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Carbon (₆C) has 14 known isotopes, from
C
to
C
as well as
C
, of which
C
and
C
are stable. The longest-lived radioisotope is
C
, with a half-life of 5.70(3)×10 years. This is also the only carbon radioisotope found in nature, as trace quantities are formed cosmogenically by the reaction
N
+
n
→
C
+
H
. The most stable artificial radioisotope is
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
C
, with a half-life of 3.5(1.4)×10 s. Light isotopes tend to decay into isotopes of boron and heavy ones tend to decay into isotopes of nitrogen.

List of isotopes

1. ( ) – Uncertainty (1σ) is given in concise form in parentheses after the corresponding last digits.
2. Modes of decay:

   EC:	Electron capture
   n:	Neutron emission
   p:	Proton emission

3. Bold symbol as daughter – Daughter product is stable.
4. ( ) spin value – Indicates spin with weak assignment arguments.
5. # – Values marked # are not purely derived from experimental data, but at least partly from trends of neighboring nuclides (TNN).
6. Subsequently decays by double proton emission to
   He
   for a net reaction of
   C
   →
   He
   + 4
   H
   
7. Immediately decays by proton emission to
   He
   for a net reaction of
   C
   → 2 
   He
   +
   H
   +
   e
   
8. Immediately decays into two
   He
   atoms for a net reaction of
   C
   → 2 
   He
   +
   H
   +
   e
   
9. Used for labeling molecules in PET scans
10. ^ Decay mode shown is energetically allowed, but has not been experimentally observed to occur in this nuclide.
11. The unified atomic mass unit is defined as 1/12 of the mass of an unbound atom of carbon-12 in its ground state.
12. Ratio of C to C used to measure biological productivity in ancient times and differing types of photosynthesis
13. Has an important use in radiodating (see carbon dating)
14. Primarily cosmogenic, produced by neutrons striking atoms of
    N
    (
    N
    +
    n
    →
    C
    +
    H
    )
15. Has 1 halo neutron
16. Has 2 halo neutrons

Carbon-11

Carbon-11 or
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. It has a half-life of 20.3402(53) min.

C
→
B
+
e
+
ν
_e + 0.96 MeV

C
+
e
→
B
+
ν
_e + 1.98 MeV

It is produced by hitting nitrogen with protons of around 16.5 MeV in a cyclotron. The causes the endothermic reaction

N
+
p
→
C
+
He
− 2.92 MeV

It can also be produced by fragmentation of
C
by shooting high-energy
C
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 DASB and Cimbi-5.

Natural isotopes

There are three naturally occurring isotopes of carbon: 12, 13, and 14.
C
and
C
are stable, occurring in a natural proportion of approximately 93:1.
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,
C
constitutes a negligible part; but, since it is radioactive with a half-life of 5.70(3)×10 years, it is radiometrically detectable. Since dead tissue does not absorb
C
, the amount of
C
is one of the methods used within the field of archeology for radiometric dating of biological material.

Paleoclimate

C
and
C
are measured as the isotope ratio δC 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 isotopes (
C
) when they convert sunlight and carbon dioxide into food. For example, large blooms of plankton (free-floating organisms) absorb large amounts of
C
from the oceans. Originally, the
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
C
from the surface, leaving the surface layers relatively rich in
C
. Where cold waters well up from the depths (such as in the North Atlantic), the water carries
C
back up with it; when the ocean was less stratified than today, there was much more
C
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

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
C
= δ
C
) is expressed as parts per thousand (‰) divergence from the ratio of a standard:

${\displaystyle \delta {\ce {^{13}C}}=\left({\frac {\left({\frac {{\ce {^{13}C}}}{{\ce {^{12}C}}}}\right)_{\text{sample}}}{\left({\frac {{\ce {^{13}C}}}{{\ce {^{12}C}}}}\right)_{\text{standard}}}}-1\right)\times 1000}$ ‰

The usual standard is Peedee Belemnite, abbreviated "PDB", a fossil belemnite. Due to shortage of the original PDB sample, artificial "virtual PDB", or "VPDB", is generally used today.

Stable carbon isotopes in carbon dioxide are utilized differentially by plants during photosynthesis. 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 δC values averaging about −26.5‰. Grasses in hot arid climates (maize in particular, but also millet, sorghum, sugar cane, and crabgrass) follow a C4 photosynthetic pathway that produces δC values averaging about −12.5‰.

