This formal charge calculator applies FC = valence electrons − (nonbonding electrons + bonding electrons ÷ 2) to any atom in a Lewis structure — the bookkeeping that ranks resonance structures.

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Electron Bookkeeping, Split Evenly Down Every Bond

Where the valence counts come from: our Electron Configuration Calculator. Part of our Chemistry Calculators collection.

Worked Examples (Verified)

  • Oxygen in H₃O⁺ (defaults): V 6, N 2, B 6 → 6 − (2 + 3) = +1 — the ion’s charge sits, on paper, on oxygen
  • Single-bonded oxygen in sulfate: V 6, N 6, B 2 → 6 − (6 + 1) = −1; the double-bonded oxygens (N 4, B 4) give 0 — the famous sulfate resonance debate starts with these two rows
  • Carbon in CO: V 4, N 2, B 6 → −1, while its oxygen (N 2, B 6) reads +1 — the “wrong-atom” charges that explain CO’s unusual bonding and toxicity-by-hemoglobin

Structure-Ranking Rules

  • Zero or minimal formal charges beat heroic charge separation — the first filter among candidate Lewis structures.
  • Negative charge, when unavoidable, belongs on the most electronegative atom (O before C before H).
  • Adjacent like charges and charges beyond ±1 are structural fiction — redraw.
  • The sum check never lies: Σ(FC) = overall charge. If your structure’s charges sum wrong, a bond or lone pair is missing.

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Frequently Asked Questions

How do you calculate formal charge?

FC = V − (N + B/2): valence electrons of the free atom, minus its nonbonding electrons, minus half its bonding electrons (bonds are split evenly between partners in this convention). Example: oxygen in H₃O⁺ → 6 − (2 + 6/2) = +1. This calculator applies the formula to any atom you describe.

What is formal charge used for?

Ranking Lewis structures. Among valid candidates, the preferred structure has the smallest formal charges, avoids adjacent like charges, and places negative charge on electronegative atoms. Formal charge is how a drawing argues that it’s the best representation.

Is formal charge the real charge on an atom?

No — it’s a bookkeeping convention that splits every bond 50/50 regardless of electronegativity. Real electron density is uneven (that’s what electronegativity and partial charges describe), and oxidation numbers are the opposite convention (every bond awarded to the more electronegative atom). Three ledgers, three purposes.

What should the formal charges add up to?

The species’ overall charge: 0 for neutral molecules, −1 for an anion like OH⁻ (whose oxygen computes to −1 with its hydrogens at 0), +1 for H₃O⁺. Any other sum means the structure has an electron-counting error.

Why does sulfate have charges in some drawings and not others?

Older drawings use two S=O double bonds (all oxygens 0, sulfur 0 — expanded octet); minimal-charge alternatives with four single bonds put −1 on each oxygen and +2 on sulfur. Textbooks differ on which fiction to teach; the formal charges are the scoreboard for the argument — compute both here and compare.

Is this formal charge calculator free?

Yes — free, instant, no sign-up.