A pure substance that cannot be broken down into other substances by chemical means is called an element.

Here's why:

* Elements are the simplest forms of matter. They are made up of only one type of atom.

* Compounds are formed when two or more elements chemically combine in a fixed ratio. These can be broken down into their constituent elements through chemical reactions.

Examples of elements:

* Oxygen (O)

* Carbon (C)

* Gold (Au)

* Hydrogen (H)

* Iron (Fe)

Let me know if you'd like more information about elements or compounds!