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Stable isotope
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Stable isotopes are chemical isotopes that are not radioactive (they have not been observed to decay, though a few of them may be theoretically unstable with exceedingly long half lives). By this definition, there are 256 known stable isotopes of the 80 elements which have one or more stable isotopes. A list of these is given at the end of this article. About 2/3rds of elements have more than one stable isotope.
Different isotopes of the same element (whether stable or unstable) have the same chemical characteristics and therefore behave almost identically.
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Encyclopedia
Stable isotopes are chemical isotopes that are not radioactive (they have not been observed to decay, though a few of them may be theoretically unstable with exceedingly long half lives). By this definition, there are 256 known stable isotopes of the 80 elements which have one or more stable isotopes. A list of these is given at the end of this article. About 2/3rds of elements have more than one stable isotope.
Different isotopes of the same element (whether stable or unstable) 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 isotopes from the 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 in order to study processes of isotope fractionation in natural systems because 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, a few tens of them are radioactive with very long half-lives. If the half life of a nuclide is comparable to or greater than the Earth's age (4.5 billion years), a significant amount will have survived since the formation of the Solar System (it will be primordial), and will contribute in that way to the natural isotopic composition of a chemical element. The shortest half lives of easily detectable primordially-present radioisotopes are around 700 million years (e.g., 235U), with a lower present limit on detection of primordial isotopes of 80 million years (e.g., 244Pu). Many radioisotopes are known in nature with still shorter half-lives, but they are made freshly by decay processes or ongoing energetic reactions, such as those produced by present bombardment of Earth by cosmic rays.
Many isotopes that are presumed to be stable (i.e. no radioactivity has been observed for them) are predicted to be radioactive with extremely long half-lives (sometimes as high as 1018 years or more). If the predicted half life falls into an experimentally accessible range, such isotopes have a chance to move from the list of stable nuclides to the radioactive category, once their activity is observed. Good examples are bismuth-209 and tungsten-180 which were formally classed as stable, but have been recently (2003) found to be alpha-active.
Most stable isotopes in the earth are believed to have been formed in processes of nucleosynthesis, either in the 'Big Bang', or in generations of stars that preceded the formation of the solar system. However, some stable isotopes also show abundance variations in the earth as a result of decay from long-lived radioactive nuclides. These decay-products are termed radiogenic isotopes, in order to distinguish them from the much larger group of 'non-radiogenic' isotopes. They play an important role in radiometric dating and isotope geochemistry.
Research areas
The Island of Stability may reveal a number of stable atoms that are heavier (and with more protons) than lead.
Stable isotope fractionation
There are three types of isotope fractionation:
List of stable isotopes
There are 80 known elements which have at least one stable isotope. As of January 2009, there were 256 known stable isotopes (isotopes which have never been observed to decay). Tin has 10 stable isotopes, more than any other element. Xenon is the only element which has 9 stable isotopes. There is no element with exactly 8 stable isotopes. Only five elements have 7 stable isotopes. There are 26 elements which have only a single stable isotope. 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 is expected that continuous improvement of experimental sensitivity will allow to find radioactivity of some isotopes that are considered stable today. For example, it wasn't until 2003 that bismuth-209 was shown to be radioactive . Many stable (or, better to say, meta-stable) nuclides have positive energy release in different kinds of radioactive decays:
Positivity of energy release in these processes means that they are allowed kinematically and, in principle, can occur. They are still not observed due to strong but not absolute suppression by spin-parity selection rules (for beta decays and isomeric transitions) or by thickness of potential barrier (for alpha and cluster decays and spontaneous fission). In the list below, the predicted (but not observed) modes of radioactive decay are noted as A for alpha decay, B for beta decay, BB for double beta decay, E for electron capture, EE for double electron capture, IT for isomeric transition.
All stable isotopes are the ground states of nuclei, with the exception of tantalum-180m, which is the nuclear isomer or excited level (the ground state of this nucleus is radioactive with a very short half-life of 8 hours); but the decay of the excited nuclear isomer is extremely strongly forbidden by spin-parity selection rules, and has never been observed and is thus included in the list. It was shown experimentally that the half-life of 180mTa is more than 1015 years. Other possible modes of 180mTa decay (beta decay, electron capture and alpha decay) have never been observed, too.
See also
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