Question

Carbon is called element while carbondioxide is a compound. give reason

Answer 1

Carbon is single element. But carbon dioxide is compound formed by carbon and oxygen

Answer 2

carbon is an element as it stands alone in it's standardised from. whereas carbon-dioxide doesn't , it stands with the help of carbon and oxygen.
in the case of carbon , it's made up of two(or more)same molecules ,since the molecules here are same , the element's properties do not vary. but in the case of carbon-dioxide, since its made up of two different elements with two different properties ,the product to form is a compound with it's own set of properties that is quite different from those of parent reactants.

{element - formed from two or more same molecules
compound - formed form two or more diversified elements}