Isotopes of carbon
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Carbon (6C) has 14 known isotopes, from 8C to 20C as well as 22C, of which only 12C and 13C are stable. The longest-lived radioisotope is 14C, with a half-life of 5700 years. This is also the only carbon radioisotope found in nature, as trace quantities are formed cosmogenically by the reaction 14N + n → 14C + 1H. The most stable artificial radioisotope is 11C, which has a half-life of 20.34 minutes. All other radioisotopes have half-lives under 20 seconds, most less than 200 milliseconds. Lighter isotopes exhibit beta-plus decay into isotopes of boron and heavier ones beta-minus decay into isotopes of nitrogen, though at the limits particle emission occurs as well. The two lightest isotopes decay into helium via short-lived isotopes of lithium, beryllium and boron.
List of isotopes
| Nuclide | Z | N | Isotopic mass (Da) | Discovery year | Half-life [resonance width] | Decay mode | Daughter isotope | Spin and parity | Isotopic abundance | |
|---|---|---|---|---|---|---|---|---|---|---|
| 8C | 6 | 2 | 8.037643(20) | 3.5(1.4) zs [230(50) keV] | 2p | 6Be | 0+ | |||
| 9C | 6 | 3 | 9.0310372(23) | 126.5(9) ms | β+ (54.1(1.7)%) | 9B | 3/2− | |||
| β+α (38.4(1.6)%) | 5Li | |||||||||
| β+p (7.5(6)%) | 8Be | |||||||||
| 10C | 6 | 4 | 10.01685322(8) | 19.3011(15) s | β+ | 10B | 0+ | |||
| 11C | 6 | 5 | 11.01143260(6) | 20.3402(53) min | β+ | 11B | 3/2− | |||
| 12C | 6 | 6 | 12 exactly | Stable | 0+ | [0.9884, 0.9904] | ||||
| 13C | 6 | 7 | 13.003354835336(252) | Stable | 1/2− | [0.0096, 0.0116] | ||||
| 14C | 6 | 8 | 14.003241989(4) | 5.70(3)×103 y | β− | 14N | 0+ | Trace | ||
| 15C | 6 | 9 | 15.0105993(9) | 2.449(5) s | β− | 15N | 1/2+ | |||
| 16C | 6 | 10 | 16.014701(4) | 750(6) ms | β−n (99.0(3)%) | 15N | 0+ | |||
| β− (1.0(3)%) | 16N | |||||||||
| 17C | 6 | 11 | 17.022579(19) | 193(6) ms | β− (71.6(1.3)%) | 17N | 3/2+ | |||
| β−n (28.4(1.3)%) | 16N | |||||||||
| β−2n ? | 15N ? | |||||||||
| 18C | 6 | 12 | 18.02675(3) | 92(2) ms | β− (68.5(1.5)%) | 18N | 0+ | |||
| β−n (31.5(1.5)%) | 17N | |||||||||
| β−2n ? | 16N ? | |||||||||
| 19C | 6 | 13 | 19.03480(11) | 46.2(2.3) ms | β−n (47(3)%) | 18N | 1/2+ | |||
| β− (46.0(4.2)%) | 19N | |||||||||
| β−2n (7(3)%) | 17N | |||||||||
| 20C | 6 | 14 | 20.04026(25) | 16(3) ms | β−n (70(11)%) | 19N | 0+ | |||
| β−2n (< 18.6%) | 18N | |||||||||
| β− (> 11.4%) | 20N | |||||||||
| 21C | 6 | 15 | 21.04900(64)# | < 30 ns | n ? | 20C ? | 1/2+# | |||
| 22C | 6 | 16 | 22.05755(25) | 6.2(1.3) ms | β−n (61(14)%) | 21N | 0+ | |||
| β−2n (< 37%) | 20N | |||||||||
| β− (> 2%) | 22N | |||||||||
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Carbon-11
Carbon-11 or 11C is a radioactive isotope of carbon that decays to boron-11 with a half-life to 20.34 minutes. This decay mainly occurs due to positron emission, with around 0.19–0.23% of decays instead occurring by electron capture.
11C → 11B + e+ + νe + 0.96 MeV
11C + e− → 11B + νe + 1.98 MeV
It is produced by hitting nitrogen with protons of around 16.5 MeV in a cyclotron. The causes the endothermic reaction
14N + p → 11C + 4He − 2.92 MeV
It can also be produced by fragmentation of 12C by shooting high-energy 12C at a target.
