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Natural abundance
Natural abundance
Natural abundance
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Relative abundance of elements in the Earth's upper crust

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 billion 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 billion 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

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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

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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]
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

See also

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References

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Natural abundance

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Natural abundance, in the of chemistry, refers to the relative proportions or percentages in which the isotopes of a given occur naturally on . These proportions vary by element and are determined by the stable isotopic compositions found in the , atmosphere, and . For most elements, only a few isotopes are stable and contribute significantly to the natural abundance, with the rest being radioactive and present in trace amounts. The natural abundance of isotopes is essential for calculating the of an element, which is the weighted average of the masses of its isotopes based on their relative abundances. For example, carbon has two primary stable isotopes: (¹²C) at approximately 98.9% abundance and (¹³C) at about 1.1%, resulting in an atomic weight of roughly 12.011 u. Similarly, consists mainly of chlorine-35 (³⁵Cl) at 75.77% and chlorine-37 (³⁷Cl) at 24.23%. These values are not uniform globally and can exhibit slight variations due to isotopic processes influenced by physical, chemical, and biological factors. Natural abundances are experimentally determined using , which separates isotopes based on their mass-to-charge ratios and quantifies their relative intensities. This measurement is crucial for applications in , where isotopic ratios affect signal intensities in techniques like (NMR) spectroscopy and (MS). Beyond basic atomic properties, natural isotopic abundances enable tracing environmental processes, such as in for studying Earth's cycles or in for metabolic pathway analysis. They also underpin practical uses, including with isotopes like (which has a very low natural abundance of about 10⁻¹⁰%) and medical imaging with stable or radioactive tracers.

Fundamentals

Definition and Scope

Natural abundance refers to the isotopic abundance of a specified of an element as found in nature, representing the average fractional abundance of each isotope in the Earth's crust, atmosphere, and oceans. This is typically expressed as a or atomic fraction, reflecting the relative proportions of isotopes in normal terrestrial materials. The scope of natural abundance encompasses stable isotopes and long-lived radioactive isotopes that persist in significant amounts in nature, which constitute the vast majority of naturally occurring atoms for most elements, while excluding short-lived radioactive nuclides with negligible presence due to rapid decay. It distinctly differs from synthetic or artificially enriched isotopes produced in laboratories or nuclear reactors, focusing solely on primordial or cosmogenic isotopes persisting in the environment. A key example is hydrogen, where the natural abundance consists of approximately 99.9885% ¹H (protium), 0.0115% ²H (deuterium), and trace amounts of ³H (tritium). Mathematically, the abundance of isotope ii is calculated as Abundance of i=(number of atoms of itotal number of atoms of the element)×100%.\text{Abundance of } i = \left( \frac{\text{number of atoms of } i}{\text{total number of atoms of the element}} \right) \times 100\%. This formula provides the standard metric for quantifying isotopic distributions across elements.

Historical Context

The understanding of natural isotopic abundance began with the early 20th-century recognition of isotopes as variants of elements sharing chemical properties but differing in . In 1913, introduced the term "" to describe such entities, observed primarily in chains where elements exhibited varying atomic weights despite identical chemical behavior, thus establishing the foundational concept for assessing natural abundances in elements. This insight resolved discrepancies in atomic weight measurements from natural sources and highlighted how isotopic mixtures contribute to observed elemental compositions. Building on Soddy's work, Francis invented the mass spectrograph in , enabling the separation and precise quantification of isotopes in non-radioactive elements for the first time. Aston's instrument deflected atomic beams through magnetic and electric fields, revealing isotopic masses near whole numbers and allowing initial abundance determinations, such as approximately 90% ^{20}Ne and 10% ^{22}Ne in . The 1920s and 1930s marked key milestones in establishing standard natural abundances, particularly for light elements, as mass spectrometry matured. Aston's subsequent measurements confirmed isotopes in elements like chlorine (^{35}Cl and ^{37}Cl in an approximate 3:1 ratio) and extended the whole-number rule for atomic masses, providing baseline data that became foundational for chemical tables. These efforts, driven by improved spectrograph designs, shifted focus from radioactive to stable isotopes, quantifying their natural distributions with increasing precision. Post-World War II, refinements accelerated through advanced spectrometry techniques developed by figures like Alfred O. C. Nier, who enhanced resolution and sensitivity to measure trace abundances more accurately across the periodic table, reducing uncertainties in standard values for elements like carbon and oxygen. Advancements in during profoundly influenced the study of natural isotopic ratios by providing a contrast to artificial alterations, positioning unaltered abundances as essential baselines for . The 1932 by explained isotopic mass differences without changing chemical identity, while the 1934 induction of artificial by Irène and demonstrated how nuclear reactions could shift ratios, emphasizing the stability of natural distributions in geochemical tracing. Harold Urey's 1932 isolation of further bridged and , revealing natural hydrogen isotopic variations (D/H ≈ 0.000155) that served as markers for environmental processes. By the 1950s, the International Union of Pure and Applied Chemistry's Commission on Atomic Weights (later the Commission on Isotopic Abundances and Atomic Weights) formalized the reporting of abundances, issuing standardized tables that accounted for isotopic compositions to ensure consistency in atomic weight calculations. This effort, building on prior measurements, addressed variations in terrestrial sources and established protocols for updating values based on new data, solidifying abundance as a core parameter in .

