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[转载]维基百科：同位素天然丰度
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同位素天然丰度
在物理学中，天然丰度（natural abundance，NA）指的是化学元素同位素在行星上自然存在时的丰度。这些同位素的相对原子质量（通过摩尔分数丰度数据加权平均）是周期表中该元素所列的原子量。同位素的丰度会因行星而异，甚至在地球上的不同地点也会有所不同，但在短期内时间上保持相对稳定。
以铀为例，它有三种天然存在的同位素：238U、235U和234U。它们各自的天然摩尔分数丰度分别为99.2739–99.2752％、0.7198–0.7202％和0.0050–0.0059％。例如，如果分析了10万个铀原子，预计会找到约99,274个238U原子，约720个235U原子，以及非常少（很可能是5或6个）的234U原子。这是因为238U比235U或234U稳定得多，每个同位素的半衰期都能表明这一点：238U的半衰期为4.468×109年，而235U的半衰期为7.038×108年，234U的半衰期为245,500年。
正是因为不同的铀同位素具有不同的半衰期，当地球年轻时，铀的同位素组成是不同的。例如，17亿年前，235U的天然丰度是3.1％，而今天是0.7％，这使得自然核裂变反应堆形成了，这是今天无法发生的事情。
然而，特定同位素的天然丰度也受到其在核合成中产生的概率的影响（如钐的情况；放射性147Sm和148Sm比稳定的144Sm丰度要高得多），以及作为自然放射性同位素的衰变产物的产生（如铅的放射性同位素的情况）。
与天然丰度的偏差通过对太阳和原始陨石的研究，现在已知太阳系最初在同位素组成上几乎是均质的。从（演变中的）银河平均值的偏差，通常可以通过质量分馏（参见质量独立分馏的文章）以及有限数量的核衰变和转化过程来解释，这些过程在太阳核燃烧开始时左右的时间内局部抽样了。还有证据表明，来自附近超新星爆炸的短寿命（现已灭绝）同位素的注入可能触发了太阳星云的坍缩。因此，地球上的天然丰度偏差通常以千分之几（‰）来测量，因为它们小于百分之一（%）。
一个例外是在原始陨石中发现的星前颗粒。这些小颗粒在演化（“垂死”）恒星的流出物中凝结而成，并且逃脱了星际介质和太阳凝聚盘（也称为太阳星云或原行星盘）中的混合和均质化过程。作为恒星凝结物（“星尘”），这些颗粒携带了它们的元素在特定核合成过程中形成的同位素特征。在这些材料中，与“天然丰度”的偏差有时以100倍因子（citation needed）来测量。
一些元素的天然同位素丰度 下表给出了一些元素的地球同位素分布。一些元素，如磷和氟，仅存在一种同位素，其天然丰度为100％
Natural abundance- From Wikipedia, the free encyclopedia
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).
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 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[edit]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%.
| Isotope | % nat. abundance | atomic mass |
|---|---|---|
| 1H | 99.985 | 1.007825 |
| 2H | 0.015 | 2.0140 |
| 12C | 98.89 | 12 (formerly by definition) |
| 13C | 1.11 | 13.00335 |
| 14N | 99.64 | 14.00307 |
| 15N | 0.36 | 15.00011 |
| 16O | 99.76 | 15.99491 |
| 17O | 0.04 | 16.99913 |
| 18O | 0.2 | 17.99916 |
| 28Si | 92.23 | 27.97693 |
| 29Si | 4.67 | 28.97649 |
| 30Si | 3.10 | 29.97376 |
| 32S | 95.0 | 31.97207 |
| 33S | 0.76 | 32.97146 |
| 34S | 4.22 | 33.96786 |
| 35Cl | 75.77 | 34.96885 |
| 37Cl | 24.23 | 36.96590 |
| 79Br | 50.69 | 78.9183 |
| 81Br | 49.31 | 80.9163 |
^ "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.
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