Hey there!
An element is a substance made up only and entirely of one particular atom.
In fact- that's what makes an element an element. It's only made of one type of atom, and that's what makes it a singular substance that's used in the production of other substances.
If it had different types of atoms, it wouldn't be that singular element. The elements we have on the periodic table are the types of atoms and elements we know in order of atomic number. We know the most prominent, least prominent, and so on- but all of these elements are scattered throughout the world.
For example, steel is not an element- or a compound. It's an alloy, it's made of several different types of metals, each can be made of other types of metals, and when you go down to what those metals are made of- you get that one substance that's made up of one singular atom- an element.
Hope this helps!