The simplest substances in nature composed of the same atoms are elements.

Here's why:

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

* Each element is made up of only one type of atom. For example, gold (Au) is made up of only gold atoms, and oxygen (O) is made up of only oxygen atoms.

Examples of elements:

* Metals: Gold (Au), Iron (Fe), Copper (Cu)

* Nonmetals: Oxygen (O), Nitrogen (N), Carbon (C)

* Noble Gases: Helium (He), Neon (Ne), Argon (Ar)

It's important to note that while elements are the simplest substances in nature, they can combine to form compounds, which are made up of two or more different elements.