Formal charge calculator
Pick the atom, then count what surrounds it in your Lewis structure: the electrons in its lone pairs and the number of bonds (a double bond counts as two). The calculator gives the formal charge, the electrons around the atom and whether that is a full octet, fewer, or an expanded octet.
Runs in your browser: nothing you enter is sent anywhere.
How to use it
- Choose the atom; its number of valence electrons is filled in. For another element, choose “Other” and enter the valence electrons (the group number for main-group elements, minus 10 for groups 13–18).
- Count the electrons in lone pairs on that atom: 2 per pair.
- Count the bonds: single 1, double 2, triple 3.
- Read the formal charge and check the electrons around the atom.
Formal charge = V − N − B ÷ 2
- V — valence electrons of the neutral atom
- N — nonbonding (lone-pair) electrons on the atom
- B — electrons in bonds to the atom (2 per bond)
The electrons around the atom are N + B; 8 is an octet, 2 a duet for hydrogen.
Worked examples
| Atom |
V |
N |
Bonds |
Formal charge |
| N in NH₄⁺ |
5 |
0 |
4 |
5 − 0 − 4 = +1 |
| O in H₂O |
6 |
4 |
2 |
6 − 4 − 2 = 0 |
| single-bonded O in O₃ |
6 |
6 |
1 |
6 − 6 − 1 = −1 |
| C in CO |
4 |
2 |
3 |
4 − 2 − 3 = −1 |
| B in BF₃ |
3 |
0 |
3 |
3 − 0 − 3 = 0, only 6 electrons |
Tips
- Check the total. Add the formal charges of every atom; the sum must equal the overall charge.
- Count electrons, not pairs. Two lone pairs are 4 nonbonding electrons.
- Common patterns. Neutral carbon has 4 bonds, nitrogen 3 bonds and one lone pair, oxygen 2 bonds and two lone pairs; one bond more or less shifts the charge by one.
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 of the bonding electrons. Since each bond holds two electrons, B ÷ 2 is simply the number of bonds, so FC = V − N − bonds.
What is the formal charge of nitrogen in NH₄⁺?
Nitrogen has 5 valence electrons, no lone pairs and four bonds: 5 − 0 − 4 = +1. That +1 is the charge of the whole ammonium ion, because all four hydrogens have a formal charge of 0.
How do formal charges help pick a Lewis structure?
The formal charges of all atoms add up to the charge of the molecule or ion. Among valid structures, prefer the one with formal charges closest to zero, and with any negative charge on the more electronegative atom.
Is formal charge the real charge on the atom?
No. It assumes bonding electrons are shared exactly equally. The real partial charges depend on electronegativity; oxidation states make the opposite assumption, giving all bonding electrons to the more electronegative atom.
Why does sulfur in SF₆ have 12 electrons?
Elements from the third period on can hold more than eight electrons in Lewis structures — an expanded octet. The calculator shows it but does not treat it as an error; for second-period atoms (C, N, O, F) more than 8 means the structure is wrong.