What is a substance made from one type of atom

Si, H, He

Explanation:

A substance of only one type of atom would be a pure substance from any of the elements in the period table. This could be nitrogen, N2, which is easily isolated, or gold, Au, which tends not to form compounds with other elements.

On earth, we have been able to refine silicon, Si, to a highly pure state. This may be the best example of a substance of only one type of atom. There may be other better answers such as some parts of a star contain only hydrogen, H, or helium, He.