A single atom is the smallest unit of an element. So, the answer is an atom of an element.

Here are some examples:

* Helium (He): A single helium atom is a noble gas.

* Hydrogen (H): A single hydrogen atom is the most abundant element in the universe.

* Carbon (C): A single carbon atom is the basis of all organic life.

It's important to note that most elements don't exist naturally in their atomic form. They tend to bond with other atoms to form molecules or compounds.