geochemistry Geochemistry is the science that uses the tools and principles of chemistry to explain the mechanisms behind major geological systems such as the Earth's crust and its oceans. The realm of geochemistry extends beyond the Earth, encompassing the e ...

geophysics Geophysics () is a subject of natural science concerned with the physical processes and Physical property, properties of Earth and its surrounding space environment, and the use of quantitative methods for their analysis. Geophysicists conduct i ...

and

nuclear physics Nuclear physics is the field of physics that studies atomic nuclei and their constituents and interactions, in addition to the study of other forms of nuclear matter. Nuclear physics should not be confused with atomic physics, which studies th ...

, primordial nuclides, also known as primordial isotopes, are

nuclide Nuclides (or nucleides, from nucleus, also known as nuclear species) are a class of atoms characterized by their number of protons, ''Z'', their number of neutrons, ''N'', and their nuclear energy state. The word ''nuclide'' was coined by the A ...

s found on

Earth Earth is the third planet from the Sun and the only astronomical object known to Planetary habitability, harbor life. This is enabled by Earth being an ocean world, the only one in the Solar System sustaining liquid surface water. Almost all ...

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 The Solar SystemCapitalization of the name varies. The International Astronomical Union, the authoritative body regarding astronomical nomenclature, specifies capitalizing the names of all individual astronomical objects but uses mixed "Sola ...

was formed, and were formed in, or after, the

Big Bang The Big Bang is a physical theory that describes how the universe expanded from an initial state of high density and temperature. Various cosmological models based on the Big Bang concept explain a broad range of phenomena, including th ...

, by

nucleosynthesis Nucleosynthesis is the process that creates new atomic nuclei from pre-existing nucleons (protons and neutrons) and nuclei. According to current theories, the first nuclei were formed a few minutes after the Big Bang, through nuclear reactions in ...

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; 286 such nuclides are known.

Stability

All of the known 251

stable nuclide Stable nuclides are isotopes of a chemical element whose nucleons are in a configuration that does not permit them the surplus energy required to produce a radioactive emission. The nuclei of such isotopes are not radioactive and unlike radionu ...

s, plus another 35 nuclides that have

half-lives Half-life is a mathematical and scientific description of exponential or gradual decay. Half-life, half life or halflife may also refer to: Film * ''Half-Life'' (film), a 2008 independent film by Jennifer Phang * '' Half Life: A Parable for t ...

long enough to have survived from the formation of the Earth, occur as primordial nuclides. These 35 primordial radionuclides represent

isotope Isotopes are distinct nuclear species (or ''nuclides'') of the same chemical element. They have the same atomic number (number of protons in their Atomic nucleus, nuclei) and position in the periodic table (and hence belong to the same chemica ...

s of 28 separate elements.

Cadmium Cadmium is a chemical element; it has chemical symbol, symbol Cd and atomic number 48. This soft, silvery-white metal is chemically similar to the two other stable metals in group 12 element, group 12, zinc and mercury (element), mercury. Like z ...

tellurium Tellurium is a chemical element; it has symbol Te and atomic number 52. It is a brittle, mildly toxic, rare, silver-white metalloid. Tellurium is chemically related to selenium and sulfur, all three of which are chalcogens. It is occasionally fou ...

xenon Xenon is a chemical element; it has symbol Xe and atomic number 54. It is a dense, colorless, odorless noble gas found in Earth's atmosphere in trace amounts. Although generally unreactive, it can undergo a few chemical reactions such as the ...

neodymium Neodymium is a chemical element; it has Symbol (chemistry), symbol Nd and atomic number 60. It is the fourth member of the lanthanide series and is considered to be one of the rare-earth element, rare-earth metals. It is a hard (physics), hard, sli ...

, samarium,

osmium Osmium () is a chemical element; it has Symbol (chemistry), symbol Os and atomic number 76. It is a hard, brittle, bluish-white transition metal in the platinum group that is found as a Abundance of elements in Earth's crust, trace element in a ...

