Stable isotope

Stable isotopes are chemical isotopes that are not radioactive (to current knowledge). Stable isotopes of the same element have the same chemical characteristics and therefore behave almost identically. The mass differences, due to a difference in the number of neutrons, result in partial separation of the light from heavy isotopes during chemical reactions (isotope fractionation). For example, the difference in mass between the two stable isotopes of hydrogen, ¹H (1 proton, no neutron, also known as protium) and ²H (1 proton, 1 neutron, also known as deuterium) is almost 100%. Therefore, a significant fractionation will occur.

Commonly analysed stable isotopes include oxygen, carbon, nitrogen, hydrogen and sulfur. These isotope systems have been under investigation for many years as they are relatively simple to measure. Recent advances in mass spectrometry (ie. multiple-collector inductively coupled plasma mass spectrometry) now enable the measurement of heavier stable isotopes, such as iron, copper, zinc, molybdenum, etc.

Stable isotopes have been used in botanical and plant biological investigations for many years, and more and more ecological and biological studies are finding stable isotopes (mostly carbon, nitrogen and oxygen) to be extremely useful. Other workers have used oxygen isotopes to reconstruct historical atmospheric temperatures, making them important tools for climate research.

Most of naturally occurring isotopes are stable; however, few tens of them are radioactive with very long half-lives. The half life of a nuclide should be comparable with (or more than) the Earth's age (4.5 billions years) to be present in the natural isotopic composition of a chemical element. The lower half life times of such the isotopes are more than 700 millions years (²³⁵U). Many isotopes which are known to be stable (i.e. no radioactivity were observed for them) are predicted to be radioactive with extremely long half-lives (as high as 10¹⁸ years or more). If the predicted half life is in the range of experimental availability, such the isotope has a chance to move from the list of stable nuclides to radioactive, if its activity would be once observed. The good examples are bismuth-209 and tungsten-180 which were recently (2003) found to be alpha active.

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Stable isotope fractionation

There are three types of isotope fractionation:

• equilibrium fractionation
• kinetic fractionation
• mass-independent fractionation

List of stable isotopes

There are exactly 80 elements which have at least 1 stable isotope. As of September 2007, there were 244 known stable isotopes. Xenon is the only element which has 9 stable isotopes. Tin has 10 stable isotopes, more than any other element. There is no element with exactly 8 stable isotopes. Every element from hydrogen to lead has at least one stable isotope with the exceptions of technetium and promethium; elements with more than 82 protons only have radioactive isotopes, although they can still occur naturally because their half-lives are of an order of magnitude not much less than that of the time since the death of a nearby star, or because they occur in a decay chain of another radioactive isotope with such a half-life. It wasn't until 2003 that bismuth-209 was shown to be radioactive.^[1] All stable isotopes are the ground states of nuclei, excluding tantalum-180m, which is the excited level (the ground state of this nucleus is radioactive), but its decay is extremely strongly forbidden by spin-parity selection rules.

See also

• isotope table (complete)
• isotope table (divided)
• isotope geochemistry
• radionuclide

References

1. ^ WWW Table of Radioactive Isotopes.