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

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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

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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

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* Stable isotope
* Environmental isotopes

## Bibliography

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1. ^ "Natural Isotopes". www.soest.hawaii.edu. Retrieved 2015-12-13.
2. ^ "Isotopes - Chemwiki". chemwiki.ucdavis.edu. 2013-10-02. Retrieved 2015-12-14.