A pure substance that is composed only of identical atoms is classified as an element.

Here's why:

* Element: A fundamental building block of matter that cannot be broken down into simpler substances by chemical means. Each element is made up of only one type of atom.

* Compound: A substance formed when two or more different elements chemically combine in a fixed ratio.

Examples of elements include:

* Gold (Au)

* Oxygen (O)

* Carbon (C)

* Helium (He)

Let me know if you have any other questions!