The 14th International Conference on

Miniaturized Systems for Chemistry and Life Sciences

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Primordial nuclide


Primordial nuclide

In geochemistry, geophysics and geonuclear physics, primordial nuclides, also known as primordial isotopes, are nuclides found on Earth that have existed in their current form since before Earth was formed. Primordial nuclides were present in the interstellar medium from which the solar system was formed, and were formed in, or after, the Big Bang, by nucleosynthesis in stars and supernovae followed by mass ejection, by cosmic ray spallation, and potentially from other processes. They are the stable nuclides plus the long-lived fraction of radionuclides surviving in the primordial solar nebula through planet accretion until the present. Only 286 such nuclides are known.

There are 252 stable primordial nuclides and 34 radioactive primordial nuclides, but only 80 primordial stable elements (1 through 82, i.e. hydrogen through lead, exclusive of 43 and 61, technetium and promethium respectively) and three radioactive primordial elements (bismuth, thorium, and uranium). Bismuth's half-life is so long that it is often classed with the 80 primordial stable elements instead, since its radioactivity is not a cause for serious concern. The number of elements is fewer than the number of nuclides, because many of the primordial elements are represented by multiple isotopes. See chemical element for more information.

As noted, these number about 252. For a list, see the article list of elements by stability of isotopes. For a complete list noting which of the "stable" 252 nuclides may be in some respect unstable, see list of nuclides and stable nuclide. These questions do not impact the question of whether a nuclide is primordial, since all "nearly stable" nuclides, with half-lives longer than the age of the universe, are primordial also.

Although it is estimated that about 34 primordial nuclides are radioactive (list below), it becomes very difficult to determine the exact total number of radioactive primordials, because the total number of stable nuclides is uncertain. There exist many extremely long-lived nuclides whose half-lives are still unknown. For example, it is predicted theoretically that all isotopes of tungsten, including those indicated by even the most modern empirical methods to be stable, must be radioactive and can decay by alpha emission, but as of 2013 this could only be measured experimentally for 180W. Similarly, all four primordial isotopes of lead are expected to decay to mercury, but the predicted half-lives are so long (some exceeding 10100 years) that this can hardly be observed in the near future. Nevertheless, the number of nuclides with half-lives so long that they cannot be measured with present instruments´and are considered from this viewpoint to be stable nuclides´is limited. Even when a "stable" nuclide is found to be radioactive, it merely moves from the stable to the unstable list of primordial nuclides, and the total number of primordial nuclides remains unchanged.

These 34 primordial nuclides represent radioisotopes of 28 distinct chemical elements (cadmium, neodymium, samarium, tellurium, uranium, and xenon each have two primordial radioisotopes). The radionuclides are listed in order of stability, with the longest half-life beginning the list. These radionuclides in many cases are so nearly stable that they compete for abundance with stable isotopes of their respective elements. For three chemical elements, indium, tellurium, and rhenium, a very long-lived radioactive primordial nuclide is found in greater abundance than a stable nuclide.


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