Ca vs calcium ion

Ca and calcium ion both are nouns.

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

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

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

Other hyponyms of Ca include factor IV.

Ca (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