A pure substance that contains only one kind of particle is called an element.

Here's why:

* Element: A fundamental substance that cannot be broken down into simpler substances by chemical means. It is composed of only one type of atom. Examples include gold (Au), oxygen (O), and carbon (C).

* Compound: A substance formed when two or more different elements are chemically bonded together. For instance, water (H₂O) is a compound formed by the chemical bonding of hydrogen and oxygen atoms.

So, to summarize, if a substance has only one type of particle, it's an element.