Helium-4
Helium-4 is a non-radioactive and light isotope of helium. It is by far the most abundant of the two naturally-occurring isotopes of Helium, making up about 99.99986% of Hthe Helium on earth. Its nucleus is an alpha particle, having two protons and two neutrons. Since the alpha decay is a common decay mode for many radiocative isotopes, this probably explains its abundance. In fact, alpha decay of heavy elements is the source of most naturally occurring Helium-4 on earth.
When Helium-4 is cooled to below 2.17 degrees Kelvin, it becomes a superfluid, with properties that are very unlike those of an ordinary liquid. For example, if Helium-4 is kept in an open vessel, a thin film will climb up the sides of the vessel and overflow. This strange behaviour is not explained chemistry of Helium, but it is explained by quantum mechanics.
Helium-3 does not display this behaviour, except at much much lower temperatures.
