text: All of the known 251 stable nuclides, plus another 35 nuclides that have half-lives long enough to have survived from the formation of the Earth, occur as primordial nuclides. These 35 primordial radionuclides represent isotopes of 28 separate elements.
  Cadmium, tellurium, xenon, neodymium, samarium, osmium, and uranium each have two primordial radioisotopes (, ; , ; , ; , ; , ; , ; and , ).
  Because the age of the Earth is  (4.6 billion years), the half-life of the given nuclides must be greater than about  (100 million years) for practical considerations. For example, for a nuclide with half-life  (60 million years), this means 77 half-lives have elapsed, meaning that for each mole () of that nuclide being present at the formation of Earth, only 4 atoms remain today.
  The seven shortest-lived primordial nuclides (i.e., the nuclides with the shortest half-lives) to have been  experimentally verified are  (),  (),  (),  (),  (),  (), and  (). 
  These are the seven nuclides with half-lives comparable to, or somewhat less than, the estimated age of the universe. (Rb, Re, Lu, and Th have half-lives somewhat longer than the age of the universe.) For a complete list of the 35 known primordial radionuclides, including the next 28 with half-lives much longer than the age of the universe, see the complete list below. For practical purposes, nuclides with half-lives much longer than the age of the universe may be treated as if they were stable. Rb, Re, Lu, Th, and U have half-lives long enough that their decay is limited over geological time scales; K and U have shorter half-lives and are hence severely depleted, but are still long-lived enough to persist significantly in nature.
  The longest-lived isotope not proven to be primordial is , which has a half-life of , followed by  () and  (). Pu has been reported to exist in nature as a primordial nuclide, although a later study did not detect it. Taking into account that all these nuclides must exist for at least , Sm must survive 45 half-lives (and hence be reduced by 2 ≈ ), Pu must survive 57 (and be reduced by a factor of 2 ≈ ), and Nb must survive 130 (and be reduced by 2 ≈ ). Mathematically, considering the likely initial abundances of these nuclides, primordial Sm and Pu should persist somewhere within the Earth today, even if they are not identifiable in the relatively minor portion of the Earth's crust available to human assays, while Nb and all shorter-lived nuclides should not. Nuclides such as Nb that were present in the primordial solar nebula but have long since decayed away completely are termed extinct radionuclides if they have no other means of being regenerated.
  Because primordial chemical elements often consist of more than one primordial isotope, there are only 83 distinct primordial chemical elements. Of these, 80 have at least one observationally stable isotope and three additional primordial elements have only radioactive isotopes (bismuth, thorium, and uranium).