Online Encyclopedia
Polyatomic ion
A polyatomic ion or molecule ion is an ion consisting of covalently bonded atoms that can be considered as acting as a single unit in the context of acid/base chemistry or in the formation of salts. For more information on polyatomic ions containing metals, and how to name these, see metal complex. Note, a polyatomic ion is also referred to in older works as a radical. In current usage the term radical refers to free radicals which are uncharged species with an unpaired electron.
Some commonly-occurring polyatomic ions and their charges are indicated below:
- aluminate Al2O42-
- carbonate CO32-
- bicarbonate HCO3- (more correctly known as hydrogen carbonate)
- tetraborate B4O72-
- chlorate ClO3-
- chromate CrO42-
- cyanide CN-
- hydroxide OH-
- nitrate NO3-
- nitrite NO2-
- permanganate MnO4-
- peroxide O22-
- phosphate PO43-
- phosphite PO33-
- pyrophosphate P2O74-
- silicate SixOy (many varieties, often polymeric)
- sulfate SO42-
- sulfite SO32-
- triflate CF3SO3- (trifluoromethanesulfonate)
- acetate CH3CO2-
- formate HCO2-
- oxalate (CO2)22-
- ammonium NH4+
- quaternary ammonium NR4+
- hydronium H3O+
- pyridinium C5H5NH+
The atoms of a polyatomic ion are always associated with each other via covalent or coordinate-covalent bonds.
A large polyatomic ion will often be considered as the conjugate acid or conjugate base of a neutral molecule, for example the conjugate acid of adrenaline at the amine group.
Note that many of the common negatively-charged (anionic) polyatomic ions are conjugate bases of acids derived from the oxides of non-metallic elements. For example sulfate or SO42- ion is derived from H2SO4 which can be regarded as SO3 + H2O.
Bicarbonate is also called hydrogen carbonate in modern chemical nomenclature.