Isotopic Composition

Standard Natural Abundances

The standard natural abundances of isotopes refer to the relative proportions of stable isotopes for each element as found in the normal terrestrial environment, averaged across global samples. These values are recommended by the International Union of Pure and Applied Chemistry (IUPAC) through its Commission on Isotopic Abundances and Atomic Weights (CIAAW), with the comprehensive update in 2021 and subsequent revisions up to 2024 representing consensus from high-precision mass spectrometric measurements. They provide a baseline for calculating standard atomic weights and are expressed as atom percent abundances, assuming no significant fractionation in bulk Earth materials. Fractional abundances, denoted as fif_i for each isotope ii, are fundamental in determining the average atomic mass AA of an element via the formula: A=∑i(mi×fi)A = \sum_i (m_i \times f_i) where mim_i is the atomic mass of isotope ii in atomic mass units (u), and the sum is over all stable isotopes. This weighted average underpins periodic table values and isotopic ratio standards in chemistry and geochemistry. The following table summarizes the standard isotopic abundances for elements 1 through 10 (hydrogen to neon), highlighting the dominant stable isotopes. Data are drawn from NIST evaluations aligned with IUPAC recommendations, using conventional single-point values for precision in applications.
ElementAtomic NumberIsotopeAbundance (atom %)Atomic Mass (u)
Hydrogen (H)1¹H99.98851.007825
²H0.01152.014102
Helium (He)2³He0.0001373.016029
⁴He99.9998634.002603
Lithium (Li)3⁶Li7.596.015123
⁷Li92.417.016004
Beryllium (Be)4⁹Be1009.012183
Boron (B)5¹⁰B19.910.012937
¹¹B80.111.009305
Carbon (C)6¹²C98.9312 (exact)
¹³C1.0713.003355
Nitrogen (N)7¹⁴N99.63614.003074
¹⁵N0.36415.000109
Oxygen (O)8¹⁶O99.75715.994915
¹⁷O0.03816.999132
¹⁸O0.20517.999160
Fluorine (F)9¹⁹F10018.998403
Neon (Ne)10²⁰Ne90.4819.992440
²¹Ne0.2720.993847
²²Ne9.2521.991385
In contrast to lighter elements, which typically feature one or two dominant isotopes comprising over 90% abundance each, heavier elements like the rare earths (lanthanides) often exhibit greater complexity, with seven or more stable isotopes per element, each having abundances generally below 10%. For example, recent 2024 IUPAC revisions updated the standard atomic weights of and based on refined isotopic abundance evaluations. These standard values serve as the reference for isotopic studies, though minor deviations can occur in specific geological or biological reservoirs.

