Elements are pure substances made up of only one kind of what?

Atom.

Explanation:

An element is the purest form of a chemical.

That is, it is composed of nothing but atoms of that type.

• If I had 1 mole of gold $\left(A u\right)$, I would have $6.022 \cdot {10}^{23}$ gold atoms.

• If I had 1 mole of silver $\left(A g\right)$, I would have $6.022 \cdot {10}^{23}$ silver
atoms.

My point is, there is nothing but atoms of that element. There are no oxygen, hydrogen or carbon atoms; theoretically, there should be nothing but atoms of whatever element you have.