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Primordial element

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Primordial element

"Primordial element" redirects here. For a concept in algebra, see Primordial element (algebra).


In geochemistry and geonuclear physics, primordial nuclides, also known as primordial isotopes, are nuclides found on the Earth that have existed in their current form since before Earth was formed. Primordial nuclides are residues from the Big Bang, from cosmogenic sources, and from ancient supernova explosions which occurred before the formation of the solar system. 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 288 such nuclides are known.

All of the known 254 stable nuclides occur as primordial nuclides, plus another 34 nuclides that have half-lives long enough to have survived from the formation of the Earth. These 34 primordial radionuclides represent isotopes of 28 separate elements. Cadmium, neodymium and uranium each have two primordial radioisotopes (113Cd, 116Cd; 144Nd, 150Nd and 235U, 238U), and tellurium and samarium have three (123Te, 128Te, 130Te and 146Sm, 147Sm, 148Sm).

Due to the age of the Earth of 4.58×109 years (4.6 billion years), this means that the half-life of the given nuclides must be greater than about 5×107 years for practical considerations. For example, for a nuclide with half-life 6×107 years, this means 77 half-lives have elapsed, meaning that for each mole (6.02×1023 atoms) of that nuclide being present at the formation of Earth, only 4 atoms remain today.

The shortest-lived primordial nuclides (i.e. nuclides with shortest half-lives) are:

..., 232Th, 238U, 40K, 235U, 146Sm and 244Pu.

These are the 6 nuclides with half-lives comparable to, or less than, the estimated age of the universe. (In the case for 232Th, it has a half life of more than 14 billion years, slightly longer than the age of the universe.) For a complete list of the 34 known primordial radionuclides, including the next 28 with half-lives much longer than the age of the universe, see the complete list in the section below.

The next longest-living nuclide after the end of the list given in the table is niobium-92 with a half-life of 3.47×107 years. (See list of nuclides for the list of all nuclides with half-lives longer than 60 minutes.) To be detected primordially, 92Nb would have to survive at least 132 half-lives since the Earth's formation, meaning its original concentration will have decreased by a factor of 1040. As of 2013, it has not been detected. It has been found that the next longer-lived nuclide, 244Pu, with a half-life of 8.08×107 years is primordial, although just barely, as its concentration in a few ores is nearly 10−18 weight parts.[1][2] Taking into account that all these nuclides must exist since at least 4.6×109 years, meaning survive 57 half-lives, their original number is now reduced by a factor of 257 which equals more than 1017.[3]

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 known 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.[4] 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, the fact merely moves it from the stable to the unstable list of primordial nuclides, and the total number of primordial nuclides remains unchanged.

Because primordial chemical elements often consist of more than one primordial isotope, there are only 84 distinct primordial chemical elements. Of these, 80 have at least one observationally stable isotope and four only have radioactive isotopes.

Naturally occurring nuclides that are not primordial

Some unstable isotopes which occur naturally (such as 14C, 3H, and 239Pu) are not primordial, as they must be constantly regenerated. This occurs by cosmic radiation (in the case of cosmogenic nuclides such as 14C and 3H), or (rarely) by such processes as geonuclear transmutation (neutron capture of uranium in the case of 239Pu). Other examples of common naturally-occurring but non-primordial nuclides are 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 thorium-232. All of such nuclides have shorter half-lives than their parent radioactive primordial nuclides.

There are about 51 nuclides which are radioactive and exist naturally on Earth but are not primordial (making a total of fewer than 340 total nuclides to be found naturally on Earth).

Primordial elements

There are 254 stable primordial nuclides and 35 radioactive primordial nuclides, but only 80 primordial stable elements (1 through 82, i.e. hydrogen through lead, exclusive of 43 and 61, i.e. technetium and promethium) and four radioactive primordial elements (bismuth, thorium, uranium, and plutonium). The numbers of elements are smaller, because many primordial elements are represented by more than one primordial nuclide. See chemical element for more information.

Naturally occurring stable nuclides

As noted, these number about 254. For a list, see the article list of stable isotopes. For a complete list noting which of the "stable" 254 nuclides may be in some respect unstable, see list of nuclides and stable isotope. 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.

List of 34 radioactive primordial nuclides and measured half-lives

These 34 primordial nuclides represent radioisotopes of 28 distinct chemical elements (cadmium, neodymium, tellurium, and uranium each have two primordial radioisotopes, and samarium has three). 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. In one case a radionuclide (Te-123) has been added and then removed from the table, as its natural decay has been reported, then re-reported as unconfirmed. In any case, Te-123's half life is longer than 5 x 1019 years, and probably much longer. [5]

In three notable cases (i.e., for three chemical elements), a very long lived radioactive primordial nuclide is nevertheless found to be the most abundant nuclide for an element that also has a stable nuclide. These unusual elements are tellurium, indium, and rhenium.

