Natural abundance

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Natural abundance

In chemistry

Chemistry

Chemistry is the science concerned with the composition, structure, and properties of matter, as well as the changes it undergoes during chemical reactions....
, natural abundance (NA) refers to the abundance isotope

Isotope

Isotopes are any of the different types of atoms of the same chemical element, each having a different atomic mass . Isotopes of an element have atomic nucleus with the same number of protons but different numbers of neutron....
s of a chemical element

Chemical element

A chemical element is a type of atom that is distinguished by its atomic number; that is, by the number of protons in its atomic nucleus. The term is also used to refer to a pure chemical Chemical substance composed of atoms with the same number of protons....
as naturally found on a planet. The relative atomic mass (a weighted average) of these isotopes is the atomic weight

Atomic weight

Atomic weight is a Dimensionless quantity physical quantity, the ratio of the average mass of atoms of an chemical element to 1/12 of the mass of an atom of carbon-12....
listed for the element in the periodic table

Periodic table

The periodic table of the chemical elements is a table method of displaying the chemical elements. Although precursors to this table exist, its invention is generally credited to Russian chemist Dmitri Mendeleev in 1869....
. The abundance of an isotope varies from planet to planet but remains relatively constant in time.

As an example, uranium

Uranium

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Encyclopedia

In chemistry

Chemistry

Isotope

Chemical element

Atomic weight

Periodic table

Uranium

Uranium is a silvery-gray metallic chemical element in the actinide series of the periodic table that has the chemical symbol U and atomic number 92....
has three naturally occurring isotopes: ²³⁸U, ²³⁵U and ²³⁴U. Their respective NA is 99.2745%, 0.72% and 0.0055%. For example, if 100,000 uranium atoms were analyzed, one would expect to find approximately 99,275 ²³⁸U atoms, 720 ²³⁵U atoms, and no more than 5 or 6 ²³⁴U atoms. This is because ²³⁸U is much more stable than ²³⁵U or ²³⁴U, as the half-life

Half-life

The half-life of a quantity whose value decreases with time is the interval required for the quantity to decay to half of its initial value. The concept originated in describing how long it takes atoms to undergo radioactive decay but also applies in a wide variety of other situations....
of each isotope reveals: 4.468×10⁹ years for ²³⁸U compared to 7.038×10⁸ years for ²³⁵U and 245,500 years for ²³⁴U. However, the natural abundance is also affected by probability of creation of given isotope in nucleosynthesis

Nucleosynthesis

Nucleosynthesis is the process of creating new atomic nuclei from preexisting nucleons . It is thought that the primordial nucleons themselves were formed from the quark-gluon plasma from the Big Bang as it cooled below ten million degrees....
(as in the case of samarium

Samarium

Samarium is a chemical element with the symbol Sm and atomic number 62....
, when radioactive ¹⁴⁷Sm and ¹⁴⁸Sm are much more abundant then stable ¹⁴⁴Sm) and by production of given isotope by natural radioactive isotopes (as in the case of radiogenic isotopes of lead

Isotopes of lead

Lead has four stable isotopes - 204Pb, 206Pb, 207Pb, 208Pb and one common radiogenic isotope 202Pb with a half-life of ~53,000 years....
)

Deviations from natural abundance

We now know from study of the sun and primitive meteorites that our solar system was initially almost homogeneous in isotopic composition. Deviations from the (evolving) galactic average, locally-sampled around the time that the sun's nuclear burning began, can generally be accounted for by mass fractionation plus a limited number of nuclear decay and transmutation processes. There is also evidence for injection of short-lived (now extinct) isotopes from a nearby supernova explosion that may have triggered solar nebula collapse. Hence deviations from natural abundance on earth are often measured in parts per thousand (per mil or ‰) because they are less than one percent (%).

The single exception to this lies with the presolar grains

Presolar grains

Presolar grains are isotopically-distinct clusters of material found in the fine-grained Matrix of primitive meteorites, whose differences from the surrounding meteorite suggest that they are older than the solar system....
found in primitive meteorites. These bypassed the homogenization, and often carry the nuclear signature of specific nucleosynthesis processes in which their elements were made. In these materials, deviations from "natural abundance" are sometimes measured in factors of 100.

Natural abundance

Deviations from natural abundance

See also

Footnotes and References

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