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

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

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

Thus an unknown substance is made only of identical atoms that cannot be broken down is an element.

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