# What is the formal charge?

Finds the formal charge on an atom in a Lewis structure from its valence electrons, lone-pair electrons and bonds: FC = valence electrons − lone-pair electrons − ½ × bonding electrons.

- Page: https://www.acalculator.org/chemistry/formal-charge-calculator
- JSON spec: https://www.acalculator.org/chemistry/formal-charge-calculator.json
- Version: 3cfe514f4c76

## Default answer

Example with the default inputs (Atom Iodine (I), 7 valence electrons, Lone-pair electrons 4, Bonds to the atom 4): The formal charge on iodine (I) is −1.

## Inputs

| Key | Label | Description |
| --- | --- | --- |
| atom | Atom | The atom whose formal charge you want; its valence electrons follow its group. |
| lp | Lone-pair electrons | Nonbonding electrons on the atom in the Lewis structure: the dots, 2 for each lone pair. |
| b | Bonds to the atom | Shared electron pairs between this atom and its neighbours: a single bond counts 1, a double bond 2, a triple bond 3. |

## Outputs

| Key | Label | Description |
| --- | --- | --- |
| charge | Formal charge | The formal charge, with its sign. |
| fc | Formal charge (number) | The formal charge as a signed whole number. |
| working | Working | Valence electrons minus lone-pair electrons minus half the bonding electrons. |
| valence | Valence electrons (free atom) | The valence electrons of the neutral atom: its group number’s last digit (helium 2). |
| bondingElectrons | Bonding electrons | Electrons in the atom’s bonds: 2 per bond. |
| assigned | Electrons assigned to the atom | Its lone-pair electrons plus one electron from each bond. |
| name | Atom | The atom’s name and symbol. |

## Method

Formal charge = valence electrons of the free atom − lone-pair (nonbonding) electrons − ½ × bonding electrons; ½ × bonding electrons is the number of bonds (a double bond counts 2).

## Assumptions

- Main-group atoms (groups 1, 2 and 13 to 18) of periods 1 to 6; the valence electrons of the free atom follow the group: 1, 2, then 3 to 8 for groups 13 to 18, and 2 for helium.
- Each bond is one shared electron pair; the atom is assigned all of its lone-pair electrons and half of its bonding electrons.
- Formal charge is bookkeeping for comparing Lewis structures, not a measured charge.

## Worked examples

1. atom = I, lp = 4, b = 4 gives fc = -1, charge = −1, assigned = 8. Source: OpenStax, Chemistry 2e, §7.4 Formal Charges and Resonance (formal charge = valence electrons − lone-pair electrons − ½ bonding electrons), https://openstax.org/books/chemistry-2e/pages/7-4-formal-charges-and-resonance, Example 7.6 Calculating Formal Charge from Lewis Structures: I: 7 − 8 = −1.
2. atom = Cl, lp = 6, b = 1 gives fc = 0, assigned = 7. Source: OpenStax, Chemistry 2e, §7.4 Formal Charges and Resonance (formal charge = valence electrons − lone-pair electrons − ½ bonding electrons), https://openstax.org/books/chemistry-2e/pages/7-4-formal-charges-and-resonance, Example 7.6: Cl: 7 − 7 = 0.
3. atom = C, lp = 2, b = 3 gives fc = -1, working = FC = 4 − 2 − ½ × 6 = −1. Source: OpenStax, Chemistry 2e, §7.4 Formal Charges and Resonance (formal charge = valence electrons − lone-pair electrons − ½ bonding electrons), https://openstax.org/books/chemistry-2e/pages/7-4-formal-charges-and-resonance, Check Your Learning after Example 7.6 (CO): C −1.
4. atom = O, lp = 2, b = 3 gives fc = 1, charge = +1. Source: OpenStax, Chemistry 2e, §7.4 Formal Charges and Resonance (formal charge = valence electrons − lone-pair electrons − ½ bonding electrons), https://openstax.org/books/chemistry-2e/pages/7-4-formal-charges-and-resonance, Check Your Learning after Example 7.6 (CO): O +1.
5. atom = Br, lp = 4, b = 3 gives fc = 0. Source: OpenStax, Chemistry 2e, §7.4 Formal Charges and Resonance (formal charge = valence electrons − lone-pair electrons − ½ bonding electrons), https://openstax.org/books/chemistry-2e/pages/7-4-formal-charges-and-resonance, Example 7.7: Br: 7 − 7 = 0.
6. atom = N, lp = 2, b = 3 gives fc = 0, valence = 5. Source: OpenStax, Chemistry 2e, §7.4 Formal Charges and Resonance (formal charge = valence electrons − lone-pair electrons − ½ bonding electrons), https://openstax.org/books/chemistry-2e/pages/7-4-formal-charges-and-resonance, Check Your Learning after Example 7.7 (NCl₃): N 0.

## FAQ

### What is the formula for formal charge?

Formal charge = valence electrons of the free atom − lone-pair electrons − ½ × bonding electrons. Half the bonding electrons is the number of bonds to the atom, so the quick form is valence electrons − dots − lines.

### How do I count lone-pair electrons and bonds?

Lone-pair electrons are the dots on the atom: 2 for each lone pair. Bonds are the lines to the atom: a single bond counts 1, a double bond 2 and a triple bond 3. In ICl₄⁻ the iodine has 2 lone pairs (4 electrons) and 4 bonds, so its formal charge is 7 − 4 − 4 = −1.

### What should the formal charges add up to?

The sum over all the atoms equals the charge of the species: 0 for a neutral molecule, and the ion’s charge for an ion. In ICl₄⁻ the iodine is −1 and each chlorine 0, which adds up to −1.

### How do formal charges help pick the best Lewis structure?

Prefer a structure whose formal charges are all zero. Otherwise prefer the smallest nonzero formal charges, structures where neighbouring formal charges are zero or of opposite sign, and negative formal charges on the more electronegative atoms. These are OpenStax’s guidelines; for nitrous oxide they pick the arrangement with oxygen at the end (NNO) over oxygen in the middle (NON).

### Is formal charge the real charge on the atom?

No. Formal charge pretends every bond is shared equally, so it is bookkeeping for comparing Lewis structures. The real distribution of charge depends on electronegativity; oxidation numbers make the opposite assumption, giving each bond’s electrons to the more electronegative atom.

### What are the formal charges in carbon monoxide?

In C≡O each atom has one lone pair and three bonds. Carbon: 4 − 2 − 3 = −1. Oxygen: 6 − 2 − 3 = +1. They add up to 0, as they must in a neutral molecule.

## Sources

- OpenStax, Chemistry 2e, §7.4 Formal Charges and Resonance: the formal charge formula, the counting steps, the sum rule, the guidelines for the most likely structure, Example 7.6 (ICl₄⁻: I −1, Cl 0) and its Check Your Learning (CO: C −1, O +1), Example 7.7 (BrCl₃: all 0) and its Check Your Learning (NCl₃: all 0). https://openstax.org/books/chemistry-2e/pages/7-4-formal-charges-and-resonance (retrieved 2026-10-19)
- OpenStax, Chemistry 2e, §6.4 Electronic Structure of Atoms (Electron Configurations): main-group valence electrons are those in the highest shell, and elements in one group have the same number of valence electrons. https://openstax.org/books/chemistry-2e/pages/6-4-electronic-structure-of-atoms-electron-configurations (retrieved 2026-10-19)