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Lewis-structure electron bookkeeping. Formal charge is not the actual partial charge on an atom.

Use electrons for an unpaired electron.

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Formal Charge Formula

The following equation is used to calculate the formal charge of an atom.

FC = V - N - (B / 2)
  • Where FC is the formal charge
  • V is the number of valence electrons of the atom (as a neutral free atom)
  • N is the number of nonbonding (lone‑pair) electrons on the atom
  • B is the number of bonding electrons around the atom (2 per single bond, 4 per double bond, 6 per triple bond)

To calculate the formal charge, divide the number of bonding electrons by 2, add the number of nonbonding (lone‑pair) electrons, then subtract this result from the number of valence electrons.

Formal Charge Definition

A formal charge is the hypothetical charge assigned to an atom in a Lewis structure, assuming bonding electrons are shared equally between bonded atoms.

Formal Charge Example

How to calculate a formal charge?

First, determine the number of valence electrons.

In this example (oxygen), the number of valence electrons is 6.

Next, determine the number of lone pair (nonbonding) electrons.

In this case, the number of lone‑pair (nonbonding) electrons is 6 (three lone pairs).

Next, determine the number of bonding electrons.

For this problem, the number of bonding electrons is 2 (one single bond).

Finally, calculate the formal charge using the formula above:

FC = V – N – (B / 2)

FC = 6 – 6 – (2 / 2)

FC = -1

FAQ

Why is calculating the formal charge of an atom important?

Calculating the formal charge of an atom is crucial for understanding the distribution of electrons in a molecule, which helps in predicting the stability, reactivity, and the structure of the molecule.

Can the formal charge of an atom be negative?

Yes, the formal charge of an atom can be negative, positive, or zero, depending on the difference between the number of valence electrons and the electrons assigned to the atom in the molecule.

How does the concept of formal charge relate to Lewis structures?

The concept of formal charge is used in drawing Lewis structures, as it helps in determining the most stable structure of a molecule by minimizing the formal charges on atoms, ideally making them as close to zero as possible.