Elemental Variations

The natural isotopic abundances of elements exhibit systematic variations across the periodic table, primarily driven by nuclear stability considerations. For lighter elements with atomic numbers Z < 20, such as hydrogen and oxygen, there are typically few stable isotopes, often just one or two, with a single dominant isotope comprising over 90% of the total abundance. For instance, ¹H accounts for 99.9885% of hydrogen, while ¹⁶O makes up 99.757% of oxygen. In contrast, heavier elements with Z > 50, like lead, possess more stable isotopes—up to four or more—with abundances distributed more evenly, reflecting broader ranges of nuclear configurations that achieve stability. Lead, for example, has four isotopes with abundances ranging from 1.4% for ²⁰⁴Pb to 52.4% for ²⁰⁸Pb. These trends arise because lighter nuclei require fewer neutrons to balance proton repulsion, leading to limited viable isotopic forms, whereas heavier nuclei demand increasing neutron-to-proton ratios for stability, allowing multiple isotopes to persist. Nuclear factors further modulate these abundances, with the odd-even rule and playing key roles. Elements with odd atomic numbers Z tend to have fewer stable isotopes compared to even-Z elements, as odd proton counts result in unpaired nucleons that reduce overall and stability; only a handful of odd-Z elements, like (Z=15), have more than one stable isotope. —specific counts of protons or neutrons (2, 8, 20, 28, 50, 82, 126)—correspond to filled nuclear shells, enhancing stability and leading to abundance peaks for isotopes achieving these configurations. Isotopes with both even Z and even neutron number N (even-even nuclei) are particularly favored, as they benefit from pairing effects that increase . These elemental variations in isotopic abundances are ultimately tied to stellar nucleosynthesis processes that govern element formation. The primordial abundances of and primarily stem from , where rapid expansion limited fusion to these light elements, resulting in their overwhelming dominance (e.g., ~75% H and ~25% He by mass in the early universe). Heavier elements, including those with multiple isotopes, were produced later through stellar fusion and explosive events like supernovae, which synthesize nuclei up to iron via processes such as the rapid neutron-capture (r-process) and slow neutron-capture (), distributing neutrons to create diverse stable isotopes. A representative example is iron (Z=26), which has five stable isotopes reflecting even-even nuclear stability. The abundances range from 5.8% for ⁵⁴Fe to 91.7% for ⁵⁶Fe, with ⁵⁶Fe's dominance attributed to its even-even configuration (26 protons, 30 neutrons) near the iron peak of , where nuclei are most stable against both fusion and fission. This distribution underscores how favors isotopes at energy minima, contributing to iron's overall cosmic prevalence.

Deviations and Variations

Causes of Isotopic Deviations

Isotopic deviations from standard natural abundances arise primarily through physical, chemical, and nuclear processes that preferentially partition between phases or alter their production rates. These mechanisms lead to measurable variations in isotope ratios within specific environmental samples or reservoirs, deviating from the globally averaged compositions established over geological timescales. Geochemical processes, such as fractional crystallization and , drive significant isotopic fractionations by exploiting mass-dependent differences in bonding energies or rates. In fractional crystallization, during magma cooling, heavier isotopes like ¹⁸O become enriched in early-forming minerals relative to the remaining melt, as lighter isotopes partition more readily into the liquid phase. Similarly, from aqueous systems enriches the residual liquid in heavier isotopes; for instance, progressive of leads to higher ¹⁸O/¹⁶O ratios in the concentrate due to the preferential loss of lighter . These equilibrium and kinetic effects are quantified by the isotope fractionation factor α\alpha, defined as α=RsampleRstandard,\alpha = \frac{R_{\text{sample}}}{R_{\text{standard}}}, where RR represents the ratio of heavy to light isotopes in the respective phases, typically close to unity but varying with and composition. Biological fractionation occurs through enzymatic and metabolic processes that favor lighter isotopes due to their lower zero-point energies, resulting in depleted heavy isotope contents in biomass. Organisms preferentially incorporate ¹²C over ¹³C during and other carbon fixation pathways, leading to biomass with δ¹³C values typically 20–30‰ lower than source materials. This kinetic discrimination extends to other elements, such as , where lighter ¹⁴N is favored in , altering ratios in biological tissues relative to inorganic reservoirs. Nuclear processes introduce deviations via production or decay that are independent of chemical . Cosmic ray in the upper atmosphere fragments and oxygen nuclei, producing trace amounts of ¹⁰Be that accumulate in sediments and cores, with production rates varying by and solar activity. Radiogenic decay, such as the of ⁸⁷Rb to ⁸⁷Sr, progressively increases the ⁸⁷Sr/⁸⁶Sr ratio in minerals and rocks over time, depending on the initial Rb/Sr ratio and age. Anthropogenic influences on isotopic abundances were negligible before 1950, but atmospheric nuclear testing in the mid-20th century dramatically elevated global ¹⁴C levels by injecting radiocarbon directly into the atmosphere, doubling concentrations by the early 1960s before declining due to the 1963 test ban treaty.

