Final answer:
Polyatomic ions need parentheses in formulas to indicate that a group of atoms within the ion acts as a single unit. The parentheses also help differentiate between individual atoms and groups of atoms in a formula.
Step-by-step explanation:
Polyatomic ions sometimes need parentheses in a formula to indicate that the entire group of atoms within the polyatomic ion acts as a single unit. This is necessary when more than one polyatomic ion is needed to balance the charges of the compound. The numerical subscript outside the parentheses indicates how many of those ions are present. For example, in Ba(NO3)2, the parentheses indicate that the entire nitrate ion (NO3) acts as a unit and there are two of them present in the compound. The parentheses help distinguish between individual atoms and groups of atoms in a formula.