Note that half-lives are in seconds, and must be divided by 3.1556926×107 to obtain half-lives in years.

The longest has a half-life of 2.2×1024 years (160 million million times the age of the Universe (the latter is about 4.32×1017 s). Only six of these 35 nuclides have half-lives shorter than, or equal to, the age of the universe. The remaining 30 have half-lives much longer. The shortest-lived primordial isotope so far detected has a half-life of only 80 million years, less than 2% of the age of the Earth and Solar System.

no nuclide energy half-life (seconds) decay mode decay energy (MeV) approx ratio half-life to age of universe
255 128Te 8.743261 6.9×1031 2 β 2.530 160 trillion
256 136Xe 8.706805 6.66×1028 2 β 2.462 150 billion
257 76Ge 9.034656 5.62×1028 2 β 2.039 130 billion
258 82Se 9.017596 3.408×1027 2 β 2.995 8 billion
259 116Cd 8.836146 9.783×1026 2 β 2.809 2 billion
260 48Ca 8.992452 7.258×1026 2 β 4.274, .0058 2 billion
261 96Zr 8.961359 6.3×1026 2 β 3.4 1 billion
262 209Bi 8.158689 5.996×1026 α 3.137 1 billion
263 130Te 8.766578 2.777×1026 2 β .868 600 million
264 150Nd 8.562594 2.493×1026 2 β 3.367 600 million
265 100Mo 8.933167 2.461×1026 2 β 3.035 600 million
266 151Eu 8.565759 1.578×1026 α 1.9644 300 million
267 180W 8.347127 5.680×1025 α 2.509 100 million
268 50V 9.055759 4.418×1024 β+ β 2.205, 1.038 10 million
269 113Cd 8.859372 2.430×1023 β .321 600,000
270 148Sm 8.607423 2.209×1023 α 1.986 500,000
271 144Nd 8.652947 7.227×1022 α 1.905 200,000
272 186Os 8.302508 6.312×1022 α 2.823 100,000
273 174Hf 8.392287 6.312×1022 α 2.497 100,000
274 115In 8.849910 1.392×1022 β .499 30,000
275 152Gd 8.562868 3.408×1021 α 2.203 8000
276 130Ba 8.742574 2.2×1021 KK 2.620 5000
277 190Pt 8.267764 2.051×1019 α 3.252 60
278 147Sm 8.610593 3.345×1018 α 2.310 8
279 138La 8.698320 3.219×1018 K β 1.737, 1.044 7
280 87Rb 9.043718 1.568×1018 β .283 4
281 187Re 8.291732 1.300×1018 β α .0026, 1.653 3
282 176Lu 8.374665 1.187×1018 β 1.193 3
283 232Th 7.918533 4.434×1017 α SF 4.083 1
284 238U 7.872551 1.410×1017 α SF 4.270 .3
285 40K 8.909707 3.938×1016 β K β+ 1.311, 1.505, 1.505 .09
286 235U 7.897198 2.222×1016 α SF 4.679 .05
287 146Sm 8.626136 3.250×1015 α 2.529 .008
288 244Pu 7.826221 2.525×1015 α SF 4.666 .006

List legends

no (number)

A running positive integer for reference. These numbers may change slightly in the future since there are 164 nuclides now classified as stable, but which are theoretically predicted to be unstable (see Stable isotope#Still-unobserved decay), so that future experiments may show that some are in fact unstable. The number starts at 255, to follow the 254 nuclides (or stable isotopes) not yet found to be radioactive.

nuclide column

Nuclide identifiers are given by their mass number A and the symbol for the corresponding chemical element (implies a unique proton number).

energy column

The column labeled "energy" denotes the mass of the average nucleon of this nuclide relative to the mass of a neutron (so all nuclides get a positive value) in MeV, formally: mnmnuclide / A.

half-life column

All times are given in seconds (3.1556926×107 s = 1 year).

decay mode column
α α decay
β β decay
K electron capture
KK double electron capture
β+ β+ decay
SF spontaneous fission
2 β double β decay
β+β+ double β+ decay
I isomeric transition
p proton emission
n neutron emission
decay energy column

Multiple values for (maximal) decay energy are mapped to decay modes in their order.

See also

References

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