calcium vs calcium ion

calcium and calcium ion both are nouns.

Word	Noun	Adjective	Verb	Adverb
calcium	Yes	No	No	No
calcium ion	Yes	No	No	No

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

calcium: a white metallic element that burns with a brilliant light; the fifth most abundant element in the earth's crust; an important component of most plants and animals
calcium ion: ion of calcium; a factor in the clotting of blood

Other hyponyms of calcium include factor IV.

calcium (noun)	calcium ion (noun)
a white metallic element that burns with a brilliant light; the fifth most abundant element in the earth's crust; an important component of most plants and animals	ion of calcium; a factor in the clotting of blood