Carbon-11 is commonly used as a radioisotope for the radioactive labeling of molecules in positron emission tomography. Among the many molecules used in this context are the radioligands [11C]DASB and [11C]Cimbi-5.
Stable isotopes
Carbon-12 and carbon-13 account for approximately 98.9% and 1.1% (respectively) of the naturally occurring carbon on Earth. However, the ratio of stable 13C and 12C in a material can vary due to differences in precursor source and isotopic fractionation induced by a variety of biogeochemical processes. The quantities of the different isotopes are commonly measured via isotope ratio mass spectrometry and expressed as parts per thousand (‰ or "per mille") divergence from the ratio of a standard:
δ C 13 = ( ( C 13 C 12 ) sample ( C 13 C 12 ) standard − 1 ) × 1000 {\displaystyle \delta {\ce {^{13}C}}=\left({\frac {\left({\frac {{\ce {^{13}C}}}{{\ce {^{12}C}}}}\right)_{\text{sample}}}{\left({\frac {{\ce {^{13}C}}}{{\ce {^{12}C}}}}\right)_{\text{standard}}}}-1\right)\times 1000} ‰
Peedee Belemnite ("PDB"), a fossil belemnite, was the original reference standard used for standardizing isotope ratio values. Due to the depletion of the original PDB, artificial "Vienna PDB", or "VPDB", is generally used today.
Paleoclimate
12C and 13C are measured as the isotope ratio δ13C in benthic foraminifera and used as a proxy for nutrient cycling and the temperature dependent air–sea exchange of CO2 (ventilation). Plants find it easier to use the lighter isotope (12C) when they convert sunlight and carbon dioxide into food. For example, large blooms of plankton (free-floating organisms) absorb large amounts of 12C from the oceans. Originally, the 12C was mostly incorporated into the seawater from the atmosphere. If the oceans that the plankton live in are stratified (meaning that there are layers of warm water near the top, and colder water deeper down), then the surface water does not mix very much with the deeper waters, so that when the plankton dies, it sinks and takes away 12C from the surface, leaving the surface layers relatively rich in 13C. Where cold waters well up from the depths (such as in the North Atlantic), the water carries 12C back up with it; when the ocean was less stratified than today, there was much more 12C in the skeletons of surface-dwelling species. Other indicators of past climate include the presence of tropical species and coral growth rings.
Tracing food sources and diets
Different photosynthetic pathways preferentially select for the lighter 12C, but their selectivity differs.[citation needed] Grasses in temperate climates (barley, rice, wheat, rye, and oats, plus sunflower, potato, tomatoes, peanuts, cotton, sugar beet, and most trees and their nuts or fruits, roses, and Kentucky bluegrass) follow a C3 photosynthetic pathway that will yield δ13C values averaging about −26.5‰.[citation needed] Grasses in hot arid climates (maize in particular, but also millet, sorghum, sugar cane, and crabgrass) follow a C4 photosynthetic pathway that produces δ13C values averaging about −12.5‰.
It follows that eating these different plants will affect the δ13C values in the consumer's body tissues. If an animal (or human) eats only C3 plants, their δ13C values will be from −18.5 to −22.0‰ in their bone collagen and −14.5‰ in the hydroxylapatite of their teeth and bones.
In contrast, C4 feeders will have bone collagen with a value of −7.5‰ and hydroxylapatite value of −0.5‰.
In case studies, millet and maize eaters can easily be distinguished from rice and wheat eaters.[citation needed] Studying how these dietary preferences are distributed geographically through time can illuminate migration paths of people and dispersal paths of different agricultural crops. However, human groups have often mixed C3 and C4 plants (northern Chinese historically subsisted on wheat and millet), or mixed plant and animal groups together (for example, southeastern Chinese subsisting on rice and fish).
Carbon-14
Carbon-14 (also called radiocarbon) occurs in trace amounts and has a half-life of 5700 years. The primary source of 14C on Earth is the reaction of 14N with thermal neutrons from cosmic radiation in the upper atmosphere; this mixes throughout the atmosphere, and biological processes such as photosynthesis incorporate the 14C into living organisms. Since organisms stop absorbing 14C upon dying, measurement of the amount of 14C in a sample may be used to estimate its age. This technique is called radiocarbon dating and is one of the principal methods of radiometric dating in the field of archaeology.
See also
Daughter products other than carbon