Helium-3 (3He) is a light, stable isotope of helium with two protons and one neutron in a nucleus called helion. It is in contrast to the more common isotope, helium-4, which has two protons and two neutrons. Helium-3 and hydrogen-1 are the only stable nuclides with more protons than neutrons. It was discovered in 1939. Helium-3 atoms are fermionic and become a superfluid at the temperature of 2.491 mK.