Observed Deviations in Nature

In geological settings, carbonate rocks often exhibit elevated ¹³C/¹²C ratios compared to , a deviation arising from biological during ancient , where autotrophs preferentially incorporate lighter ¹²C into , leaving relatively enriched in ¹³C that then precipitates as carbonates. This results in typical δ¹³C values of around 0‰ for marine carbonates versus -20‰ to -30‰ for associated kerogens, providing a preserved over billions of years. Similarly, helium isotope ratios in volcanic gases show deviations from atmospheric norms, with elevated ³He/⁴He (up to 30–40 times the atmospheric ratio of ~1.4 × 10⁻⁶) signaling input from primordial mantle plumes rather than radiogenic crustal sources. These high ratios, observed in hotspots like and , indicate deeper mantle upwelling and help delineate plume dynamics. Atmospheric displays significant isotopic enrichment in ¹⁸O, with δ¹⁸O values exceeding +20‰ relative to source oxygen, primarily due to mass-dependent during UV photolysis, where lighter isotopologues dissociate more readily, concentrating heavier isotopes in the remaining O₃. This anomaly, most pronounced in the , extends to mass-independent effects for ¹⁷O, influencing the broader . In contrast, atmospheric ¹⁴C experiences seasonal variations, with Δ¹⁴C declining by up to 20‰ in winter due to the —dilution by ¹⁴C-free CO₂ from fossil fuel combustion, amplified by seasonal emission patterns and reduced biospheric exchange. These fluctuations, observed globally since the mid-20th century, reflect anthropogenic impacts on the . In oceanic environments, nitrate δ¹⁵N exhibits depth gradients, increasing from ~5‰ at the surface to over 20‰ in mid-depth oxygen minimum zones (150–400 m), driven by denitrification, which preferentially reduces lighter ¹⁴N, enriching the residual nitrate pool. This isotopic signal, prominent in regions like the Arabian Sea and Eastern Tropical Pacific, traces nitrogen loss and redox conditions, with gradients persisting into deeper waters via water mass mixing. Extraterrestrial materials, such as lunar grains, show deviations in oxygen isotopes, with underabundance of ¹⁶O (δ¹⁶O depleted by ~50‰ relative to terrestrial standards) attributed to implantation and from particles, which preferentially deliver heavier isotopes during and reimplantation processes. A remarkable example of isotopic deviation is the Oklo natural in , where ores from ~2 billion years ago display depleted ²³⁵U abundances of 0.3–0.6% (versus the current natural standard of 0.720%), resulting from ancient fission reactions that consumed fissile ²³⁵U beyond expected . In reactor zone 13, the ²³⁵U/²³⁸U ratio reaches as low as 0.3655%, confirming self-sustaining criticality under past geochemical conditions. These deviations highlight rare natural nuclear processes and inform .

