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Relative abundance of elements.png

In chemistry, 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 to 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 billion years ago the NA of 235U was 3.1% compared to today's 0.7%, and for that reason a natural nuclear fission reactor was able 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 by natural radioactive isotopes (as in the case of radiogenic isotopes of lead).

Deviations from natural abundance[edit]

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 mil or ‰‰) because they are less than one percent (%). The single exception to this lies with the presolar grains found in primitive meteorites. These bypassed the homogenization, and often carry the nuclear signature of specific nucleosynthesis processes in which their elements were made.[4] In these materials, deviations from "natural abundance" are sometimes measured in factors of 100.

See also[edit]

Footnotes and References[edit]

  1. ^ Uranium Isotopes, retrieved 14 March 2012 
  2. ^ Robert N. Clayton (1978) Isotopic anomalies in the early solar system, Annual Review of Nuclear and Particle Science 28:501–522.
  3. ^ Ernst Zinner (2003) An isotopic view of the early solar system, Science 300:5617, 265–267.
  4. ^ Ernst Zinner (1998) Stellar nucleosynthesis and the isotopic composition of presolar grains from primitive meteorites, Annual Review of Earth and Planetary Sciences 26:147–188.

External links[edit]

Original courtesy of Wikipedia: http://en.wikipedia.org/wiki/Natural_abundance — Please support Wikipedia.
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445 news items

Montreal Gazette

Montreal Gazette
Fri, 17 Apr 2015 13:03:45 -0700

Unfortunately it had a natural abundance of only 0.7 per cent and technology to separate it from the far more abundant but seemingly useless U-238 was not available. Still, since U-235 releases neutrons through natural radioactivity, the possibility ...
Oregon Natural Resources Report
Mon, 13 Apr 2015 05:06:17 -0700

America has rewritten the rules of the global energy economy, thanks in large part to the tremendous surge in production of shale oil and natural gas. And now there is a growing body of evidence that exporting our natural abundance of energy resources ...


Wed, 15 Apr 2015 07:03:45 -0700

Such a reactor can function with natural abundance uranium because the heavy water slows down neutrons, increasing the chance that they will cause fission when they encounter the relatively rare uranium-235 atoms. But in such a plant, slow neutrons can ...


Thu, 09 Apr 2015 14:22:55 -0700

Strain said that Louisiana and the Southeast's natural abundance of water, if managed properly, helps give the region a huge advantage in the future. "Water is going to be the number one commodity that will determine productivity, economic growth and ...

EurekAlert (press release)

EurekAlert (press release)
Wed, 15 Apr 2015 05:56:15 -0700

Mapping monoclonal antibody structure by 2D 13C NMR at natural abundance. Analytical Chemistry, 87: 3556-3561 (2015). DOI: 10.1021/ac504804m. ** The monoclonal antibody used in this experiment is NISTmAb, an immunoglobulin G type 1 donated by ...


Mon, 13 Apr 2015 06:53:34 -0700

457-466), which concluded: “Given the low natural abundance of PEA it the plant materials, it appears nearly impossible to achieve the amounts of [BM]PEA found in the dietary supplements by formulating them with plant material or extracts of A. rigidula.
The Australian
Fri, 03 Apr 2015 06:05:28 -0700

We're so fortunate to be surrounded by such natural abundance here, so close. Even in Sydney you don't have to go far to find a vista of what the land must have looked like before white settlement. “Scholars, I plead with you, Where are your ...
Press Release Rocket
Thu, 16 Apr 2015 08:22:30 -0700

Presently, stability in the supply of rare earth permanent magnets has led the market to recover its losses and regain its stability. China being the leading manufacturer of Permanent Magnet (PM) motors, owing to the country's natural abundance in rare ...

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