Formal Charge
Definition and meaning of Formal Charge in chemistry.
Formal charge is the hypothetical charge an atom would have in a Lewis structure if all bonding electrons were shared exactly equally between the bonded atoms, regardless of their actual electronegativities.
In more detail
It is calculated as FC = V − N − B/2, where V is the number of valence electrons in the free atom, N is the number of nonbonding (lone pair) electrons on that atom in the structure, and B is the number of electrons it shares in bonds. Chemists use formal charge to compare competing, valid Lewis structures for the same molecule or ion: the most reasonable structure generally has formal charges as close to zero as possible, with any negative formal charge placed on the more electronegative atom. Formal charge is a bookkeeping tool, not a measure of true charge distribution, which is better described by electronegativity differences or computed partial charges.
Key facts
| Field | General Chemistry |
|---|---|
| Formula | FC = V − N − B/2 |
| Units | Dimensionless (expressed as a signed integer charge) |
| Key rule | Best Lewis structure minimizes formal charges |
In the nitrate ion (NO3−), drawing one N=O double bond and two N–O single bonds gives nitrogen a formal charge of +1, the doubly bonded oxygen a formal charge of 0, and each singly bonded oxygen a formal charge of −1; the sum (+1 + 0 − 1 − 1) equals the ion's overall charge of −1.
Frequently asked questions
Is formal charge the same as the real charge on an atom?
No. Formal charge assumes perfectly equal electron sharing in every bond, so it is a formalism for comparing Lewis structures, not an experimental measure of actual charge distribution, which depends on electronegativity.
How is formal charge used to choose between resonance or Lewis structures?
The preferred structure is usually the one with formal charges closest to zero, and when nonzero charges are unavoidable, negative formal charge should sit on the more electronegative atom.