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 ...

geophysics Geophysics () is a subject of natural science concerned with the physical processes and physical properties of the Earth and its surrounding space environment, and the use of quantitative methods for their analysis. The term ''geophysics'' som ...

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 the ...

, primordial nuclides, also known as primordial isotopes, are nuclides found on

Earth Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's surf ...

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 The Big Bang event is a physical theory that describes how the universe expanded from an initial state of high density and temperature. Various cosmological models of the Big Bang explain the evolution of the observable universe from t ...

, 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 Cosmic ray spallation, also known as the x-process, is a set of naturally occurring nuclear reactions causing nucleosynthesis; it refers to the formation of chemical elements from the impact of cosmic rays on an object. Cosmic rays are highly ener ...

, 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 nuclides that are not radioactive and so (unlike radionuclides) do not spontaneously undergo radioactive decay. When such nuclides are referred to in relation to specific elements, they are usually termed stable isotopes. ...

s, plus another 35 nuclides that have half-lives long enough to have survived from the formation of the Earth, occur as primordial nuclides. These 35 primordial radionuclides represent

isotope Isotopes are two or more types of atoms that 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), and that differ in nucleon numbers ( mass number ...

s of 28 separate

elements Element or elements may refer to: Science * Chemical element, a pure substance of one type of atom * Heating element, a device that generates heat by electrical resistance * Orbital elements, parameters required to identify a specific orbit of ...

Cadmium Cadmium is a chemical element with the symbol Cd and atomic number 48. This soft, silvery-white metal is chemically similar to the two other stable metals in group 12, zinc and mercury. Like zinc, it demonstrates oxidation state +2 in most of ...

tellurium Tellurium is a chemical element with the 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 fo ...

xenon Xenon is a chemical element with the 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, samarium, osmium, and

uranium Uranium is a chemical element with the 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. Uranium is weakly ...

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, or core formation, or of the material from which Earth formed. This dating is based on evidence from radiometric age-dating of ...

is (4.6 billion years), the

half-life Half-life (symbol ) is the time required for a quantity (of substance) to reduce to half of its initial value. The term is commonly used in nuclear physics to describe how quickly unstable atoms undergo radioactive decay or how long stable at ...

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 four shortest-lived primordial nuclides (i.e., the nuclides with the shortest half-lives) to have been experimentally verified are (), (), (), and (). These are the four 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 time elapsed since the Big Bang. Astronomers have derived two different measurements of the age of the universe: a measurement based on direct observations of an early state of the universe, ...

. (²³²Th has a half life slightly longer than the age of the universe.) For a complete list of the 35 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 these is , with a half-life of . It has been reported to exist in nature as a primordial nuclide, although a later study did not detect it. The second-longest-lived isotope not proven to be primordial is , which has a half-life of , about double that of the third-longest-lived such isotope (). Taking into account that all these nuclides must exist for at least , ²⁴⁴Pu must survive 57 half-lives (and hence be reduced by a factor of 2⁵⁷ ≈ ), ¹⁴⁶Sm must survive 67 (and be reduced by 2⁶⁷ ≈ ), and ⁹²Nb must survive 130 (and be reduced by 2¹³⁰ ≈ ). Mathematically, considering the likely initial abundances of these nuclides, primordial ²⁴⁴Pu and ¹⁴⁶Sm 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 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 element A chemical element is a species of atoms that have a given number of protons in their nuclei, including the pure substance consisting only of that species. Unlike chemical compounds, chemical elements cannot be broken down into simpler sub ...

s. Of these, 80 have at least one

observationally stable Stable nuclides are nuclides that are not radioactive and so (unlike radionuclides) do not spontaneously undergo radioactive decay. When such nuclides are referred to in relation to specific elements, they are usually termed stable isotopes. T ...

isotope and three additional primordial elements have only radioactive isotopes (

bismuth Bismuth is a chemical element with the 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 ...

thorium Thorium is a weakly radioactive metallic chemical element with the symbol Th and atomic number 90. Thorium is silvery and tarnishes black when it is exposed to air, forming thorium dioxide; it is moderately soft and malleable and has a high ...

, 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 (in the case of cosmogenic nuclides 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, ...

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 with the symbol Rn and atomic number 86. It is a radioactive, colourless, odourless, tasteless noble gas. It occurs naturally in minute quantities as an intermediate step in the normal radioactive decay chains through ...

polonium Polonium is a chemical element with the symbol Po and atomic number 84. Polonium is a chalcogen. A rare and highly radioactive metal with no stable isotopes, polonium is chemically similar to selenium and tellurium, though its metallic characte ...

, and

radium Radium is a chemical element with the symbol Ra and atomic number 88. It is the sixth element in group 2 of the periodic table, also known as the alkaline earth metals. Pure radium is silvery-white, but it readily reacts with nitrogen (rathe ...

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

argon Argon is a chemical element with the 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 a ...

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 gas or layers of gases that envelop a planet, and is held in place by the gravity of the planetary body. A planet retains an atmosphere when the gravity is great and the temperature of the atmosphere is low. ...

