ammonia vs ammonium ion

ammonia and ammonium ion both are nouns.

Word	Noun	Adjective	Verb	Adverb
ammonia	Yes	No	No	No
ammonium ion	Yes	No	No	No

As nouns, ammonium ion is a hyponym of ammonia; that is, ammonium ion is a word with a more specific, narrower meaning than ammonia:

ammonia: a pungent gas compounded of nitrogen and hydrogen (NH3)
ammonium ion: the ion NH4 derived from ammonia; behaves in many respects like an alkali metal ion

Other hyponyms of ammonia include ammonium.

ammonia (noun)	ammonium ion (noun)
a pungent gas compounded of nitrogen and hydrogen (NH3)	the ion NH4 derived from ammonia; behaves in many respects like an alkali metal ion
a water solution of ammonia