeeping tool for picking the best Lewis structure.
What is the best Lewis structure?
The one with formal charges closest to zero, negative charges on the most electronegative atoms, and no adjacent like charges. A structure with all-zero formal charges is usually correct.
Can formal charge be fractional?
No, it is always an integer. If you get a fraction, recheck your electron counting. Each bond contributes 1 to the bonding-electron half-count per atom.
Is formal charge the same as oxidation state?
No. Formal charge assumes equal sharing; oxidation state assumes the more electronegative atom takes all bonding electrons. In CO, carbon has formal charge -1 but oxidation state +2.
Why does ozone have formal charges?
Ozone (O3) must place a positive charge on the central oxygen and negative on one terminal oxygen to satisfy octets. The real molecule is a resonance hybrid with the charge spread over both ends.