A substance composed only of one type of atom is called an element.

Here's why:

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

* Atoms are the smallest unit of an element that retains the chemical properties of that element.

Examples of elements include:

* Gold (Au): Composed only of gold atoms.

* Oxygen (O): Composed only of oxygen atoms.

* Iron (Fe): Composed only of iron atoms.