View source for Isotopes of carbon

[[Carbon]] has two stable, naturally occurring [[isotope]]s, [[carbon-12]] and [[carbon-13]]. There is also a naturally-occuring radioactive isotope, [[carbon-14]] (''t''<sub>½</sub>&nbsp;= 5700&nbsp;years), that is formed by the action of [[cosmic ray]]s on the [[Terrestrial atmosphere|Earth's atmosphere]]. [[Carbon-11]] is an artificial isotope used in [[positron emission tomography]] (PET), a medical imaging technique.

==Atomic weight==
{| class="wikitable" align=right style="margin: 0.2em 0 0.3em 0.5em;"
|-
|+ ''A''<sub>r</sub>(<sub>6</sub>C) = 12.0107(8)
|-
! Isotope
! Mass/u
! Amount<br/>fraction
! Normal<br/>range
|-
| <sup>12</sup>C
| 12 (by definition)<ref group="note" name="C-12">The [[relative atomic mass]] of [[carbon-12]] is exactly twelve [[atomic mass unit]]s, without any [[measurement uncertainty]], by the definition of the atomic mass unit.</ref>
| 0.9893(8)
| 0.9885–0.9902
|-
| <sup>13</sup>C
| 13.003 354 837 78(98)
| 0.0107(8)
| 0.0115–0.0098
|-
| colspan=4 | References:<ref name="AME"/>
|-
|}
The standard [[atomic weight]] of carbon has been fixed at its current value since 1995. The range of natural [[isotopic composition]]s makes a more precise standard value unobtainable, and some terrestrial geological specimens may show compositions outside of the normal range. Natural amounts of [[carbon-14]] (''x''&nbsp;<&nbsp;10<sup>−12</sup>) are insignificant in the calculation of atomic weight.

Carbon-13 is systematically depleted in samples of biological origin (compared to atmospheric [[carbon dioxide]] and most [[Carbonate mineral|carbonate rocks]]) due to a [[kinetic isotope effect]] that is particularly pronounced in [[Photosynthesis|photosynthetic reactions]].<ref group="note">Marine organisms that obtain carbonate from sea water to construct exoskeletons do not show the depletion of <sup>13</sup>C ''in the exoskeletons'': indeed, when such exoskeletons become carbonate rocks, they often show anomalously ''high'' amount fractions of <sup>13</sup>C, an indication that the carbon did not pass through a photosynthetic pathway.</ref> The effect is strongest in [[C4-Pathway|C<sub>4</sub>-plants]], which are predominant in tropical and subtropical climates, and measurements of carbon isotopic compositions are important in [[geochemistry]], [[paleoclimatology]] and [[paleoceanography]]. Such measurements have also attracted interest for detecting cases of "doping" in sport:<ref>{{citation | first1 = Rodrigo | last1 = Aguilera | first2 = Caroline K. | last2 = Hatton | first3 = Don H. | last3 = Catlin | title = Detection of Epitestosterone Doping by Isotope Ratio Mass Spectrometry | journal = Clin. Chem. | volume = 48 | pages = 629–36 | year = 2002}}.</ref> steroid hormones produced by biotechnological methods would be expected to have higher amount fractions of <sup>13</sup>C than those produced by an athlete's body.<ref group="note">It was shown by Aguilera, Hatton & Catlin (2002) that this was the case in their 2002 samples: however, it is possible to "artificially deplete" the <sup>13</sup>C levels in artificial steroid hormones.</ref>

==Radiologically significant isotopes==
{| class="wikitable"
|-
! Isotope
! Half life
! Decay mode
! <u>Energy</u><br/>MeV
! Weighted mean<br/>energies/MeV
! Daughter<br/>nuclide
! Ref.
|-
| rowspan=4 | <sup>11</sup>C
| rowspan=4 | 1223.1(12) s<br/>(20.39(2) min)
| β<sup>+</sup> (99.759(15)%)
| 0.3856(4)
| rowspan=4 | γ, X: 1.02<br/>β, Auger: 0.385
| rowspan=4 | {{nuclide|Z=5|A=11}} (stable)
| rowspan=4 | <ref>{{citation | title = ENSDF Decay Data in the MIRD (Medical Internal Radiation Dose) Format for <sup>11</sup>C | url = http://www.nndc.bnl.gov/useroutput/11c_mird.html | publisher = National Nuclear Data Center | accessdate = 2010-03-13}}.</ref><ref>{{citation | first = F. | last = Ajzenberg-Selove | year = 1990 | title = Energy levels of light nuclei ''A'' = 11–12 | journal = Nucl. Phys. A | volume = 506 | issue = 1 | pages = 1–158 | doi = 10.1016/0375-9474(90)90271-M}}.</ref>
|-
| γ<sup>±</sup> (2×99.759%)
| 0.5110
|-
| X (ec) (0.0002%)
| 1.0{{e|−4}}
|-
| K-Auger (0.241(15)%)
| 1.7{{e|−4}}
|-
| <sup>14</sup>C
| 5.70(3){{e|3}} a
| β<sup>−</sup> (100%)
| 0.04947
| β: 0.04947
| {{nuclide|Z=7|A=14}} (stable)
| <ref>{{citation | title = ENSDF Decay Data in the MIRD (Medical Internal Radiation Dose) Format for <sup>14</sup>C | url = http://www.nndc.bnl.gov/useroutput/14c_mird.html | publisher = National Nuclear Data Center | accessdate = 2010-03-13}}.</ref><ref>{{citation | first = F. | last = Ajzenberg-Selove | year = 1991 | title = Energy levels of light nuclei ''A'' = 13–15 | journal = Nucl. Phys. A | volume = 523 | issue = 1 | pages = 1–196 | doi = 10.1016/0375-9474(91)90446-D}}.</ref>
|-
|}

