What are Helium-4 and helium-3?

Helium is a gaseous element with atomic number 2, that is it has two protons in its nucleus. It has two isotopes, helium-3 with one neutron and helium-4 with two neutrons. Helium-4 is much the most frequent one, helium-3 is only 0.000137 percent of the total. Both are stable.