It follows that eating these different plants will affect the δC values in the consumer's body tissues. If an animal (or human) eats only C3 plants, their δC 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 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).

See also

• Cosmogenic isotopes
• Environmental isotopes
• Isotopic signature
• Radiocarbon dating

References

1. "Standard Atomic Weights: Carbon". CIAAW. 2009.
2. Prohaska, Thomas; Irrgeher, Johanna; Benefield, Jacqueline; Böhlke, John K.; Chesson, Lesley A.; Coplen, Tyler B.; Ding, Tiping; Dunn, Philip J. H.; Gröning, Manfred; Holden, Norman E.; Meijer, Harro A. J. (2022-05-04). "Standard atomic weights of the elements 2021 (IUPAC Technical Report)". Pure and Applied Chemistry. doi:10.1515/pac-2019-0603. ISSN 1365-3075.
3. Wang, Meng; Huang, W.J.; Kondev, F.G.; Audi, G.; Naimi, S. (2021). "The AME 2020 atomic mass evaluation (II). Tables, graphs and references*". Chinese Physics C. 45 (3): 030003. doi:10.1088/1674-1137/abddaf.
4. ^ Kondev, F. G.; Wang, M.; Huang, W. J.; Naimi, S.; Audi, G. (2021). "The NUBASE2020 evaluation of nuclear properties" (PDF). Chinese Physics C. 45 (3): 030001. doi:10.1088/1674-1137/abddae.
5. ^ "Atomic Weight of Carbon". CIAAW.
6. Scobie, J.; Lewis, G. M. (1 September 1957). "K-capture in carbon 11". Philosophical Magazine. 2 (21): 1089–1099. Bibcode:1957PMag....2.1089S. doi:10.1080/14786435708242737.
7. Campbell, J. L.; Leiper, W.; Ledingham, K. W. D.; Drever, R. W. P. (1967-04-11). "The ratio of K-capture to positron emission in the decay of C". Nuclear Physics A. 96 (2): 279–287. Bibcode:1967NuPhA..96..279C. doi:10.1016/0375-9474(67)90712-9.
8. "Carbon-11 Production and Transformation". Scholarly Community Encyclopedia.
9. Lu, Shuiyu; et al. (Jan 18, 2024). "Gas Phase Transformations in Carbon-11 Chemistry". Int. J. Mol. Sci. 25 (2): 1167. doi:10.3390/ijms25021167. PMC 10816134. PMID 38256240.
10. Daria Boscolo; et al. (Sep 2024). "First image-guided treatment of a mouse tumor with radioactive ion beams". arXiv:2409.14898.
11. Lynch-Stieglitz, Jean; Stocker, Thomas F.; Broecker, Wallace S.; Fairbanks, Richard G. (1995). "The influence of air-sea exchange on the isotopic composition of oceanic carbon: Observations and modeling". Global Biogeochemical Cycles. 9 (4): 653–665. Bibcode:1995GBioC...9..653L. doi:10.1029/95GB02574. S2CID 129194624.
12. Tim Flannery The weather makers: the history & future of climate change, The Text Publishing Company, Melbourne, Australia. ISBN 1-920885-84-6
13. Miller, Charles B.; Wheeler, Patricia (2012). Biological oceanography (2nd ed.). Chichester, West Sussex: John Wiley & Sons, Ltd. p. 186. ISBN 9781444333022. OCLC 794619582.
14. Faure, Gunter; Mensing, Teresa M. (2005). "27 Carbon". Isotopes: Principles and Applications (Third ed.). Hoboken, NJ: Wiley. ISBN 978-81-265-3837-9.
15. O'Leary, Marion H. (May 1988). "Carbon Isotopes in Photosynthesis" (PDF). BioScience. 38 (5): 328–336. doi:10.2307/1310735. JSTOR 1310735. S2CID 29110460. Retrieved 17 November 2022.
16. Tycot, R. H. (2004). M. Martini; M. Milazzo; M. Piacentini (eds.). "Stable isotopes and diet: you are what you eat" (PDF). Proceedings of the International School of Physics "Enrico Fermi" Course CLIV.
17. Richard, Hedges (2006). "Where does our protein come from?". British Journal of Nutrition. 95 (6): 1031–2. doi:10.1079/bjn20061782. PMID 16768822.

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Table of nuclides Categories: Isotopes Tables of nuclides Metastable isotopes Isotopes by element

• Isotopes of carbon
• Carbon
• Lists of isotopes by element