Formal Charge Calculator

Formal charge is a bookkeeping tool that helps you evaluate Lewis structures. It tells you how the electron distribution in a molecule compares to the free atom.

The Formula

Formal Charge = Valence Electrons - Lone Pair Electrons - (Bonding Electrons / 2)

Or equivalently: FC = V - L - B/2

Oxygen in Water (H2O)

• Valence electrons for O = 6
• Lone pair electrons = 4 (two lone pairs)
• Bonding electrons = 4 (two O-H bonds, each with 2 electrons)
• FC = 6 - 4 - 4/2 = 6 - 4 - 2 = 0

Oxygen in water has zero formal charge.

Nitrogen in Ammonium (NH4+)

• Valence electrons for N = 5
• Lone pair electrons = 0
• Bonding electrons = 8 (four N-H bonds)
• FC = 5 - 0 - 8/2 = 5 - 0 - 4 = +1

Rules for Best Lewis Structures

1. Minimize formal charges -- structures with zero formal charges are preferred
2. Negative formal charges should go on the more electronegative atom
3. Avoid placing formal charges of the same sign on adjacent atoms