Polyatomic Ions

Polyatomic ions are ions that consist of multiple atoms and both ionic and covalent bonding. These polyatomic ions will act as discrete units (a group of bonded atoms with an overall charge). For example, the common OH hydroxide ion can form ionic bonds with group 1 or 2 cations (i.e. Na+ and Ca2+) just like a halogen 1- anion of Cl or Br. Most university chemistry courses will assume you will have had exposure to the majority of the polyatomic ions in the past and will require your familiarity with some.

NameFormulaRelated AcidFormula
acetateCH3COOacetic acidCH3COOH
cyanideCNhydrocyanic acidHCN
azideN3hydrazoic acidHN3
carbonateCO32−carbonic acidH2CO3
nitrateNO3nitric acidHNO3
nitriteNO2nitrous acidHNO2
sulfateSO42−sulfuric acidH2SO4
hydrogen sulfateHSO4  
sulfiteSO32−sulfurous acidH2SO3
hydrogen sulfiteHSO3  
phosphatePO43−phosphoric acidH3PO4
hydrogen phosphateHPO42−  
dihydrogen phosphateH2PO4  
perchlorateClO4perchloric acidHClO4
chlorateClO3chloric acidHClO3
chloriteClO2chlorous acidHClO2
hypochloriteClOhypochlorous acidHClO
chromateCrO42−chromic acidH2CrO4
dichromateCr2O72−dichromic acidH2Cr2O7
permanganateMnO4permanganic acidHMnO4
Polyatomic ions and acids derived from them

Note that there is a system for naming some polyatomic ions; -ate and -ite are suffixes designating polyatomic ions containing more or fewer oxygen atoms. Per- (short for “hyper”) and hypo- (meaning “under”) are prefixes meaning more oxygen atoms than -ate and fewer oxygen atoms than -ite, respectively. For example, perchlorate is ClO4, chlorate is ClO3, chlorite is ClO2, and hypochlorite is ClO. Unfortunately, the number of oxygen atoms corresponding to a given suffix or prefix is not consistent; for example, nitrate is NO3, while sulfate is SO42−.

Naming is discussed more in the nomenclature section – for ionic compounds and acids.