Isotopes of Helium

Helium has eight known isotopes but only two are stable: ³He and ⁴He. ³He is found in only very small quantities compared to ⁴He. It is produced in trace amounts by the beta decay of tritium. This form is found in abundance in stars, as a product of nuclear fusion. Extraplanetary materials have trace amounts of ³He from solar winds. ⁴He is produced by the alpha decay of heavier radioactive elements on Earth. It is an unusually stable isotope because its nucleons are arranged in complete shells.