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Formal Charge and Hybridization Calculator
In short: Enter a molecule or ion and get the formal charge of every atom, worked out as FC = V − N − B/2, plus its lone pairs, steric number, hybridization (sp, sp², sp³, sp³d, sp³d²), electron geometry, shape and approximate bond angle. It is free, works in your web browser and needs no download.
Paste a SMILES string, type a formula such as NO3- or SO4^2-, or pick an example. For every atom the table shows the valence electrons, lone-pair and bonding electrons, the formal charge with the arithmetic, the steric number, the hybridization and the shape around it.
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Manual formal charge check
For one atom in a Lewis structure you drew yourself: pick the element, count the atoms attached to it, the bonds (a double bond counts as 2) and the lone pairs.
Formal charge and hybridization formulas
Formal charge FC = V − N − B/2 V = valence electrons, N = nonbonding electrons, B = bonding electrons
Steric number SN = (atoms bonded) + (lone pairs) 2 → sp, 3 → sp², 4 → sp³, 5 → sp³d, 6 → sp³d²
V is the group number for main-group elements (C = 4, N = 5, O = 6, halogens = 7). Each lone pair is 2 nonbonding electrons. Each single bond adds 2 bonding electrons, a double bond 4 and a triple bond 6. The formal charges of all atoms add up to the overall charge.
Steric number, hybridization and shape
| Steric number | Hybridization | Electron geometry | Shapes (lone pairs 0, 1, 2…) | Example |
|---|---|---|---|---|
| 2 | sp | linear | linear (180°) | C in CO₂, HC≡CH |
| 3 | sp² | trigonal planar | trigonal planar (120°), bent | C in C=O, N in NO₃⁻, central O in O₃ |
| 4 | sp³ | tetrahedral | tetrahedral (109.5°), trigonal pyramidal (≈107°), bent (≈104.5°) | CH₄, NH₃, H₂O |
| 5 | sp³d | trigonal bipyramidal | trigonal bipyramidal, seesaw, T-shaped, linear | PCl₅, SF₄, ClF₃, I₃⁻ |
| 6 | sp³d² | octahedral | octahedral, square pyramidal, square planar | SF₆, BrF₅, XeF₄ |
Lone pairs next to π bonds
A nitrogen or oxygen with a lone pair next to a double bond (or next to an empty p orbital) usually puts that lone pair in a p orbital so it can delocalise. The atom is then sp² and flat even though the simple count gives 4. Examples: the N of amides, aniline and pyrrole, the O of esters, furan and phenol, and the O⁻ of carboxylates. The tool marks these atoms with an asterisk.
Worked examples
Ammonium, NH₄⁺
N: V = 5, no lone pairs, four N–H bonds (B = 8).
FC = 5 − 0 − 8/2 = +1. Four σ bonds, steric number 4.
N is +1, sp³, tetrahedral (109.5°)
Ozone, O₃
Central O: 1 lone pair, one double and one single bond (B = 6): 6 − 2 − 3 = +1. Single-bonded end O: 6 − 6 − 1 = −1.
Central O +1, sp², bent (≈117°)
Cyanide, CN⁻
C: 1 lone pair, triple bond (B = 6): 4 − 2 − 3 = −1. N: 1 lone pair: 5 − 2 − 3 = 0.
Charge sits on C; both atoms sp
Nitromethane, CH₃NO₂
N: no lone pairs, B = 8: 5 − 0 − 4 = +1. O⁻: 6 − 6 − 1 = −1. Writing N with two N=O bonds would give N 10 electrons, which is not allowed.
N +1 (sp²), one O −1, CH₃ carbon sp³
Sulfate, SO₄²⁻ (JEE / NEET style)
With two S=O and two S–O⁻: S = 6 − 0 − 12/2 = 0, each O⁻ = −1. In the all-single-bond octet form S = +2.
S is sp³, tetrahedral; total −2
Acetamide, CH₃CONH₂
N: 1 lone pair and three σ bonds, so the simple count is 4 (sp³). The lone pair is next to the C=O π bond and delocalises.
Amide N is sp², planar
Common mistakes
- Counting bonds instead of bonding electrons. Use B/2 (the number of bonds), not B. A double bond adds 2 to B/2.
- Forgetting lone pairs. N is the number of nonbonding electrons, so each lone pair counts as 2.
- Counting a double bond as two domains. For the steric number, a double or triple bond is one domain.
- Giving N or O more than 8 electrons. Second-period atoms cannot expand their octet. Write nitro groups as N⁺–O⁻, not N(=O)=O.
- Calling amide or aniline N sp³. Lone pairs in conjugation sit in p orbitals, so these atoms are sp².
- Mixing up formal charge and oxidation number. They use different electron-counting rules and are usually different numbers.
Draw it properly. Open the structure in the free MolDraw editor to add lone pairs, formal charges and arrows, then export a PNG or SVG for notes and slides.
Open MolDraw editorFrequently asked questions
How do you calculate formal charge?
Use FC = V − N − B/2, where V is the number of valence electrons of the free atom, N is the number of nonbonding (lone-pair) electrons on the atom and B is the number of bonding electrons around it. For the N in nitrate: FC = 5 − 0 − 8/2 = +1.
What is the formal charge of each atom in CO2?
All three atoms have a formal charge of 0 in O=C=O. Carbon: 4 − 0 − 8/2 = 0. Each oxygen: 6 − 4 − 4/2 = 0. Carbon is sp hybridized and the molecule is linear.
How do I find the hybridization of an atom?
Count the steric number: the atoms bonded to it plus its lone pairs. 2 = sp, 3 = sp², 4 = sp³, 5 = sp³d, 6 = sp³d². Double and triple bonds count once. A lone pair next to a π bond is usually in a p orbital, which makes the atom sp².
Why is the nitrogen in an amide sp2 and not sp3?
The nitrogen lone pair is next to the C=O π bond, so it delocalises into it (resonance). To overlap, the lone pair must sit in a p orbital, so the nitrogen is sp² and planar. The same is true for aniline, pyrrole and enamine nitrogens.
Is formal charge the same as oxidation number?
No. Formal charge assumes the bonding electrons are shared equally. Oxidation number assumes they all go to the more electronegative atom. In CO2 the carbon has a formal charge of 0 but an oxidation number of +4.
Which Lewis structure is best when there are several?
Prefer the structure with formal charges closest to zero, with any negative formal charge on the more electronegative atom, and never put more than 8 electrons on a second-period atom such as C, N or O.
Does the sum of formal charges equal the charge of the ion?
Yes. The formal charges of all atoms add up to the overall charge. In sulfate, SO4 2−, they add up to −2; in ammonium, NH4+, to +1.
Related chemistry tools
Read the full guide: How to calculate formal charge and hybridization.
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