* Helium-4 (⁴He) is the most common isotope, making up almost all of the helium found naturally. It has two protons and two neutrons.

* Helium-3 (³He) is much rarer, containing two protons and only one neutron.

Both isotopes of helium are stable, meaning they don't decay into other elements. However, they have slightly different properties due to the difference in their neutron count.