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).

What defines a natural isotope

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).

Naturally occurring radioisotopes

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.

See also

• Stable isotope
• Environmental isotopes

Bibliography

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