Polyatomic ions contain more than one atom covalently bonded to one another. The structure of four polyatomic anions and a polyatomic cation are shown below.
Nitrate, sulfate and carbonate are examples of oxoanions. Oxoanions have a central atom surrounded by either three or four oxygen atoms. The oxygen of the structural element =O is referred to as an oxo group.
Chemists represent oxoanions either by their formulae or their name. Note that names of oxoanions
Other polyatomic ions result from combination of H+ with molecular substances (NH3 or H2O) or an anion (O2– or CO3 2–). The structure of the ammonium and hydrogen carbonate ion is shown.
Nitrate, sulfate and carbonate are examples of oxoanions. Oxoanions have a central atom surrounded by either three or four oxygen atoms. The oxygen of the structural element =O is referred to as an oxo group.
 SO42– sulfate |  CO32– carbonate |  NO3– nitrate |
Chemists represent oxoanions either by their formulae or their name. Note that names of oxoanions
- contain all (such as carbon) or part (such as sulf) of the central element name
- end in -ate.
Compare with -ide ending on monatomic anion names (Cl– is chloride, for example).
Other polyatomic ions result from combination of H+ with molecular substances (NH3 or H2O) or an anion (O2– or CO3 2–). The structure of the ammonium and hydrogen carbonate ion is shown.
 NH4+ ammonium |  HCO3– hydrogen carbonate |  OH– hydroxide |
NH4+ is similar to H3O+ that is formed from water by reaction with H+.
Note that the names of both of these polyatomic cations end in -ium.
Hydroxide (usually written OH–) results from H+ combining with O2–.
It may be more appropriate to write HO–.
In hydrogencarbonate (HCO3–) the H+ is bonded to one of the oxygens that was negative in CO32– .
Note that the names of both of these polyatomic cations end in -ium.
Hydroxide (usually written OH–) results from H+ combining with O2–.
It may be more appropriate to write HO–.
In hydrogencarbonate (HCO3–) the H+ is bonded to one of the oxygens that was negative in CO32– .