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 1.9×10¹⁹ years, potassium-40 with a half-life of 1.251(3)×10⁹ years), daughter products of those isotopes (such as ²³⁴Th, with a half-life of 24 days) or cosmogenic elements.^[1] 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.^[2] 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).

Natural isotopes must be either stable, have a half-life exceeding about 7×10⁷ years (there are 35 isotopes in this category, see stable isotope for more details) or are generated in large amounts cosmogenically (such as ¹⁴C, which has a half-life of only 6000 years but is made by cosmic rays colliding with ¹⁴N).

Some radioisotopes occur in nature with a half-life of less than 7×10⁷ 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.

• Stable isotope
• Environmental isotopes