The simplest pure substance on Earth is hydrogen, represented by the chemical symbol H.

Here's why:

* Atomic Structure: Hydrogen has the simplest atomic structure of all elements, consisting of only one proton and one electron.

* Abundance: Hydrogen is the most abundant element in the universe, and it is found in many compounds on Earth, like water (H2O).

* Diatomic Nature: Hydrogen exists as a diatomic molecule (H2), meaning two hydrogen atoms bond together to form a stable molecule.

While other elements like helium (He) also have simple atomic structures, hydrogen's abundance and the existence of its diatomic form make it the simplest pure substance on Earth.