There are 145 stable even–even nuclides, forming ~58% of the 251 stable nuclides. There are also 22 primordial long-lived even–even nuclides. As a result, many of the 41 even-numbered elements from 2 to 82 have many primordial isotopes. Half of these even-numbered elements have six or more stable isotopes. The lightest stable even-even isotope is 2 He and the heaviest is 82 Pb. These are also the lightest and heaviest known doubly magic nuclides. 82 Pb is the final decay product of 90 Th, a primordial radionuclide with an even proton and neutron number. 92 U is another notable primordial radionuclide with a half life of 4.468 billion years, and produces almost half of all radioactive heat within the Earth.