How to Represent Compounds with Polyatomic Ions

Some chemical formulas contain groups of atoms that act as a unit.
When several groups of the same kind are present, their formula is set off in parentheses with a subscript to indicate the number of that group.
Many of these groups of atoms have a charge associated with them and are called polyatomic ions.
To determine the total number of each type of atom in a compound containing a group within parentheses, multiply the subscript outside the parentheses by the subscript for each atom inside the parentheses.