Answer: Element
Explanation:
Element is a pure substance which is composed of identical atoms. It can not be decomposed into simpler constituents using chemical reactions.Example: Silver \((Ag)\)
Compound is a pure substance which is made from atoms of different elements combined together in a fixed ratio by mass.It can be decomposed into simpler constituents using chemical reactions. Example: carbon dioxide \((CO_2)\)
Thus an unknown substance is made only of identical atoms that cannot be broken down is an element.
