Elements are pure substances made up of only one kind of what?
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
#(Au)#, I would have #6.022*10^23#gold atoms.
If I had 1 mole of silver
#(Ag)#, I would have #6.022*10^23#silver
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.