Important Polyatomic Ions These are some of the ions that you should know the names of, in no particular order: CN - cyanide OCN - cyanate SCN - thiocyanate NH 4 + ammonium NO3 - nitrate CO3² - carbonate SO3² - sulfite HSO 4 - hydrogen sulfate HS - hydrosulfide PO 4 3- phosphate H2PO 4 3- dihydrogen phosphate ClO - hypochlorite ClO3 - chlorate CH3COO - acetate Cr2O 7 2- dichromate O2² - peroxide C2O 4 2- oxalate NO2 - nitrite IO3 - iodate HCO3 - hydrogen carbonate SO 4 2- sulfate OH - hydroxide H - hydride HSO3 - hydrogen sulfite HPO 4 2- hydrogen phosphate ClO2 - chlorite ClO 4 - perchlorate CrO 4 2- chromate HC2O 4 - hydrogen oxalate