An Etymological Dictionary of Astronomy and Astrophysics

1. A suffix occurring in names of → chemical elements, such as → barium, → sodium, → titanium, → uranium. Variant -um, as in → lanthanum.
   

2. It also occurs in names of chemical radicals (ammonium, carbonium) and ions of complex cations (imidazolium, nitrosylium, pyridinium).

See also: From L. suffix forming certain neuter nouns (odium, tedium, colloquium; equilibrium, millennium; collegium, consortium), from Gk. -ion.