calcium ion vs Ca

calcium ion and Ca both are nouns.

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

As nouns, Ca is a hypernym of calcium ion; that is, Ca is a word with a broader meaning than calcium ion:

calcium ion: ion of calcium; a factor in the clotting of blood
Ca: 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

Other hypernyms of calcium ion include atomic number 20, calcium, clotting factor, coagulation factor.

calcium ion (noun)	Ca (noun)
ion of calcium; a factor in the clotting of blood	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