, and

uranium Uranium is a chemical element; it has chemical symbol, symbol U and atomic number 92. It is a silvery-grey metal in the actinide series of the periodic table. A uranium atom has 92 protons and 92 electrons, of which 6 are valence electrons. Ura ...

each have two primordial radioisotopes (, ; , ; , ; , ; , ; , ; and , ). Because the

age of the Earth The age of Earth is estimated to be 4.54 ± 0.05 billion years. This age may represent the age of Earth's accretion (astrophysics), accretion, or Internal structure of Earth, core formation, or of the material from which Earth formed. This dating ...

is (4.58 billion years), the

half-life Half-life is a mathematical and scientific description of exponential or gradual decay. Half-life, half life or halflife may also refer to: Film * Half-Life (film), ''Half-Life'' (film), a 2008 independent film by Jennifer Phang * ''Half Life: ...

of the given nuclides must be greater than about (100 million years) for practical considerations. For example, for a nuclide with half-life (60 million years), 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 seven shortest-lived primordial nuclides (i.e., the nuclides with the shortest half-lives) to have been experimentally verified are (), (), (), (), (), (), and (). These are the seven nuclides with half-lives comparable to, or somewhat less than, the estimated

age of the universe In physical cosmology, the age of the universe is the cosmological time, time elapsed since the Big Bang: 13.79 billion years. Astronomers have two different approaches to determine the age of the universe. One is based on a particle physics ...

. (⁸⁷Rb, ¹⁸⁷Re, ¹⁷⁶Lu, and ²³²Th have half-lives somewhat longer than the age of the universe.) For a complete list of the 35 known primordial radionuclides, including the next 28 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. ⁸⁷Rb, ¹⁸⁷Re, ¹⁷⁶Lu, 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 longest-lived isotope not proven to be primordial is , which has a half-life of , followed by () and (). ²⁴⁴Pu was reported to exist in nature as a primordial nuclide in 1971, but this detection could not be confirmed by further studies in 2012 and 2022. Taking into account that all these nuclides must exist for at least , ¹⁴⁶Sm must survive 50 half-lives (and hence be reduced by 2⁵⁰ ≈ ), ²⁴⁴Pu must survive 57 (and be reduced by a factor of 2⁵⁷ ≈ ), and ⁹²Nb must survive 130 (and be reduced by 2¹³⁰ ≈ ). Mathematically, considering the likely initial abundances of these nuclides, primordial ¹⁴⁶Sm and ²⁴⁴Pu should persist somewhere within the Earth today, even if they are not identifiable in the relatively minor portion of the Earth's crust available to human assays, while ⁹²Nb and all shorter-lived nuclides should not. Nuclides such as ⁹²Nb that were present in the primordial solar nebula but have long since decayed away completely are termed

extinct radionuclide An extinct radionuclide is a radionuclide that was formed by nucleosynthesis before the formation of the Solar System, about 4.6 billion years ago, but has since decayed to virtually zero abundance and is no longer detectable as a primordial nu ...

s if they have no other means of being regenerated. As for ²⁴⁴Pu, calculations suggest that as of 2022, sensitivity limits were about one order of magnitude away from detecting it as a primordial nuclide. Because primordial chemical elements often consist of more than one primordial isotope, there are only 83 distinct primordial

chemical element A chemical element is a chemical substance whose atoms all have the same number of protons. The number of protons is called the atomic number of that element. For example, oxygen has an atomic number of 8: each oxygen atom has 8 protons in its ...

s. Of these, 80 have at least one

observationally stable Stable nuclides are isotopes of a chemical element whose nucleons are in a configuration that does not permit them the surplus energy required to produce a radioactive emission. The nuclei of such isotopes are not radioactive and unlike radionuc ...

isotope and three additional primordial elements have only radioactive isotopes (

bismuth Bismuth is a chemical element; it has symbol Bi and atomic number 83. It is a post-transition metal and one of the pnictogens, with chemical properties resembling its lighter group 15 siblings arsenic and antimony. Elemental bismuth occurs nat ...

thorium Thorium is a chemical element; it has symbol Th and atomic number 90. Thorium is a weakly radioactive light silver metal which tarnishes olive grey when it is exposed to air, forming thorium dioxide; it is moderately soft, malleable, and ha ...

