ammonium polyatomic cation ammonium ion derivative A derivative of ammonium, NH4(+), in which one (or more) of the hydrogens bonded to the nitrogen have been replaced with univalent organyl groups. The substituting carbon of the organyl group must not itself be directly attached to a heteroatom (thereby excluding protonated amides, hemiaminals, etc). ammonium ion derivatives azanium ion derivative azanium ion derivatives nitrogen molecular entity