In chemistry, an onium ion is a cation formally obtained by the protonation of mononuclear parent hydride of a pnictogen (group 15 of the periodic table), chalcogen (group 16), or halogen (group 17). The oldest-known onium ion, and the namesake for the class, is ammonium, NH4+, the protonated derivative of ammonia, NH3. The name onium is also used for cations that would result from the substitution of hydrogen atoms in those ions by other groups, such as organic groups, or halogens; such as tetraphenylphosphonium, (C6H5)4P+. The substituent groups may be divalent or trivalent, yielding ions such as iminium and nitrilium. A simple onium ion has a charge of +1. A larger ion that has two onium ion subgroups is called a double onium ion, and has a charge of +2. A triple onium ion has a charge of +3, and so on. Compounds of an onium cation and some other anion are known as onium compounds or onium salts. Onium ions and onium compounds are inversely analogous to -ate ions and ate complexes:

Simple onium cations (hydrides with no substitutions)

Group 13 (boron group) onium cations

Group 14 (carbon group) onium cations

Group 15 (pnictogen) onium cations

Group 16 (chalcogen) onium cations

Hydrogen onium cation

Group 17 (halogen) onium cations, halonium ions, H2X+ (protonated hydrogen halides)

Pseudohalogen onium cations

Group 18 (noble gas) onium cations

Onium cations with monovalent substitutions

Onium cations with polyvalent substitutions

Double onium dications

Enium cations

The extra bond is added to a less-common parent hydride, a carbene analog, typically named -ene or -ylene, which is neutral with 2 fewer bonds than the more-common hydride, typically named -ane or -ine.

Substituted eniums

Ynium cations