Here's a breakdown:

* Elements are the simplest pure substances. They cannot be broken down further by chemical means.

* Atoms are the fundamental building blocks of elements. They are the smallest unit of an element that retains the chemical properties of that element.

* Substances are any matter with a definite composition and distinct properties.

So, if a substance is made of only one element, it's essentially a collection of the same type of atom. This means:

* The substance has a uniform chemical composition – it's made of only one type of atom.

* The substance is pure – it doesn't contain any other elements or compounds.

Here are some examples of substances made up of only one element:

* Gold (Au) is a pure element, meaning it is composed entirely of gold atoms.

* Oxygen (O2) is a diatomic element, meaning it is composed of two oxygen atoms bonded together.

* Iron (Fe) is a pure element, meaning it is composed entirely of iron atoms.

It's important to note that even though a substance is made up of only one element, it can exist in different forms:

* Allotropes are different structural forms of the same element. For example, carbon can exist as graphite, diamond, and buckminsterfullerene.

* Isotopes are atoms of the same element that have the same number of protons but different numbers of neutrons. For example, carbon-12 and carbon-14 are both isotopes of carbon.

So, while a substance made of only one element might seem simple, there's still a lot of interesting chemistry going on at the atomic level!