A polyatomic ion is a group of atoms with an associated charge. Occasionally, more than one Lewis structure may exist for a given polyatomic ion or molecule. To determine which one actually occurs, the concept of formal charge is used. The formal charge is the number of valence electrons of the atom minus the number of unbonded electrons minus half the total number of electrons participating in bonds. When determining the correct Lewis structure, the structure is chosen such that the formal charge on each of the atoms is minimized.
Example:
Determine the Lewis structure of .
Since the polyatomic ion has a charge of -2, the ion must contain two extra electrons. It is reasonable to assume that the sulfur atom is at the center of the ion. If the sulfur has a single bond with each of the oxygen atoms, the following structure results.
