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.
| Name | Formula | Related Acid | Formula |
|---|---|---|---|
| ammonium | NH4+ | ||
| hydronium | H3O+ | ||
| peroxide | O22− | ||
| hydroxide | OH− | ||
| acetate | CH3COO− | acetic acid | CH3COOH |
| cyanide | CN− | hydrocyanic acid | HCN |
| azide | N3− | hydrazoic acid | HN3 |
| carbonate | CO32− | carbonic acid | H2CO3 |
| bicarbonate | HCO3− | ||
| nitrate | NO3− | nitric acid | HNO3 |
| nitrite | NO2− | nitrous acid | HNO2 |
| sulfate | SO42− | sulfuric acid | H2SO4 |
| hydrogen sulfate | HSO4− | ||
| sulfite | SO32− | sulfurous acid | H2SO3 |
| hydrogen sulfite | HSO3− | ||
| phosphate | PO43− | phosphoric acid | H3PO4 |
| hydrogen phosphate | HPO42− | ||
| dihydrogen phosphate | H2PO4− | ||
| perchlorate | ClO4− | perchloric acid | HClO4 |
| chlorate | ClO3− | chloric acid | HClO3 |
| chlorite | ClO2− | chlorous acid | HClO2 |
| hypochlorite | ClO− | hypochlorous acid | HClO |
| chromate | CrO42− | chromic acid | H2CrO4 |
| dichromate | Cr2O72− | dichromic acid | H2Cr2O7 |
| permanganate | MnO4− | permanganic acid | HMnO4 |
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.