Formal Charge
Formal charge is the charge an atom would have in a molecule if every bonding pair of electrons were shared perfectly equally. It's a bookkeeping tool that tells you where the charge sits in a Lewis structure and which of several structures is the most likely.
It is not a real, measurable charge — it assumes equal sharing, ignoring electronegativity. Its job is to compare Lewis structures.
The formula
— where V = valence electrons of the free atom, N = non-bonding (lone-pair) electrons, and B = bonding electrons.
— since each bond contributes one shared electron to the atom.
Worked example — carbon dioxide, O=C=O
Apply the formula to every atom. If it's a neutral molecule the formal charges must add up to 0; for an ion they add up to the ion's charge.
Atom | Valence (V) | Lone-pair e⁻ | Bonds | Formal charge |
|---|---|---|---|---|
C | 4 | 0 | 4 (two double bonds) | 4 − 0 − 4 = 0 |
each O | 6 | 4 | 2 (one double bond) | 6 − 4 − 2 = 0 |
Every atom has a formal charge of 0, and they sum to 0 — this confirms O=C=O is the best Lewis structure for CO₂.
Worked example — where does the charge sit in NH₄⁺?
The ammonium ion NH₄⁺ has nitrogen bonded to four hydrogens. Use formal charge to show which atom carries the ion's +1 charge.
Nitrogen: valence V = 5. In NH₄⁺ it has 4 bonds and 0 lone pairs.
Apply the formula: FC(N) = 5 − 0 − 4 = +1.
Each hydrogen: V = 1, with 1 bond and 0 lone pairs → FC(H) = 1 − 0 − 1 = 0.
Sum the formal charges: (+1) + 4×(0) = +1, which matches the ion's overall charge. ✓
The +1 charge sits on the nitrogen (FC = +1); every hydrogen is 0.
Choosing the best Lewis structure
When a molecule can be drawn several ways, formal charge picks the winner:
the best structure has formal charges as close to zero as possible.
if some atoms must carry charge, put any negative formal charge on the most electronegative atom.
Common mistakes
Counting a double bond as one bond — it counts as 2 bonds (and a triple bond as 3) in the formula.
Treating formal charge as a real charge — it assumes equal sharing and ignores electronegativity, so it is only a comparison tool, not an actual measured charge.
Forgetting to check the total — formal charges must sum to 0 for a molecule, or to the ion charge for an ion.
Frequently asked questions
How do you calculate formal charge?
Use formal charge = valence electrons − lone-pair electrons − number of bonds. Count each atom's normal (free-atom) valence electrons, subtract the electrons in its lone pairs, and subtract one for each bond it makes (so a double bond counts as 2). The values across a whole species must add up to its overall charge.
What is the difference between formal charge and oxidation number?
Formal charge assumes bonding electrons are shared equally between the two atoms. Oxidation number does the opposite — it gives both bonding electrons to the more electronegative atom. They answer different questions: formal charge helps choose Lewis structures; oxidation number tracks electron transfer in redox.
Why is formal charge useful?
It shows where charge is located in a molecule or ion and lets you compare competing Lewis structures. The most likely structure is the one with formal charges closest to zero, with any negative charge placed on the most electronegative atom.
This concept in your exam
Open your board's guide for exam-specific technique and past-paper practice:
Draw structures and assign charges with step-by-step model answers — free to try.
Start practicing →