In physics, natural abundance (NA) refers to the abundance of isotopes of a chemical element as naturally found on a planet. The relative atomic mass (a weighted average, weighted by mole-fraction abundance figures) of these isotopes is the atomic weight listed for the element in the periodic table. The abundance of an isotope varies from planet to planet, and even from place to place on the Earth, but remains relatively constant in time (on a short-term scale).
As an example, uranium has three naturally occurring isotopes: 238U, 235U, and 234U. Their respective natural mole-fraction abundances are 99.2739–99.2752%, 0.7198–0.7202%, and 0.0050–0.0059%.[1] For example, if 100,000 uranium atoms were analyzed, one would expect to find approximately 99,274 238U atoms, approximately 720 235U atoms, and very few (most likely 5 or 6) 234U atoms. This is because 238U is much more stable than 235U or 234U, as the half-life of each isotope reveals: 4.468 × 109 years for 238U compared with 7.038 × 108 years for 235U and 245,500 years for 234U.
Exactly because the different uranium isotopes have different half-lives, when the Earth was younger, the isotopic composition of uranium was different. As an example, 1.7×109 years ago the NA of 235U was 3.1% compared with today's 0.7%, and that allowed a natural nuclear fission reactor to form, something that cannot happen today.
However, the natural abundance of a given isotope is also affected by the probability of its creation in nucleosynthesis (as in the case of samarium; radioactive 147Sm and 148Sm are much more abundant than stable 144Sm) and by production of a given isotope as a daughter of natural radioactive isotopes (as in the case of radiogenic isotopes of lead).
It is now known from study of the Sun and primitive meteorites that the 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 (see the article on mass-independent fractionation) plus a limited number of nuclear decay and transmutation processes.[2] There is also evidence for injection of short-lived (now-extinct) isotopes from a nearby supernova explosion that may have triggered solar nebula collapse.[3] Hence deviations from natural abundance on Earth are often measured in parts per thousand (per mille or ‰) because they are less than one percent (%).
An exception to this lies with the presolar grains found in primitive meteorites. These small grains condensed in the outflows of evolved ("dying") stars and escaped the mixing and homogenization processes in the interstellar medium and the solar accretion disk (also known as the solar nebula or protoplanetary disk).[4][clarification needed] As stellar condensates ("stardust"), these grains carry the isotopic signatures of specific nucleosynthesis processes in which their elements were made.[5] In these materials, deviations from "natural abundance" are sometimes measured in factors of 100.[citation needed][4]
Natural isotope abundance of some elementsEdit
The next table gives the terrestrial isotope distributions for some elements. Some elements, such as phosphorus and fluorine, only exist as a single isotope, with a natural abundance of 100%.
Natural isotope abundance of some elements on Earth[6]
^Zinner, Ernst (2003). "An isotopic view of the early solar system". Science. 300 (5617): 265–267. doi:10.1126/science.1080300. PMID 12690180. S2CID 118638578.
^ abAnders, Edward; Zinner, Ernst (1993). "Interstellar Grains in Primitive Meteorites: Diamond, Silicon Carbide, and Graphite". Meteoritics. 28 (4): 490–514. Bibcode:1993Metic..28..490A. doi:10.1111/j.1945-5100.1993.tb00274.x.
^Zinner, Ernst (1998). "Stellar nucleosynthesis and the isotopic composition of presolar grains from primitive meteorites". Annual Review of Earth and Planetary Sciences. 26: 147–188. Bibcode:1998AREPS..26..147Z. doi:10.1146/annurev.earth.26.1.147.
^Lide, D. R., ed. (2002). CRC Handbook of Chemistry and Physics (83rd ed.). Boca Raton, FL: CRC Press. ISBN0-8493-0483-0.
