Helium-4
The most common isotope, 4He, is produced on Earth by alpha decay of heavier radioactive elements; the alpha particles that emerge are fully ionized 4He nuclei. 4He is an unusually stable nucleus because its nucleons are arranged into complete shells. It was also formed in enormous quantities during Big Bang nucleosynthesis.
Terrestrial helium consists almost exclusively (99.99986%) of this isotope. Its boiling point of 4.2 K is the lowest of any known substance. When cooled further to 2.17 K, it transforms to a unique superfluid state of zero viscosity. It solidifies only at pressures above 25 atmospheres, where its melting point is 0.95 K.
Read more about this topic: Isotopes Of Helium