title: Natural isotopes
text: # Natural isotopes
  
  Natural isotopes are either stable isotopes or radioactive isotopes that have a sufficiently long half-life to allow them to exist in substantial concentrations in the Earth (such as bismuth-209, with a half-life of $1.9×10^{19}$ years, potassium-40 with a half-life of $1.251(3)×10^9$ years), daughter products of those isotopes (such as $^{234}Th,$ with a half-life of 24 days) or cosmogenic elements. The heaviest stable isotope is lead-208, but the heaviest 'natural' isotope is U-238.
  
  Many elements have both natural and artificial isotopes. For example, hydrogen has three natural isotopes and another four known artificial isotopes. A further distinction among stable natural isotopes is division into primordial (existed when the Solar System formed) and cosmogenic (created by cosmic ray bombardment or other similar processes).
  
  ## What defines a natural isotope
  
  Natural isotopes must be either stable, have a half-life exceeding about $7×10^7$ years (there are 35 isotopes in this category, see stable isotope for more details) or are generated in large amounts cosmogenically (such as $^{14}C,$ which has a half-life of only 6000 years but is made by cosmic rays colliding with $^{14}N).$
  
  ## Naturally occurring radioisotopes
  
  Some radioisotopes occur in nature with a half-life of less than $7×10^7$ years (carbon-14: 5,730 ± 40 years, tritium: 12.32 years etc.). They are synthesised all the time by cosmic radiation. A practical use is radiocarbon dating with carbon-14.
  
  