Primordial nuclide


In geochemistry, geophysics and nuclear 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.

Stability

All of the known 252 stable nuclides, plus another 34 nuclides that have half-lives long enough to have survived from the formation of the Earth, occur as primordial nuclides. These 34 primordial radionuclides represent isotopes of 28 separate elements.
Cadmium, tellurium, xenon, neodymium, samarium and uranium each have two primordial radioisotopes.
Because the age of the Earth is , the half-life of the given nuclides must be greater than about for practical considerations. For example, for a nuclide with half-life , this means 77 half-lives have elapsed, meaning that for each mole of that nuclide being present at the formation of Earth, only 4 atoms remain today.
The four shortest-lived primordial nuclides are,,, and.
These are the 4 nuclides with half-lives comparable to, or less than, the estimated age of the universe. For a complete list of the 34 known primordial radionuclides, including the next 30 with half-lives much longer than the age of the universe, see the complete list below. For practical purposes, nuclides with half-lives much longer than the age of the universe may be treated as if they were stable. 232Th and 238U have half-lives long enough that their decay is limited over geological time scales; 40K and 235U have shorter half-lives and are hence severely depleted, but are still long-lived enough to persist significantly in nature.
The next longest-living nuclide after the end of the list given in the table is, with a half-life of. It has been reported to exist in nature as a primordial nuclide, although later studies could not detect it. Likewise, the second-longest-lived non-primordial has a half-life of, about double that of the third-longest-lived non-primordial . Taking into account that all these nuclides must exist for at least, 244Pu must survive 57 half-lives, 146Sm must survive 67, and 92Nb must survive 130. Considering the likely initial abundances of these nuclides, possibly measurable quantities of 244Pu and 146Sm should persist today, while they should not for 92Nb and all shorter-lived nuclides. Nuclides such as 92Nb that were present in the primordial solar nebula but have long since decayed away completely are termed extinct radionuclides if they have no other means of being regenerated.
Because primordial chemical elements often consist of more than one primordial isotope, there are only 83 distinct primordial chemical elements. Of these, 80 have at least one observationally stable isotope and three additional primordial elements have only radioactive isotopes.

Naturally occurring nuclides that are not primordial

Some unstable isotopes which occur naturally are not primordial, as they must be constantly regenerated. This occurs by cosmic radiation, or by such processes as geonuclear transmutation. Other examples of common naturally occurring but non-primordial nuclides are isotopes of radon, polonium, and radium, which are all radiogenic nuclide daughters of uranium decay and are found in uranium ores. A similar radiogenic series is derived from the long-lived radioactive primordial nuclide 232Th. These nuclides are described as :wikt:geogenic|geogenic, meaning that they are decay or fission products of uranium or other actinides in subsurface rocks. All such nuclides have shorter half-lives than their parent radioactive primordial nuclides. Some other geogenic nuclides do not occur in the decay chains of 232Th, 235U, or 238U but can still fleetingly occur naturally as products of the spontaneous fission of one of these three long-lived nuclides, such as 126Sn, which makes up about 10−14 of all natural tin.

Primordial elements

There are 252 stable primordial nuclides and 34 radioactive primordial nuclides, but only 80 primordial stable elements and three radioactive primordial elements. 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.

Naturally occurring stable nuclides

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 also primordial.

Radioactive primordial nuclides

Although it is estimated that about 34 primordial nuclides are radioactive, 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 this could only be measured experimentally for. Similarly, all four primordial isotopes of lead are expected to decay to mercury, but the predicted half-lives are so long 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.

List of 34 radioactive primordial nuclides and measured half-lives

These 34 primordial nuclides represent radioisotopes of 28 distinct chemical elements. 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.
The longest-lived radionuclide has a half-life of, which is 160 trillion times the age of the Universe. Only four of these 34 nuclides have half-lives shorter than, or equal to, the age of the universe. Most of the remaining 30 have half-lives much longer. The shortest-lived primordial isotope, 235U, has a half-life of 704 million years, about one sixth of the age of the Earth and the Solar System.
No.NuclideEnergyHalf-
life
Decay
mode		Decay energy
Approx. ratio
half-life to
age of universe
253128Te8.7432612 β−2.530160 trillion
254124Xe8.778264KK2.8641 trillion
25578Kr9.022349KK2.846670 billion
256136Xe8.7068052 β−2.462150 billion
25776Ge9.0346562 β−2.039130 billion
258130Ba8.742574KK2.62090 billion
25982Se9.0175962 β−2.9958 billion
260116Cd8.8361462 β−2.8092 billion
26148Ca8.9924522 β−4.274,.00582 billion
26296Zr8.9613592 β−3.41 billion
263209Bi8.158689α3.1371 billion
264130Te8.7665782 β−.868600 million
265150Nd8.5625942 β−3.367600 million
266100Mo8.9331672 β−3.035600 million
267151Eu8.565759α1.9644300 million
268180W8.347127α2.509100 million
26950V9.055759β+ or β−2.205, 1.03810 million
270113Cd8.859372β−.321600,000
271148Sm8.607423α1.986500,000
272144Nd8.652947α1.905200,000
273186Os8.302508α2.823100,000
274174Hf8.392287α2.497100,000
275115In8.849910β−.49930,000
276152Gd8.562868α2.2038000
277190Pt8.267764α3.25247
278147Sm8.610593α2.3107.7
279138La8.698320K or β−1.737, 1.0447.4
28087Rb9.043718β−.2833.6
281187Re8.291732β−.00263
282176Lu8.374665β−1.1932.7
283232Th7.918533α or SF4.0831
284238U7.872551α or SF or 2 β−4.2700.3
28540K8.909707β− or K or β+1.311, 1.505, 1.5050.09
286235U7.897198α or SF4.6790.05

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