Here's why:

* Elements are the simplest pure substances that cannot be broken down further by chemical means. They are made up of only one type of atom.

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

* Mixtures are combinations of two or more substances where each substance retains its own properties. Mixtures can be homogeneous (uniform throughout) or heterogeneous (non-uniform).

Barium is found on the periodic table, making it a pure element.