, and uranium).

Naturally occurring nuclides that are not primordial

Some unstable isotopes which occur naturally (such as , , and ) are not primordial, as they must be constantly regenerated. This occurs by

cosmic radiation Cosmic rays or astroparticles are high-energy particles or clusters of particles (primarily represented by protons or atomic nuclei) that move through space at nearly the speed of light. They originate from the Sun, from outside of the Sol ...

(in the case of

cosmogenic nuclide Cosmogenic nuclides (or cosmogenic isotopes) are rare nuclides (isotopes) created when a high-energy cosmic ray interacts with the nucleus of an '' in situ'' Solar System atom, causing nucleons (protons and neutrons) to be expelled from the atom ...

s such as and ), or (rarely) by such processes as geonuclear transmutation (

neutron capture Neutron capture is a nuclear reaction in which an atomic nucleus and one or more neutrons collide and merge to form a heavier nucleus. Since neutrons have no electric charge, they can enter a nucleus more easily than positively charged protons, wh ...

of uranium in the case of and ). Other examples of common naturally occurring but non-primordial nuclides are isotopes of

radon Radon is a chemical element; it has symbol Rn and atomic number 86. It is a radioactive noble gas and is colorless and odorless. Of the three naturally occurring radon isotopes, only Rn has a sufficiently long half-life (3.825 days) for it to b ...

polonium Polonium is a chemical element; it has symbol Po and atomic number 84. A rare and highly radioactive metal (although sometimes classified as a metalloid) with no stable isotopes, polonium is a chalcogen and chemically similar to selenium and tel ...

, and

radium Radium is a chemical element; it has chemical symbol, symbol Ra and atomic number 88. It is the sixth element in alkaline earth metal, group 2 of the periodic table, also known as the alkaline earth metals. Pure radium is silvery-white, ...

, which are all radiogenic nuclide daughters of uranium decay and are found in uranium ores. The stable

argon Argon is a chemical element; it has symbol Ar and atomic number 18. It is in group 18 of the periodic table and is a noble gas. Argon is the third most abundant gas in Earth's atmosphere, at 0.934% (9340 ppmv). It is more than twice as abu ...

isotope ⁴⁰Ar is actually more common as a radiogenic nuclide than as a primordial nuclide, forming almost 1% of the Earth's

atmosphere An atmosphere () is a layer of gases that envelop an astronomical object, held in place by the gravity of the object. A planet retains an atmosphere when the gravity is great and the temperature of the atmosphere is low. A stellar atmosph ...

, which is regenerated by the

beta decay In nuclear physics, beta decay (β-decay) is a type of radioactive decay in which an atomic nucleus emits a beta particle (fast energetic electron or positron), transforming into an isobar of that nuclide. For example, beta decay of a neutron ...

of the extremely long-lived radioactive primordial isotope ⁴⁰K, whose half-life is on the order of a billion years and thus has been generating argon since early in the Earth's existence. (Primordial argon was dominated by the

alpha process The alpha process, also known as alpha capture or the alpha ladder, is one of two classes of nuclear fusion reactions by which stars convert helium into heavier elements. The other class is a cycle of reactions called the triple-alpha process, w ...

nuclide ³⁶Ar, which is significantly rarer than ⁴⁰Ar on Earth.) A similar radiogenic series is derived from the long-lived radioactive primordial nuclide ²³²Th. These nuclides are described as 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 chain In nuclear science a decay chain refers to the predictable series of radioactive disintegrations undergone by the nuclei of certain unstable chemical elements. Radioactive isotopes do not usually decay directly to stable isotopes, but rather ...

s of ²³²Th, ²³⁵U, or ²³⁸U but can still fleetingly occur naturally as products of the

spontaneous fission Spontaneous fission (SF) is a form of radioactive decay in which a heavy atomic nucleus splits into two or more lighter nuclei. In contrast to induced fission, there is no inciting particle to trigger the decay; it is a purely probabilistic proc ...

of one of these three long-lived nuclides, such as ¹²⁶Sn, which makes up about 10⁻¹⁴ of all natural tin. Another, ⁹⁹Tc, has also been detected. There are five other long-lived fission products known.

