A substance that contains only a single type of atom is

element

Explanation:

Atoms are the building blocks of all matter. When a substance contains only one type of atom, it is called an element. Each element has a special name and unique properties that are different from all the rest of the elements. All known elements are organized on a chart called the periodic table of elements.

An element

Explanation:

An element is a substance consisting the same type of atoms.

oxygen, selenium, tellurium, and polonium.

they're all from the same group as sulfur (group 16).