Formal Charge Formula
Determine charge distribution in molecules
Formula
FC = V - N - B/2
- FC = formal charge
- V = valence electrons in free atom
- N = non-bonding electrons (lone pairs)
- B = bonding electrons (shared)
Example: CO
Lewis structure: :C≡O:
Carbon: V = 4, N = 2 (one lone pair), B = 6 (triple bond)
FC(C) = 4 - 2 - 6/2 = 4 - 2 - 3 = -1
Oxygen: V = 6, N = 2 (one lone pair), B = 6 (triple bond)
FC(O) = 6 - 2 - 6/2 = 6 - 2 - 3 = +1
Answer: FC(C) = -1, FC(O) = +1
Tips
- Sum of all formal charges = overall molecular charge.
- Best structures minimize formal charges and place negative FC on more electronegative atoms.
- Count each bond electron once when determining B (e.g., double bond = 4 bonding electrons).