Primordial elements

A primordial element is a

with at least one primordial nuclide. There are 251 stable primordial nuclides and 35 radioactive primordial nuclides, but only 80 primordial stable ''elements''—hydrogen through lead, atomic numbers 1 to 82, except for

technetium Technetium is a chemical element; it has Symbol (chemistry), symbol Tc and atomic number 43. It is the lightest element whose isotopes are all radioactive. Technetium and promethium are the only radioactive elements whose neighbours in the sense ...

(43) and

promethium Promethium is a chemical element; it has Symbol (chemistry), symbol Pm and atomic number 61. All of its isotopes are Radioactive decay, radioactive; it is extremely rare, with only about 500–600 grams naturally occurring in the Earth's crust a ...

(61)—and three radioactive primordial ''elements''—bismuth (83), thorium (90), and uranium (92). If plutonium (94) turns out to be primordial (specifically, the long-lived isotope Pu), then it would be a fourth radioactive primordial, though practically speaking it would still be more convenient to produce synthetically. Bismuth's half-life is so long that it is often classed with the 80 stable elements instead, since its radioactivity is not a cause for concern. The number of elements is smaller than the number of nuclides, because many of the primordial elements are represented by multiple

isotopes Isotopes are distinct nuclear species (or ''nuclides'') of the same chemical element. They have the same atomic number (number of protons in their nuclei) and position in the periodic table (and hence belong to the same chemical element), but ...

. See

for more information.

Naturally occurring stable nuclides

As noted, this number is about 251. For a list, see the article

list of elements by stability of isotopes Of the first 82 chemical elements in the periodic table, 80 have isotopes considered to be stable. Overall, there are 251 known stable isotopes in total. Background Atomic nuclei consist of protons and neutrons, which attract each other throu ...

. For a complete list noting which of the "stable" 251 nuclides may be in some respect unstable, see

list of nuclides This list of nuclides shows observed nuclides that either are stable or, if radioactive, have half-lives longer than one hour. This includes isotopes of the first 105 elements, except for 87 (francium), 102 (nobelium) and 104 (rutherfordium). At ...

and

. 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

Though it is estimated that about 35 primordial nuclides are

radioactive Radioactive decay (also known as nuclear decay, radioactivity, radioactive disintegration, or nuclear disintegration) is the process by which an unstable atomic nucleus loses energy by radiation. A material containing unstable nuclei is conside ...

(list below), it becomes very hard to determine the exact total number of radioactive primordials, because the total number of stable nuclides is uncertain. There are many extremely long-lived nuclides whose half-lives are still unknown; in fact, all nuclides heavier than dysprosium-164 are theoretically radioactive. 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

alpha decay Alpha decay or α-decay is a type of radioactive decay in which an atomic nucleus emits an alpha particle (helium nucleus). The parent nucleus transforms or "decays" into a daughter product, with a mass number that is reduced by four and an a ...

, but this could only be measured experimentally for W. Likewise, all four primordial isotopes of lead are expected to decay to mercury, but the predicted half-lives are so long (some exceeding 10 years) that such decays could 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

s—is limited. Even when a "stable" nuclide is found to be radioactive, it merely moves from the ''stable'' to the ''unstable'' list of primordials, and the total number of primordial nuclides remains unchanged. For practical purposes, these nuclides may be considered stable for all purposes outside specialized research.

List of 35 radioactive primordial nuclides and measured half-lives

These 35 primordial radionuclides are isotopes of 28 elements (cadmium, neodymium, osmium, samarium, tellurium, uranium, and xenon each have two primordial radioisotopes). These nuclides are listed in order of decreasing stability. Many of them are so nearly stable that they compete for abundance with stable isotopes of their respective elements. For three elements (

indium Indium is a chemical element; it has Symbol (chemistry), symbol In and atomic number 49. It is a silvery-white post-transition metal and one of the softest elements. Chemically, indium is similar to gallium and thallium, and its properties are la ...

