Polyatomic ions are ions that consist of more than one kind of atom. (Ions form when an atom either gains or loses valence electrons in pursuit of filling its valence shell.) The following table lists the most common polyatomic ions, grouped by ionic charge. They'll pop up frequently and annoy you until you simply buckle down and memorize them.

Common Polyatomic Ions
–1 Charge –2 Charge –3 Charge +1 Charge
Dihydrogen phosphate (H2PO4) Hydrogen phosphate (HPO42–) Phosphite (PO33–) Ammonium (NH4+)
Acetate (C2H3O2) Oxalate (C2O42–) Phosphate (PO43–)
Hydrogen sulfite (HSO3) Sulfite (SO32–)
Hydrogen sulfate (HSO4) Sulfate (SO42–)
Hydrogen carbonate (HCO3) Carbonate (CO32–)
Nitrite (NO2) Chromate (CrO42–)
Nitrate (NO3) Dichromate (Cr2O72–)
Cyanide (CN) Silicate (SiO32–)
Hydroxide (OH)
Permanganate (MnO4)
Hypochlorite (ClO)
Chlorite (ClO2)
Chlorate (ClO3)
Perchlorate (ClO4)