Here's why:

* Element: A pure substance made up of only one type of atom. Carbon (C) is the chemical symbol for the element carbon.

* Compound: A substance made up of two or more different elements chemically bonded together.

* Mixture: A combination of two or more substances that are not chemically bonded. Mixtures can be homogeneous (evenly mixed) or heterogeneous (unevenly mixed).

Diamond is made entirely of carbon atoms, making it a pure element.