Here's why:

* Atomic Number: Hydrogen has an atomic number of 1, meaning it only has one proton in its nucleus. Helium has an atomic number of 2, with two protons.

* Atomic Mass: Hydrogen's atomic mass is approximately 1 atomic mass unit (amu), while helium's is approximately 4 amu. This difference in mass is due to the additional protons and neutrons in the helium atom.