What is the formal charge?
Pick the atom, then count its lone-pair electrons and its bonds in your Lewis structure. The calculator gives the atom’s formal charge, the working, and how many electrons the atom is assigned.
- Formal charge
- −1
The formal charge on iodine (I) is −1.
- Formal charge (number)
- −1
- Working
- FC = 7 − 4 − ½ × 8 = −1
- Valence electrons (free atom)
- 7
- Bonding electrons
- 8
- Electrons assigned to the atom
- 8
- Atom
- iodine (I)
Formal charge: −1. The formal charge on iodine (I) is −1.
How to calculate
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.
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.
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).
- 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
Each example is checked against the calculator on every build.
- Atom Iodine (I), 7 valence electrons, Lone-pair electrons 4, Bonds to the atom 4 gives Formal charge (number) -1, Formal charge −1, Electrons assigned to the atom 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
- Atom Chlorine (Cl), 7 valence electrons, Lone-pair electrons 6, Bonds to the atom 1 gives Formal charge (number) 0, Electrons assigned to the atom 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
- Atom Carbon (C), 4 valence electrons, Lone-pair electrons 2, Bonds to the atom 3 gives Formal charge (number) -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
- Atom Oxygen (O), 6 valence electrons, Lone-pair electrons 2, Bonds to the atom 3 gives Formal charge (number) 1, Formal 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
- Atom Bromine (Br), 7 valence electrons, Lone-pair electrons 4, Bonds to the atom 3 gives Formal charge (number) 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
- Atom Nitrogen (N), 5 valence electrons, Lone-pair electrons 2, Bonds to the atom 3 gives Formal charge (number) 0, Valence electrons (free atom) 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
How it works
Formula (OpenStax §7.4).
Formal charge = V − L − ½ × B
- V is the number of valence electrons of the free (neutral) atom.
- L is the number of lone-pair (nonbonding) electrons on the atom in the Lewis structure.
- B is the number of bonding electrons around the atom. Each bond is one shared pair, so ½ × B is the number of bonds: a single bond counts 1, a double bond 2, a triple bond 3.
The page asks for L and the number of bonds, and writes the working as FC = V − L − ½ × B.
Valence electrons of the free atom. Main-group atoms of periods 1 to 6 only (groups 1, 2 and 13 to 18). V is the number of electrons in the atom’s outer shell, which is the same for a whole group: 1 for group 1, 2 for group 2, 3 to 8 for groups 13 to 18, and 2 for helium. For example H 1, B 3, C 4, N 5, O 6, F, Cl, Br and I 7, Xe 8.
Other outputs.
- Bonding electrons: 2 × bonds.
- Electrons assigned to the atom: L + bonds (all of its lone-pair electrons and one electron from each bond). The formal charge is V minus this number.
- The charge is written with its sign: +1, −1, 0.
Inputs. L is a whole number from 0 to 8, and bonds a whole number from 0 to 8.
Worked examples by hand
Iodine in ICl₄⁻ (OpenStax Example 7.6). V = 7, L = 4 (two lone pairs), 4 bonds (B = 8). FC = 7 − 4 − ½ × 8 = −1. Electrons assigned: 4 + 4 = 8.
Chlorine in ICl₄⁻. V = 7, L = 6, 1 bond. FC = 7 − 6 − 1 = 0. The ion’s total is −1 + 4 × 0 = −1, its charge.
Carbon monoxide (Check Your Learning). C: 4 − 2 − 3 = −1. O: 6 − 2 − 3 = +1.
Bromine in BrCl₃ (Example 7.7). V = 7, L = 4, 3 bonds: 7 − 4 − 3 = 0.
Nitrogen in NCl₃ (Check Your Learning). V = 5, L = 2, 3 bonds: 5 − 2 − 3 = 0.
Other questions people ask
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.