In geochemistry and geonuclear physics, primordial nuclides or primordial isotopes are nuclides found on the earth that have existed in their current form since before Earth was formed. Only 288 such nuclides are known. All of the known 255 stable nuclides occur as primordial nuclides, plus another 33 nuclides that have half-lives long enough to have survived from the formation of the Earth.
Due to the age of the Earth of 4.58×109 yr, this means that the half-life of the given nuclides must be greater than about 5×107 yr for practical considerations. For example, for a nuclide with half-life 6×107 yr, 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 6 atoms remain today.
The shortest-lived isotopes (i.e. isotopes with shortest half-lives) in the list of 33 radioactive primordial nuclides are:
These are the 6 nuclides with half-lives comparable to, or less than, the estimated age of the universe. For a complete list of the 33 known primordial radionuclides, including the next 27 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 yr. (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. To date, it has not been detected. It has been found that the next longer-lived nuclide, 244Pu, with a half-life of 8.08×107 yr is primordial, although just barely, as its concentration in a few ores is nearly 10−18 weight parts. Taking into account that all these nuclides must exist since at least 4.6×109 yr, meaning survive 57 half-lives, their original number is now reduced by a factor of 257 which equals more than 1017.
Although it is estimated that about 33 primordial nuclides are radioactive (list below), it becomes very difficult to determine the exact total number of radioactive primoridals, because the total number of stable nuclides is uncertain. There exist many extremely long-lived isotopes 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 2009[update] this could only be measured experimentally for 180W. 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.
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).
Naturally occurring stable nuclides
As noted, these number about 255. For a list, see the article list of stable isotopes. For a complete list noting which of the "stable" 255 nuclides may be in some respect unstable, see list of nuclides. 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 33 radioactive primordial nuclides and measured half-lives
These are listed in order of stability, with the longest half-life first. Note that half-lives are in seconds, and must be divided by 3.1556926 x 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, which is about 4.32 x 1017 seconds). Only six of these 33 nuclides have half-lives shorter than, or equal to, the age of the universe. The remaining 27 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 (number)
A running positive integer for reference. These numbers may change slightly in the future as more of the 165 theoretically unstable nuclides are actually found to be unstable experimentally. The number starts at 256, to follow the 255 nuclides not yet found 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). In the rare case that this is not the ground state, this is indicated by a m for metastable appended to the mass 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: mn − mnuclide / A.
- half-life column
All times are given in seconds (3.1556926×107 s = 1 year).
- decay mode column
|KK||double electron capture|
|2||double β− decay|
|β+β+||double β+ decay|
- decay energy column
Multiple values for (maximal) decay energy are mapped to decay modes in their order.
|no||nuclide||energy||half-life (seconds)||decay mode||decay energy (MeV)||approx ratio half-life to age of universe|
|256||128Te||8.743261||6.9×1031||2||2.530||160 million million|
|260||48Ca||8.992452||7.258×1026||2 β−||4.274 , 0.0058||2,000 million|
|261||96Zr||8.961359||6.3×1026||2 β−||3.4||1,000 million|
|268||50V||9.055759||4.418×1024||β+ β−||2.205 , 1.038||10 million|
|279||138La||8.698320||3.219×1018||K β−||1.737 , 1.044||7|
|281||187Re||8.291732||1.300×1018||β− α||0.0026 , 1.653||3|
|285||40K||8.909707||3.938×1016||β− K β+||1.311 , 1.505 , 1.505||0.09|
- Primordial element
- Table of nuclides sorted by half-life
- Table of nuclides
- Isotope geochemistry
- Mononuclidic element
- Monoisotopic element
- Stable isotope
- List of elements by stability of isotopes
- List of elements by nuclear stability