Measurement and Applications

Techniques for Determination

The determination of natural isotopic abundances has evolved significantly since the early , beginning with Francis Aston's development of the mass spectrograph in , which enabled the first precise measurements of atomic masses and the discovery of isotopes in elements like . This instrument used magnetic deflection of positive ions to separate species by mass-to-charge ratio, achieving resolutions sufficient to distinguish isotopes differing by about 1 part in 300. Subsequent refinements, such as Aston's later designs incorporating photographic detection, laid the foundation for quantitative isotopic analysis. Modern systems, including mass spectrometers, now provide ultra-high resolution exceeding 100,000, allowing separation of fine isotopic structures in complex mixtures without prior purification. Mass spectrometry remains the cornerstone for accurate isotopic abundance measurements, with thermal ionization mass spectrometry (TIMS) particularly suited for solid samples like minerals and metals. In TIMS, samples are loaded onto a heated filament, where ionizes atoms with high for elements like and , enabling isotope ratio precisions of 0.001% or better through multi-collector detection that simultaneously measures multiple to minimize effects. For liquid or dissolved samples, (ICP-MS) offers versatility, ionizing aerosols in a high-temperature plasma and achieving similar precisions of around 0.001% for ratios in elements such as and lead when using multi-collector configurations. Complementary techniques address specific challenges, such as (NMR) for isotopes in organic compounds. Natural abundance ²H NMR exploits the low of to quantify ¹H/²H ratios directly in solution, providing site-specific abundances with precisions typically around 1-5% relative standard deviation, though it requires longer acquisition times due to low sensitivity. For in-situ analysis of solids like rocks or tissues, ICP-MS ablates material with a focused beam, transporting particles to the plasma for ionization, which allows spatially resolved isotopic measurements with precisions of 0.1-0.5% without sample preparation. Calibration against international standards ensures and comparability across measurements. For carbon isotopes, the Vienna Pee Dee Belemnite (VPDB) standard, derived from a belemnite , defines the δ¹³C scale with an assigned ¹³C/¹²C ratio of 0.011113 ± 0.000022 (95% confidence), facilitating corrections for mass-dependent in techniques like . () extends detection to ultra-trace levels, particularly for cosmogenic isotopes like ¹⁴C, achieving sensitivities down to attomolar concentrations (equivalent to ¹⁴C/C ratios of ~10⁻¹⁵) by accelerating ions to MeV energies to eliminate molecular interferences and count individual atoms. These methods collectively enable the identification of subtle isotopic deviations in natural samples, such as those arising from environmental processes.

Practical Uses in Science

In geochronology, the ratio of ²³⁸U to ²³⁵U serves as a foundation for U-Pb dating, enabling precise age determinations of geological materials such as zircon crystals from ancient rocks, which track Earth's formative processes over billions of years. This method leverages the distinct decay paths of ²³⁸U to ²⁰⁶Pb (half-life 4.468 billion years) and ²³⁵U to ²⁰⁷Pb (half-life 704 million years), producing concordant ages that resolve events like continental crust formation. Similarly, the decay of ⁸⁷Rb to ⁸⁷Sr (half-life 48.8 billion years) allows Rb-Sr dating to establish chronologies for igneous and metamorphic rocks, as demonstrated in analyses of the Duluth Gabbro Complex, yielding ages around 1.1 billion years that align with broader tectonic histories. In , variations in the δ¹⁸O isotope ratio within cores provide a proxy for reconstructing past temperatures, as heavier ¹⁸O preferentially condenses in colder conditions, preserving signals of glacial-interglacial cycles. By combining δ¹⁸O with excess measurements and models, researchers quantify and source temperatures, revealing effects in records spanning hundreds of thousands of years. Astrophysicists use solar system isotopic abundances, derived from meteorites and solar spectroscopy, to validate models, confirming how processes like the s- and r-processes in stars and supernovae produced heavy elements matching observed ratios. These comparisons refine predictions of chemical in the , linking the solar system's composition to the from which it formed 4.567 billion years ago. Stable isotope ratios find applications in forensics for tracing food origins, where δ¹³C distinguishes C3 from C4 plant sources to detect adulteration, such as unauthorized cane sugar in wines, and δ¹⁸O maps geographical provenance via water signatures. In medicine, stable isotopes like ¹³C and ²H label drugs for studies, enabling non-invasive tracking of pharmacokinetic pathways through without radiation risks, particularly in vulnerable populations like pregnant individuals. A specific example is the ¹³C/¹²C breath test for metabolic disorders, which assesses by measuring the recovery of ¹³CO₂ after ingesting ¹³C-glucose; reduced oxidation in insulin-resistant states correlates with altered breath ratios, offering a non-invasive diagnostic tool for early detection and monitoring.

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