In
chemistry, an
onium compound, or
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, , the protonated derivative of
ammonia, .