Here's why:

* Elements are the simplest pure substances. They are made up of only one type of atom. For example, gold (Au) is an element because it's made up of only gold atoms.

* Compounds are made up of two or more different elements chemically combined. For example, water (H₂O) is a compound because it's made up of hydrogen (H) and oxygen (O) atoms.

* Mixtures are combinations of different substances that are not chemically bonded. They can be separated by physical means.

Let me know if you have any other questions!