, and

rhenium Rhenium is a chemical element; it has symbol Re and atomic number 75. It is a silvery-gray, heavy, third-row transition metal in group 7 of the periodic table. With an estimated average concentration of 1 part per billion (ppb), rhenium is one ...

) a very long-lived radioactive primordial nuclide is more abundant than a stable nuclide. The longest-lived radionuclide known, Te, has a half-life of : 1.6 × 10 times the

age of the Universe In physical cosmology, the age of the universe is the cosmological time, time elapsed since the Big Bang: 13.79 billion years. Astronomers have two different approaches to determine the age of the universe. One is based on a particle physics ...

. Only four of these 35 nuclides have half-lives shorter than, or equal to, the age of the universe. Most of the other 30 have half-lives much longer. The shortest-lived primordial, U, has a half-life of 703.8 million years, about 1/6 the age of the Earth and

. Many of these nuclides decay by

double beta decay In nuclear physics, double beta decay is a type of radioactive decay in which two neutrons are simultaneously transformed into two protons, or vice versa, inside an atomic nucleus. As in single beta decay, this process allows the atom to move cl ...

, though some like Bi decay by other means such as

. At the end of the list, are two more nuclides: Sm and Pu. They have not been confirmed as primordial, but their half-lives are long enough that minute quantities should persist today. {, class="wikitable sortable" style="text-align:right" ! No. ! data-sort-type="number" , Nuclide ! Energy ! Half-
life
(years) ! Decay
mode !

Decay energy The decay energy is the energy change of a nucleus having undergone a radioactive decay. Radioactive decay is the process in which an unstable atomic nucleus loses energy by emitting ionizing particles and radiation. This decay, or loss of energ ...

