Here's why:

* Elements are pure substances made up of only one type of atom (like copper, Cu, or oxygen, O).

* Compounds are formed when two or more different elements chemically combine in a fixed ratio.

Copper oxide is formed by the chemical combination of copper (Cu) and oxygen (O). There are actually different forms of copper oxide, like copper(I) oxide (Cu₂O) and copper(II) oxide (CuO), each with a different ratio of copper and oxygen atoms.