Here's why:

* Elements are the fundamental building blocks of matter. They are pure substances that cannot be broken down into simpler substances by chemical means.

* Atoms are the smallest unit of an element that retains the chemical properties of that element.

Therefore, each element is made up of only one type of atom. For example, gold (Au) is made up of only gold atoms, oxygen (O) is made up of only oxygen atoms, and so on.