Worked example 1
Calculate the formal charge on nitrogen in NH₄⁺.
Try it first: Identify V, N and B for nitrogen before substituting.
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Molecular StructureFormal ChargeContent level: Core 20 min
Formal Charge: Definition and Calculation
What you'll be able to do: Calculate the formal charge on any atom in a Lewis structure and check that the values sum correctly.
Best after: Drawing Lewis Structures Step by Step
Introduction
Formal charge is a bookkeeping tool that tells you whether an atom is carrying its fair share of electrons.
These are recommended, not required. You can start this lesson at any time.
Learning objectives
- State and apply the formal charge formula
- Compute formal charges for every atom in a structure
- Verify that formal charges sum to the overall charge
- Distinguish formal charge from oxidation number and from real partial charge
Lesson
The formula
Formal charge = valence electrons - non-bonding electrons - half the bonding electrons. An equivalent phrasing is valence minus lone pair electrons minus the number of bonds, because each bond contributes one electron to the atom under this scheme.
Use the group valence count of the free atom, not the electron count you drew around it.
Working through an atom
For the oxygen in water: valence 6, non-bonding 4, bonding 4, so 6 - 4 - 2 = 0. For each hydrogen: 1 - 0 - 1 = 0. Neutral molecules drawn well usually give zero everywhere, which is a fast sanity check.
The sum rule
The formal charges of all atoms must add to the total charge on the species: zero for a neutral molecule, -1 for a 1- ion, +1 for a 1+ ion. If your values do not add up, either the arithmetic or the structure is wrong.
What formal charge is not
Formal charge splits every bond exactly in half and ignores electronegativity. Oxidation number does the opposite, awarding both electrons to the more electronegative atom. Real partial charges lie somewhere between the two. In water, oxygen has formal charge 0, oxidation number -2, and a genuine partial negative charge.
Both are bookkeeping schemes. Neither is a measured quantity.
Key ideas
Definition
Formal charge
Valence electrons minus non-bonding electrons minus half of the bonding electrons.
Rule
Shortcut
Formal charge = valence - lone pair electrons - number of bonds.
Rule
Sum rule
Formal charges must add up to the overall charge on the molecule or ion.
Key concept
Ammonium nitrogen
5 - 0 - 4 = +1, which matches the 1+ charge on NH₄⁺.
Equation
Formal charge
FC = V - N - B/2
Worked examples
Worked example 2
Find the formal charges in the standard triple bonded structure of carbon monoxide, C triple bond O, with one lone pair on each atom.
Try it first: Treat each atom separately with the same formula.
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Common mistakes
Using the electron count drawn around the atom as the valence number V.
Why it's wrong: V is a property of the free atom taken from the periodic table.
Check instead: Read V off the group before substituting.
Counting all bonding electrons instead of half.
Why it's wrong: Formal charge splits each bonding pair evenly between the two atoms.
Check instead: Use the shortcut: subtract the number of bonds, not the number of bonding electrons.
Confusing formal charge with oxidation number.
Why it's wrong: Oxidation number assigns both bonding electrons to the more electronegative atom.
Check instead: Ask which convention the question wants before calculating.
Practice this skill
No practice questions are available for this topic yet. You can still practice the whole unit.
What you should now know
Formal charge equals the valence electrons of the free atom minus the non-bonding electrons minus half the bonding electrons. It assumes every bond is shared perfectly evenly, so it is bookkeeping rather than real charge, and the values must sum to the overall charge on the species.
- Formal charge equals V minus N minus half of B
- The shortcut subtracts lone pair electrons and the number of bonds
- Formal charges must sum to the overall charge
- Formal charge assumes perfectly even sharing
- It is different from oxidation number and from real partial charge
Sources and further reading
- Chemistry 2e, Section 7.4: Formal Charges and ResonanceOpenStax · Rice University · 7.4View source
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