* Element: An element is a pure substance made up of only one type of atom. It cannot be broken down into simpler substances by chemical means. Examples: gold (Au), oxygen (O), carbon (C).

* Compound: A compound is formed when two or more different elements chemically combine in a fixed ratio. It can be broken down into its constituent elements through chemical reactions. Example: water (H₂O) is a compound made of hydrogen and oxygen.

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

* Neutron: A neutron is a subatomic particle found in the nucleus of an atom. It's not a substance itself.

Therefore, the only pure substance that cannot be broken down by chemical reactions is an element.