External linksEdit
(archived 2015)
Exact Masses of the Elements and Isotopic Abundances, Scientific Instrument Services
(archived 2011)
October 15, 2023
natural, abundance, physics, natural, abundance, refers, abundance, isotopes, chemical, element, naturally, found, planet, relative, atomic, mass, weighted, average, weighted, mole, fraction, abundance, figures, these, isotopes, atomic, weight, listed, element. In physics natural abundance NA refers to the abundance of isotopes of a chemical element as naturally found on a planet The relative atomic mass a weighted average weighted by mole fraction abundance figures of these isotopes is the atomic weight listed for the element in the periodic table The abundance of an isotope varies from planet to planet and even from place to place on the Earth but remains relatively constant in time on a short term scale As an example uranium has three naturally occurring isotopes 238U 235U and 234U Their respective natural mole fraction abundances are 99 2739 99 2752 0 7198 0 7202 and 0 0050 0 0059 1 For example if 100 000 uranium atoms were analyzed one would expect to find approximately 99 274 238U atoms approximately 720 235U atoms and very few most likely 5 or 6 234U atoms This is because 238U is much more stable than 235U or 234U as the half life of each isotope reveals 4 468 109 years for 238U compared with 7 038 108 years for 235U and 245 500 years for 234U Exactly because the different uranium isotopes have different half lives when the Earth was younger the isotopic composition of uranium was different As an example 1 7 109 years ago the NA of 235U was 3 1 compared with today s 0 7 and that allowed a natural nuclear fission reactor to form something that cannot happen today However the natural abundance of a given isotope is also affected by the probability of its creation in nucleosynthesis as in the case of samarium radioactive 147Sm and 148Sm are much more abundant than stable 144Sm and by production of a given isotope as a daughter of natural radioactive isotopes as in the case of radiogenic isotopes of lead Contents 1 Deviations from natural abundance 2 Natural isotope abundance of some elements 3 See also 4 References 5 External linksDeviations from natural abundance EditIt is now known from study of the Sun and primitive meteorites that the 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 see the article on mass independent fractionation plus a limited number of nuclear decay and transmutation processes 2 There is also evidence for injection of short lived now extinct isotopes from a nearby supernova explosion that may have triggered solar nebula collapse 3 Hence deviations from natural abundance on Earth are often measured in parts per thousand per mille or because they are less than one percent An exception to this lies with the presolar grains found in primitive meteorites These small grains condensed in the outflows of evolved dying stars and escaped the mixing and homogenization processes in the interstellar medium and the solar accretion disk also known as the solar nebula or protoplanetary disk 4 clarification needed As stellar condensates stardust these grains carry the isotopic signatures of specific nucleosynthesis processes in which their elements were made 5 In these materials deviations from natural abundance are sometimes measured in factors of 100 citation needed 4 Natural isotope abundance of some elements EditThe next table gives the terrestrial isotope distributions for some elements Some elements such as phosphorus and fluorine only exist as a single isotope with a natural abundance of 100 Natural isotope abundance of some elements on Earth 6 Isotope nat abundance atomic mass1H 99 985 1 0078252H 0 015 2 014012C 98 89 12 formerly by definition 13C 1 11 13 0033514N 99 64 14 0030715N 0 36 15 0001116O 99 76 15 9949117O 0 04 16 9991318O 0 2 17 9991628Si 92 23 27 9769329Si 4 67 28 9764930Si 3 10 29 9737632S 95 0 31 9720733S 0 76 32 9714634S 4 22 33 9678635Cl 75 77 34 9688537Cl 24 23 36 9659079Br 50 69 78 918381Br 49 31 80 9163See also EditAbundance of the chemical elements Decay product Isotope Presolar grains RadionuclideReferences Edit Uranium Isotopes GlobalSecurity org Retrieved 14 March 2012 Clayton Robert N 1978 Isotopic anomalies in the early solar system Annual Review of Nuclear and Particle Science 28 501 522 Bibcode 1978ARNPS 28 501C doi 10 1146 annurev ns 28 120178 002441 Zinner Ernst 2003 An isotopic view of the early solar system Science 300 5617 265 267 doi 10 1126 science 1080300 PMID 12690180 S2CID 118638578 a b Anders Edward Zinner Ernst 1993 Interstellar Grains in Primitive Meteorites Diamond Silicon Carbide and Graphite Meteoritics 28 4 490 514 Bibcode 1993Metic 28 490A doi 10 1111 j 1945 5100 1993 tb00274 x Zinner Ernst 1998 Stellar nucleosynthesis and the isotopic composition of presolar grains from primitive meteorites Annual Review of Earth and Planetary Sciences 26 147 188 Bibcode 1998AREPS 26 147Z doi 10 1146 annurev earth 26 1 147 Lide D R ed 2002 CRC Handbook of Chemistry and Physics 83rd ed Boca Raton FL CRC Press ISBN 0 8493 0483 0 External links EditBerkeley Isotopes Project Interactive Table archived 2015 Exact Masses of the Elements and Isotopic Abundances Scientific Instrument Services Tools to compute low and high precision isotopic distribution archived 2011 Retrieved from https en wikipedia org w index php title Natural abundance amp oldid 1171090969, wikipedia, wiki, book, books, library,
