Carbon (6C) has 14 known isotopes, from 8
C
to 20
C
as well as 22
C
, of which 12
C
and 13
C
are stable. The longest-lived radioisotope is 14
C
, with a half-life of 5.70(3)×103 years. This is also the only carbon radioisotope found in nature, as trace quantities are formed cosmogenically by the reaction 14
N
+
n
→ 14
C
+ 1
H
. The most stable artificial radioisotope is 11
C
, which has a half-life of 20.3402(53) min. All other radioisotopes have half-lives under 20 seconds, most less than 200 milliseconds. The least stable isotope is 8
C
, with a half-life of 3.5(1.4)×10−21 s. Light isotopes tend to decay into isotopes of boron and heavy ones tend to decay into isotopes of nitrogen.
| |||||||||||||||||||||||||||||||
| Standard atomic weight Ar°(C) | |||||||||||||||||||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
List of isotopes
| Nuclide | Z | N | Isotopic mass (Da)[3] [n 1] | Half-life[4] [resonance width] | Decay mode[4] [n 2] | Daughter isotope [n 3] | Spin and parity[4] [n 4][n 5] | Natural abundance (mole fraction) | |||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Normal proportion[4] | Range of variation | ||||||||||||||||||
| 8 C | 6 | 2 | 8.037643(20) | 3.5(1.4) zs [230(50) keV] | 2p | 6 Be [n 6] | 0+ | ||||||||||||
| 9 C | 6 | 3 | 9.0310372(23) | 126.5(9) ms | β+ (54.1(1.7)%) | 9 B | 3/2− | ||||||||||||
| β+α (38.4(1.6)%) | 5 Li [n 7] | ||||||||||||||||||
| β+p (7.5(6)%) | 8 Be [n 8] | ||||||||||||||||||
| 10 C | 6 | 4 | 10.01685322(8) | 19.3011(15) s | β+ | 10 B | 0+ | ||||||||||||
| 11 C [n 9] | 6 | 5 | 11.01143260(6) | 20.3402(53) min | β+ | 11 B | 3/2− | ||||||||||||
| 11m C | 12160(40) keV | p ?[n 10] | 10 B ? | 1/2+ | |||||||||||||||
| 12 C | 6 | 6 | 12 exactly[n 11] | Stable | 0+ | [0.9884, 0.9904][5] | |||||||||||||
| 13 C [n 12] | 6 | 7 | 13.003354835336(252) | Stable | 1/2− | [0.0096, 0.0116][5] | |||||||||||||
| 14 C [n 13] | 6 | 8 | 14.003241989(4) | 5.70(3)×103 y | β− | 14 N | 0+ | Trace[n 14] | < 10−12 | ||||||||||
| 14m C | 22100(100) keV | IT | 14 C | (2−) | |||||||||||||||
| 15 C | 6 | 9 | 15.0105993(9) | 2.449(5) s | β− | 15 N | 1/2+ | ||||||||||||
| 16 C | 6 | 10 | 16.014701(4) | 750(6) ms | β−n (99.0(3)%) | 15 N | 0+ | ||||||||||||
| β− (1.0(3)%) | 16 N | ||||||||||||||||||
| 17 C | 6 | 11 | 17.022579(19) | 193(6) ms | β− (71.6(1.3)%) | 17 N | 3/2+ | ||||||||||||
| β−n (28.4(1.3)%) | 16 N | ||||||||||||||||||
| β−2n ?[n 10] | 15 N ? | ||||||||||||||||||
| 18 C | 6 | 12 | 18.02675(3) | 92(2) ms | β− (68.5(1.5)%) | 18 N | 0+ | ||||||||||||
| β−n (31.5(1.5)%) | 17 N | ||||||||||||||||||
| β−2n ?[n 10] | 16 N ? | ||||||||||||||||||
| 19 C [n 15] | 6 | 13 | 19.03480(11) | 46.2(2.3) ms | β−n (47(3)%) | 18 N | 1/2+ | ||||||||||||
| β− (46.0(4.2)%) | 19 N | ||||||||||||||||||
| β−2n (7(3)%) | 17 N | ||||||||||||||||||
| 20 C | 6 | 14 | 20.04026(25) | 16(3) ms | β−n (70(11)%) | 19 N | 0+ | ||||||||||||
| β−2n (< 18.6%) | 18 N | ||||||||||||||||||
| β− (> 11.4%) | 20 N | ||||||||||||||||||
| 22 C [n 16] | 6 | 16 | 22.05755(25) | 6.2(1.3) ms | β−n (61(14)%) | 21 N | 0+ | ||||||||||||
| β−2n (< 37%) | 20 N | ||||||||||||||||||
| β− (> 2%) | 22 N | ||||||||||||||||||
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Carbon-11
Carbon-11 or 11
C
is a radioactive isotope of carbon that decays to boron-11. This decay mainly occurs due to positron emission, with around 0.19–0.23% of decays instead occurring by electron capture.[6][7] It has a half-life of 20.3402(53) min.
It is produced from nitrogen in a cyclotron by the reaction
- 14
N
+
p
→ 11
C
+ 4
He
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 [11
C
]DASB and [11
C
]Cimbi-5.
Natural isotopes
There are three naturally occurring isotopes of carbon: 12, 13, and 14. 12
C
and 13
C
are stable, occurring in a natural proportion of approximately 93:1. 14
C
is produced by thermal neutrons from cosmic radiation in the upper atmosphere, and is transported down to earth to be absorbed by living biological material. Isotopically, 14
C
constitutes a negligible part; but, since it is radioactive with a half-life of 5.70(3)×103 years, it is radiometrically detectable. Since dead tissue does not absorb 14
C
, the amount of 14
C
is one of the methods used within the field of archeology for radiometric dating of biological material.
Paleoclimate
12
C
and 13
C
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).[8] Plants find it easier to use the lighter isotopes (12
C
) when they convert sunlight and carbon dioxide into food. For example, large blooms of plankton (free-floating organisms) absorb large amounts of 12
C
from the oceans. Originally, the 12
C
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 12
C
from the surface, leaving the surface layers relatively rich in 13
C
. Where cold waters well up from the depths (such as in the North Atlantic), the water carries 12
C
back up with it; when the ocean was less stratified than today, there was much more 12
C
in the skeletons of surface-dwelling species. Other indicators of past climate include the presence of tropical species and coral growth rings.[9]
Tracing food sources and diets
The quantities of the different isotopes can be measured by mass spectrometry and compared to a standard; the result (e.g., the delta of the 13
C
= δ13
C
) is expressed as parts per thousand (‰):[10]
‰
Stable carbon isotopes in carbon dioxide are utilized differentially by plants during photosynthesis.[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‰.[11]
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.[12]
In contrast, C4 feeders will have bone collagen with a value of −7.5‰ and hydroxylapatite value of −0.5‰.
In actual case studies, millet and maize eaters can easily be distinguished from rice and wheat eaters. 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).[13]