Here's why:

* Pure substance: A pure substance is made up of only one type of atom or molecule.

* Element: Barium (Ba) is an element, meaning it's found on the periodic table and cannot be broken down into simpler substances.

Let's contrast that with compounds and mixtures:

* Compound: A compound is formed when two or more different elements chemically combine in a fixed ratio (e.g., water, H₂O).

* Mixture: A mixture is a combination of two or more substances that are not chemically bonded and can be separated by physical means (e.g., salt and pepper).