The formal charge on an atom is defined as valence electrons minus the sum of which terms?

• A. Valence electrons minus lone-pair electrons
• B. Valence electrons minus oxidation state
• C. Valence electrons minus (lone-pair electrons + covalent bonds)
• D. Valence electrons minus the number of protons

Answer: (C)

Formal charge is found by comparing the valence electrons of the atom with the electrons assigned to it in the bonded structure. Electrons assigned consist of all nonbonding electrons on the atom (lone pairs) plus the electrons the atom shares in covalent bonds. In this bookkeeping, each covalent bond contributes one electron to the atom’s share, so you subtract the number of lone-pair electrons and the number of covalent bonds from the valence electrons. This is why the formal charge equals valence electrons minus (lone-pair electrons + covalent bonds). For example, carbon in methane has four valence electrons, no lone pairs, and four bonds to hydrogen, giving a formal charge of 0. This approach also yields zero for nitrogen in ammonia, since 5 − (2 + 3) = 0.