Al vs ammonium alum

Al and ammonium alum both are nouns.

Word	Noun	Adjective	Verb	Adverb
Al	Yes	No	No	No
ammonium alum	Yes	No	No	No

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

Al: a silvery ductile metallic element found primarily in bauxite
ammonium alum: a white crystalline double sulfate of aluminum: the ammonium double sulfate of aluminum

Other hyponyms of Al include alum, potash alum, potassium alum, alum, ammonia alum.

Al (noun)	ammonium alum (noun)
a silvery ductile metallic element found primarily in bauxite	a white crystalline double sulfate of aluminum: the ammonium double sulfate of aluminum