The elements that can be broken down into simpler substances are compounds.

Here's why:

* Elements are the simplest pure substances. They are made up of only one type of atom and cannot be broken down into simpler substances by chemical means. Examples include gold (Au), oxygen (O), and carbon (C).

* Compounds are formed when two or more different elements chemically combine in a fixed ratio. They can be broken down into their constituent elements through chemical reactions. Examples include water (H₂O), salt (NaCl), and carbon dioxide (CO₂).

To summarize: Elements are the building blocks of matter, while compounds are formed from the combination of elements.