Here is a sample calculation using the formal charge formula of C in CO2:

The Formal Charge Formula:

You can then perform the same calculation on the oxygen atoms and sum all the formal charges if you are asked for the total charge of a molecule. Remember that each bond counts as two shared electrons on an atom. The result of the formal charge calculation for CO2 is 0 for each atom, summing them gives a total molecular charge of 0.

F = Z - (S/2) - U


F: Formal Charge


Z: Group number of the atom being calculated


S: Number of electrons being shared with other atoms


U: Number of unshared electrons on the atom being calculated


YouTube Video: How to calculate formal charge

How to Calculate Formal Charge


Definition of formal charge:Formal charge is a hypothetical charge on an individual atom in a molecule described by the formal charge formula and is easily calculated by looking at the Lewis Structure. These contribute to the overall charge of a molecule: just sum the formal charges on each of the atoms to determine the overall charge of a molecule.

Knowing how to calculate formal charge is important to determine a resonance structure for a molecule that contributes most to actual hybrid resonance.