Periodic table colored according to the number of stable isotopes. It can be seen that odd-atomic number elements have fewer stable isotopes than their even neighbours.

Stable isotopes are chemical isotopes that are not radioactive. About 2/3rds of elements have more than one stable isotope. Different 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 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 primoridal), 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 energetic reasons such as those produced 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:

• equilibrium fractionation
• kinetic fractionation
• mass-independent fractionation

List of stable isotopes

See also: List of elements by nuclear stability

There are 80 known elements which have at least one stable isotope. As of September 2007, there were 250 known stable isotopes. 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. Mononuclidic elements are those that have a single isotope (stable or very long-lived) in their natural abundance; there are 27 of these. 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

• Table of nuclides
• Isotope geochemistry
• Radionuclide

References

1. ^ "WWW Table of Radioactive Isotopes".