Question

An unknown substance is made only of identical atoms that cannot be broken down. This substance is a ________.

Answer 1

The substance is an element off of the periodic table. 

Answer 2

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.