, which is regenerated by the

beta decay In nuclear physics, beta decay (β-decay) is a type of radioactive decay in which a beta particle (fast energetic electron or positron) is emitted from an atomic nucleus, transforming the original nuclide to an isobar of that nuclide. For e ...

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 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, the decay chain refers to a series of radioactive decays of different radioactive decay products as a sequential series of transformations. It is also known as a "radioactive cascade". Most radioisotopes do not decay dire ...

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 that is found only in very heavy chemical elements. The nuclear binding energy of the elements reaches its maximum at an atomic mass number of about 56 (e.g., iron-56); spontaneous breakd ...

of one of these three long-lived nuclides, such as ¹²⁶Sn, which makes up about 10⁻¹⁴ of all natural tin.

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, with the exceptions of technetium (43) and

promethium Promethium is a chemical element with the symbol Pm and atomic number 61. All of its isotopes are radioactive; it is extremely rare, with only about 500–600 grams naturally occurring in Earth's crust at any given time. Promethium is one of onl ...

(61)—and three radioactive primordial ''elements''—bismuth (83), thorium (90), and uranium (92). 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 smaller than the number of nuclides, because many of the primordial elements are represented by multiple isotopes. See

for more information.

Naturally occurring stable nuclides

As noted, these number about 251. For a list, see the article list of elements by stability of isotopes. For a complete list noting which of the "stable" 251 nuclides may be in some respect unstable, see list of nuclides 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

Although 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 consi ...

(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 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 (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

s—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. 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 nuclides represent radioisotopes of 28 distinct chemical elements (cadmium, neodymium, osmium, 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 Indium is a chemical element with the symbol In and atomic number 49. Indium is the softest metal that is not an alkali metal. It is a silvery-white metal that resembles tin in appearance. It is a post-transition metal that makes up 0.21 par ...

, and rhenium, a very long-lived radioactive primordial nuclide is found in greater abundance than a stable nuclide. The longest-lived radionuclide, ¹²⁸Te, has a half-life of , which is 160 trillion times the

age of the Universe In physical cosmology, the age of the universe is the time elapsed since the Big Bang. Astronomers have derived two different measurements of the age of the universe: a measurement based on direct observations of an early state of the universe, ...

. Only four of these 35 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 703.8 million years, about one sixth of the age of the Earth and the

Solar System The Solar System Capitalization 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 "Solar ...

. Many of these nuclides decay by double beta decay, although some like ²⁰⁹Bi decay by other methods such as

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

. At the end of the list, two more nuclides have been added: ²⁴⁴Pu and ¹⁴⁶Sm. 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
(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, , ¹³⁰Te, , 8.766578, , , , align=center, 2 β⁻ , , align=center, .868, , align=center, 640 million , - , , 264, , ¹⁵⁰Nd, , 8.562594, , , , align=center, 2 β⁻ , , align=center, 3.367, , align=center, 570 million , - , , 265, , ¹⁰⁰Mo, , 8.933167, , , , align=center, 2 β⁻ , , align=center, 3.035, , align=center, 570 million , - , , 266, , ¹⁵¹Eu, , 8.565759, , , , align=center, α , , align=center, 1.9644, , align=center, 360 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, , ¹¹³Cd, , 8.859372, , , , align=center, β⁻ , , align=center, .321, , align=center, 560,000 , - , , 270, , ¹⁴⁸Sm, , 8.607423, , , , align=center, α , , align=center, 1.986, , align=center, 510,000 , - , , 271, , ¹⁴⁴Nd, , 8.652947, , , , align=center, α , , align=center, 1.905, , align=center, 170,000 , - , , 272, , ¹⁸⁶Os, , 8.302508, , , , align=center, α , , align=center, 2.823, , align=center, 150,000 , - , , 273, , ¹⁷⁴Hf, , 8.392287, , , , align=center, α , , align=center, 2.497, , 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, 8000 , - , , 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, 47 , - , , 278, , ¹⁴⁷Sm, , 8.610593, , , , align=center, α , , align=center, 2.310, , align=center, 7.7 , - , , 279, , ¹³⁸La, , 8.698320, , , , align=center, K or β⁻ , , align=center, 1.737, 1.044, , 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, 0.3 , - , , 285, , ⁴⁰K, , 8.909707, , , , align=center, β⁻ or K or β⁺ , , align=center, 1.311, 1.505, 1.505, , align=center, 0.09 , - , , 286, , ²³⁵U, , 7.897198, , , , align=center, α or SF , , align=center, 4.679, , align=center, 0.05 , - bgcolor=#ffc0c0 , , 287, , ²⁴⁴Pu, , 7.826221, , , , align=center, α or SF , , align=center, 4.666, , align=center, 0.006 , - bgcolor=#ffc0c0 , , 288, , ¹⁴⁶Sm, , 8.626136, , , , align=center, α , , align=center, 2.529, , align=center, 0.005

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References

{{reflist Geochemistry Radiometric dating Isotopes Metrology