Flexi Says: The charge is balanced by the presence of two nitrate ions and one calcium ion. Parentheses are used around the nitrate ion because more than one of the polyatomic ion is needed. If only one polyatomic ion is in a formula, parentheses are not used. As an example, the formula for calcium carbonate is CaCO3. The carbonate ion carries a 2− charge and so exactly balances the 2+ charge of the calcium ion.