Here's why:

* Element: A pure substance that cannot be broken down into simpler substances by chemical means (e.g., gold, oxygen).

* Compound: A substance formed when two or more elements are chemically combined in a fixed ratio (e.g., water, salt).

* Mixture: A combination of two or more substances that are not chemically bonded. The substances in a mixture retain their individual properties (e.g., sand and water).

Polystyrene is made up of the elements carbon (C), hydrogen (H), and sometimes oxygen (O). These elements are chemically bonded together in a specific repeating pattern to form long chains, making it a compound.