Which option expresses the formal charge formula?

Which option expresses the formal charge formula?

• A. valence electrons - nonbonding electrons + bonds
• B. valence electrons - bonding electrons
• C. valence electrons - nonbonding electrons - bonds
• D. valence electrons + nonbonding electrons - bonds

Answer: (C)

Formal charge is found by counting the electrons that belong to the atom as if all bonds were shared equally, then subtracting those from the atom’s valence electrons. You start with the valence electrons (V), subtract the nonbonding (lone pair) electrons on the atom (N), and subtract the electrons that the atom shares in bonds. In formula form this is FC = V − N − (bonding electrons)/2. In many writeups, the number of bonds around the atom is used for the last term, since each bond carries two electrons and half of those belong to the atom. So subtracting the number of bonds from V − N gives the same result as subtracting half of the bonding electrons. For example, look at oxygen in a water molecule. Oxygen has six valence electrons, it has four nonbonding electrons (two lone pairs), and it forms two bonds (two O–H bonds). The formal charge is 6 − 4 − 2 = 0. This shows how the expression works: you remove the electrons that stay with the atom (nonbonding) and the electrons it shares in bonds (counted as bonds here), from the valence count. That’s why the option that uses valence electrons minus nonbonding electrons minus the bonding contribution matches the formal charge calculation.

Formal charge is found by counting the electrons that belong to the atom as if all bonds were shared equally, then subtracting those from the atom’s valence electrons. You start with the valence electrons (V), subtract the nonbonding (lone pair) electrons on the atom (N), and subtract the electrons that the atom shares in bonds. In formula form this is FC = V − N − (bonding electrons)/2. In many writeups, the number of bonds around the atom is used for the last term, since each bond carries two electrons and half of those belong to the atom. So subtracting the number of bonds from V − N gives the same result as subtracting half of the bonding electrons.

For example, look at oxygen in a water molecule. Oxygen has six valence electrons, it has four nonbonding electrons (two lone pairs), and it forms two bonds (two O–H bonds). The formal charge is 6 − 4 − 2 = 0. This shows how the expression works: you remove the electrons that stay with the atom (nonbonding) and the electrons it shares in bonds (counted as bonds here), from the valence count.

That’s why the option that uses valence electrons minus nonbonding electrons minus the bonding contribution matches the formal charge calculation.