(MeV) ! Approx. ratio
half-life to
age of universe , - , , 252, , ¹²⁸Te, , 8.743261, , , , align=center, 2 β⁻ , , align=center, 2.530, , align=center, 160 trillion , - , , 253, , ¹²⁴Xe, , 8.778264, , , , align=center , KK , , align=center , 2.864 , , align=center, 1.3 trillion , - , , 254, , ⁷⁸Kr, , 9.022349, , , , align=center , KK , , align=center, 2.846, , align=center, 670 billion , - , , 255, , ¹³⁶Xe, , 8.706805, , , , align=center, 2 β⁻ , , align=center, 2.462, , align=center, 160 billion , - , , 256, , ⁷⁶Ge, , 9.034656, , , , align=center, 2 β⁻ , , align=center, 2.039, , align=center, 130 billion , - , , 257, , data-sort-value="130.5", ¹³⁰Ba, , 8.742574, , , , align=center, KK , , align=center, 2.620, , align=center, 87 billion , - , , 258, , ⁸²Se, , 9.017596, , , , align=center, 2 β⁻ , , align=center, 2.995, , align=center, 8.0 billion , - , , 259, , ¹¹⁶Cd, , 8.836146, , , , align=center, 2 β⁻ , , align=center, 2.809, , align=center, 2.3 billion , - , , 260, , ⁴⁸Ca, , 8.992452, , , , align=center, 2 β⁻ , , align=center, 4.274, .0058, , align=center, 1.7 billion , - , , 261, , ²⁰⁹Bi, , 8.158689, , , , align=center, α , , align=center, 3.137, , align=center, 1.5 billion , - , , 262, , ⁹⁶Zr, , 8.961359, , , , align=center, 2 β⁻ , , align=center, 3.4, , align=center, 1.5 billion , - , , 263, , ¹⁵⁰Nd, , 8.562594, , , , align=center, 2 β⁻ , , align=center, 3.367, , align=center, 671 million , - , , 264, , ¹³⁰Te, , 8.766578, , , , align=center, 2 β⁻ , , align=center, .868, , align=center, 640 million , - , , 265, , ¹⁰⁰Mo, , 8.933167, , , , align=center, 2 β⁻ , , align=center, 3.035, , align=center, 510 million , - , , 266, , ¹⁵¹Eu, , 8.565759, , , , align=center, α , , align=center, 1.9644, , align=center, 333 million , - , , 267, , ¹⁸⁰W, , 8.347127, , , , align=center, α , , align=center, 2.509, , align=center, 130 million , - , , 268, , ⁵⁰V, , 9.055759, , , , align=center, β⁺ or β⁻ , , align=center, 2.205, 1.038, , align=center, 10 million , - , , 269, , ¹⁷⁴Hf, , 8.392287, , , , align=center, α , , align=center, 2.497, , align=center, 5.1 million , - , , 270, , ¹¹³Cd, , 8.859372, , , , align=center, β⁻ , , align=center, .321, , align=center, 560,000 , - , , 271, , ¹⁴⁸Sm, , 8.607423, , , , align=center, α , , align=center, 1.986, , align=center, 510,000 , - , , 272, , ¹⁴⁴Nd, , 8.652947, , , , align=center, α , , align=center, 1.905, , align=center, 170,000 , - , , 273, , ¹⁸⁶Os, , 8.302508, , , , align=center, α , , align=center, 2.823, , align=center, 150,000 , - , , 274, , ¹¹⁵In, , 8.849910, , , , align=center, β⁻ , , align=center, .499, , align=center, 32,000 , - , , 275, , ¹⁵²Gd, , 8.562868, , , , align=center, α , , align=center, 2.203, , align=center, 7,800 , - , , 276, , ¹⁸⁴Os, , 8.311850, , , , align=center, α , , align=center, 2.963, , align=center, 810 , - , , 277, , ¹⁹⁰Pt, , 8.267764, , , , align=center, α , , align=center, 3.252, , align=center, 35 , - , , 278, , ¹⁴⁷Sm, , 8.610593, , , , align=center, α , , align=center, 2.310, , align=center, 7.7 , - , , 279, , ¹³⁸La, , 8.698320, , , , align=center, β⁻ or K or β⁺ , , align=center, 1.044, 1.737, 1.737, , align=center, 7.4 , - , , 280, , ⁸⁷Rb, , 9.043718, , , , align=center, β⁻ , , align=center, .283, , align=center, 3.6 , - , , 281, , ¹⁸⁷Re, , 8.291732, , , , align=center, β⁻, , align=center, .0026, , align=center, 3.0 , - , , 282, , ¹⁷⁶Lu, , 8.374665, , , , align=center, β⁻ , , align=center, 1.193, , align=center, 2.7 , - , , 283, , ²³²Th, , 7.918533, , , , align=center, α or SF , , align=center, 4.083, , align=center, 1.0 , - , , 284, , ²³⁸U, , 7.872551, , , , align=center, α or SF or 2 β⁻ , , align=center, 4.270, , align=center, 0.32 , - , , 285, , ⁴⁰K, , 8.909707, , , , align=center, β⁻ or K or β⁺ , , align=center, 1.311, 1.505, 1.505, , align=center, 0.091 , - , , 286, , ²³⁵U, , 7.897198, , , , align=center, α or SF , , align=center, 4.679, , align=center, 0.051 , - bgcolor=#ffc0c0 , , 287, , ¹⁴⁶Sm, , 8.626136, , , , align=center, α , , align=center, 2.529, , align=center, 0.0067 , - bgcolor=#ffc0c0 , , 288, , ²⁴⁴Pu, , 7.826221, , , , align=center, α or SF , , align=center, 4.666, , align=center, 0.0059

List legends

References

{{reflist Geochemistry Radiometric dating Isotopes Metrology

Stability

Naturally occurring nuclides that are not primordial

Primordial elements

Naturally occurring stable nuclides

Radioactive primordial nuclides

List of 35 radioactive primordial nuclides and measured half-lives

List legends

See also

References