==All isotopes==
{| class="wikitable" style="white-space:nowrap;" width=100%
! rowspan=2 | Symbol
! Z([[proton|p]])
! N([[neutron|n]])
! Mass/u
! rowspan=2 | <u>Excess energy</u><br/>MeV
! rowspan=2 | <u>Binding energy/A</u><br/>MeV
! rowspan=2 | <u>β<sup>−</sup>-decay energy</u><br/>MeV
! rowspan=2 | Spin
! rowspan=2 | Half life
! rowspan=2 | Decay mode,<br/>proportion
|-
! colspan="3" | Excitation energy/MeV
|-
| <sup>8</sup>C
| 6
| 2
| 8.037 675(25)
| 35.094(23)
| 3.0978(29)
| —
| align=center | 0
| 2.0(4) zs
| 2p (100%)
|-
| <sup>9</sup>C
| 6
| 3
| 9.031 0367(23)
| 28.9105(21)
| 4.337 48(24)
| —
| align=center | (−{{frac|3|2}})
| 126.5(9) ms
| β<sup>+</sup> (100%)
|-
| <sup>10</sup>C
| 6
| 4
| 10.016 853 23(43)
| 15.698 68(40)
| 6.032 041(40)
| −23.10(40)
| align=center | 0
| 19.290(12) s
| β<sup>+</sup> (100%)
|-
| <sup>11</sup>C
| 6
| 5
| 11.011 433 61(102)
| 10.650 34(95)
| 6.676 370(86)
| −13.653(46)
| align=center | −{{frac|3|2}}
| 20.39(2) min
| β<sup>+</sup> (100%)
|-
| <sup>12</sup>C
| 6
| 6
| 12 (by definition)<ref group="note" name="C-12"/>
| 0 (by definition)<ref group="note" name="C-12"/>
| 7.680 144
| −17.338 08(100)
| align=center | 0
| colspan=2 | S<small>TABLE</small>
|-
| <sup>13</sup>C
| 6
| 7
| 13.003 354 837 78(98)
| 3.125 011 29(91)
| 7.469 849
| −2.220 47(27)
| align=center | −{{frac|2}}
| colspan=2 | S<small>TABLE</small>
|-
| <sup>14</sup>C
| 6
| 8
| 14.003 241 9887(41)
| 3.019 8931(38)
| 7.520 319
| 0.156 476(4)
| align=center | 0
| 5.70(3){{e|3}} a
| β<sup>−</sup> (100%)
|-
| <sup>15</sup>C
| 6
| 9
| 15.010 5992(86)
| 9.873 14(80)
| 7.100 169(53)
| 9.771 71(80)
| align=center | +{{frac|2}}
| 2.449(5) s
| β<sup>−</sup> (100%)
|-
| <sup>16</sup>C
| 6
| 10
| 16.014 7013(38)
| 13.6941(36)
| 6.922 05(22)
| 8.0105(44)
| align=center | 0
| 747(8) ms
| β<sup>−</sup> (100%)
|-
| <sup>17</sup>C
| 6
| 11
| 17.022 5861(187)
| 21.0388(174)
| 6.557 62(102)
| 13.167(23)
| align=center | (+{{frac|3|2}})
| 193(5) ms
| β<sup>−</sup> (100%)
|-
| <sup>18</sup>C
| 6
| 12
| 18.026 759(32)
| 24.926(30)
| 6.425 75(167)
| 11.812(35)
| align=center | 0
| 92(2) ms
| β<sup>−</sup> (100%)
|-
| <sup>19</sup>C
| 6
| 13
| 19.034 805(106)
| 32.421(98)
| 6.1179(52)
| 16.559(99)
| align=center | (+{{frac|2}})
| 46.2(23) ms
| β<sup>−</sup> (100%)
|-
| <sup>20</sup>C
| 6
| 14
| 20.040 32(26)
| 37.56(24)
| 5.9587(120)
| 15.79(25)
| align=center | 0
| 16(3) ms
| β<sup>−</sup> (100%)
|-
| <sup>21</sup>C
| 6
| 15
| 21.049 34(54)#
| 45.96(50)#
| 5.659(24)#
| 20.71(51)#
| align=center | +{{frac|2}}#
| <30 ns
| ?n
|-
| <sup>22</sup>C
| 6
| 16
| 22.057 20(97)#
| 53.28(90)#
| 5.436(41)#
| 21.24(92)#
| align=center | 0
| 6.2(13) ms
| β<sup>−</sup> (100%)
|-
| colspan=10 | Values marked # are estimated from systematic trends rather than experimentally measured.<br/>Spins quoted in parentheses are uncertain in value and/or parity.<br/>''Sources'': Except as otherwise noted, 
:isotopic masses and associated energies are taken from the AME 2003 dataset;<ref name="AME">{{AME 2003}}</ref>
:nuclear spins and decay properties are taken from N<small>UBASE</small> 2003.<ref name="Nubase">{{Nubase 2003}}.</ref>
|}

==Notes and references==
===Notes===
{{reflist|group=note}}

===References===
{{reflist|2}}

[[Category:Isotopes of carbon|*]]

{{CC-BY-3.0}}