About the Formal Charge Calculator
This formal charge calculator works out the formal charge on a single atom in a Lewis structure. Choose the element (or type its valence electrons), then enter how many nonbonding electrons sit on the atom and how many bonds it forms — either as a bond count (a double bond counts as 2) or as the number of shared electrons.
Formal charge is the bookkeeping tool chemistry students use to pick the best resonance structure, check that a Lewis drawing adds up to the ion’s overall charge, and spot where positive and negative charge sit in molecules such as CO, NO₃⁻ or NH₄⁺. Run each atom through the calculator and add the results: the sum must equal the charge on the molecule or ion.
Formal charge assumes every bond is shared perfectly equally, so it is not the same as the real partial charge or the oxidation number. Structures where formal charges are as close to zero as possible — and any negative charge sits on the more electronegative atom — are usually the most stable.
With the default inputs, the formal charge is -1. Change any value above to recalculate instantly.
How to use the formal charge calculator
- 1Draw the Lewis structure and pick one atom to analyse.
- 2Select the element so its valence electrons are filled in.
- 3Count the lone-pair electrons on that atom (two per pair) and enter them.
- 4Enter the number of bonds, counting a double bond as 2 and a triple bond as 3.
- 5Read the formal charge, then repeat for each atom and check the sum matches the total charge.
Formula and method
Formal charge compares how many valence electrons an atom brings as a free neutral atom (V) with how many it “owns” in the Lewis structure. The atom is given all of its nonbonding (lone-pair) electrons N and exactly half of the electrons in each bond it forms, B/2, as if every bond were shared perfectly equally. If it owns fewer electrons than it started with, its formal charge is positive; if more, negative.
Because each bond holds two electrons, B/2 equals the number of bonds, so the rule is often stated as FC = valence electrons − dots − lines. The formal charges of all atoms always add up to the overall charge of the molecule or ion.
- FC
- Formal charge on the atom
- V
- Valence electrons of the neutral atom (its group number for main-group elements)
- N
- Nonbonding electrons on the atom (2 per lone pair)
- B
- Bonding electrons shared by the atom (2 per bond)
Worked examples
Oxygen in hydroxide (OH⁻)
Oxygen has 6 valence electrons, three lone pairs (6 nonbonding electrons) and one bond. FC = 6 − 6 − 2/2 = −1, which accounts for the whole charge of the hydroxide ion.
Nitrogen in ammonium (NH₄⁺)
Nitrogen brings 5 valence electrons and has four N–H bonds with no lone pairs. FC = 5 − 0 − 8/2 = +1, and each hydrogen is 0, so the ion carries +1 overall.
Carbon in carbon monoxide (C≡O)
Carbon has one lone pair (2 electrons) and shares 6 electrons in the triple bond. FC = 4 − 2 − 6/2 = −1. Oxygen in CO works out to +1, so the molecule is neutral overall.
Frequently asked questions
How do you calculate formal charge?+
Subtract the number of nonbonding electrons and half the bonding electrons from the atom’s valence electrons: FC = V − N − B/2. A shortcut is valence electrons minus dots minus lines in the Lewis structure.
What is the difference between formal charge and oxidation number?+
Formal charge splits every bond electron equally between the two atoms. Oxidation number gives all the bonding electrons to the more electronegative atom. They answer different questions and often give different values for the same atom.
How does formal charge help choose the best Lewis structure?+
The preferred structure usually has formal charges closest to zero, avoids like charges on adjacent atoms, and puts any negative formal charge on the most electronegative atom.
Should formal charges add up to the ion charge?+
Yes. The sum of the formal charges of every atom always equals the overall charge: 0 for a neutral molecule, −1 for nitrate, +1 for ammonium. If it does not, recount electrons in the structure.
Can formal charge be a fraction?+
For a single Lewis structure the formal charge is always a whole number. Fractions appear only when you average over resonance structures, such as −⅔ on each oxygen in the carbonate ion.