Polyatomic ions are a group of atoms that act as a collective unit or one ion. They are considered to be ONE unit and act as single ions. Therefore, the formula, charge, and name of each must be memorized.
There is a pattern that some follow, however, which can ease the memorization process. For example, notice that almost all of the polyatomic ions have oxygen atoms and share a common central atom such as sulfur in SO42- and SO32-. The formulas vary by the number of oxygen atoms present. The version with fewer oxygen atoms ends in “ite” and the one with more oxygen atoms end in “ate” as in sulfate (SO42-) and sulfite (SO32-).
A comprehensive list of common polyatomic ions follows:
ChemistryBytes.com 