Editing Carbon (section)

===Isotopes===
{{Main|Isotopes of carbon}}
[[Isotope]]s of carbon are [[atomic nuclei]] that contain six [[proton]]s plus a number of [[neutron]]s (varying from 2 to 16). Carbon has two stable, naturally occurring isotopes.<ref name="WebElements">{{cite web |url=http://www.webelements.com/webelements/elements/text/C/isot.html |title=Carbon&nbsp;– Naturally occurring isotopes |publisher=WebElements Periodic Table |access-date=2008-10-09 |url-status=live |archive-url=https://web.archive.org/web/20080908030327/http://www.webelements.com/webelements/elements/text/C/isot.html |archive-date=2008-09-08}}</ref> The isotope [[carbon-12]] ({{sup|12}}C) forms 98.93% of the carbon on Earth, while [[carbon-13]] ({{sup|13}}C) forms the remaining 1.07%.<ref name="WebElements" /> The concentration of {{sup|12}}C is further increased in biological materials because biochemical reactions discriminate against {{sup|13}}C.<ref>{{cite journal |last1=Gannes |first1=Leonard Z. |last2=Del Rio |first2=Carlos Martı́nez |last3=Koch |first3=Paul |title=Natural Abundance Variations in Stable Isotopes and their Potential Uses in Animal Physiological Ecology |journal=Comparative Biochemistry and Physiology&nbsp;– Part A: Molecular & Integrative Physiology |volume=119 |issue=3 |pages=725–737 |date=1998 |doi=10.1016/S1095-6433(98)01016-2 |pmid=9683412}}</ref> In 1961, the [[International Union of Pure and Applied Chemistry]] (IUPAC) adopted the isotope carbon-12 as the basis for [[atomic weight]]s.<ref>{{cite web |url=http://www.bipm.org/en/si/base_units/ |title=Official SI Unit definitions |access-date=2007-12-21 |url-status=live |archive-url=https://web.archive.org/web/20071014094602/http://www.bipm.org/en/si/base_units/ |archive-date=2007-10-14}}</ref> Identification of carbon in [[nuclear magnetic resonance]] (NMR) experiments is done with the isotope {{sup|13}}C.

[[Carbon-14]] ({{sup|14}}C) is a naturally occurring [[radioisotope]], created in the [[upper atmosphere]] (lower [[stratosphere]] and upper [[troposphere]]) by interaction of nitrogen with cosmic rays.<ref>{{cite book |first=S. |last=Bowman |date=1990 |title=Interpreting the past: Radiocarbon dating |publisher=British Museum Press |isbn=978-0-7141-2047-8}}</ref> It is found in trace amounts on Earth of 1 part per [[10^12|trillion]] (0.0000000001%) or more, mostly confined to the atmosphere and superficial deposits, particularly of peat and other organic materials.<ref>{{cite web |last=Brown |first=Tom |date=March 1, 2006 |url=http://www.llnl.gov/str/March06/Brown.html |title=Carbon Goes Full Circle in the Amazon |publisher=Lawrence Livermore National Laboratory |access-date=2007-11-25 |url-status=live |archive-url=https://web.archive.org/web/20080922031202/https://www.llnl.gov/str/March06/Brown.html |archive-date=September 22, 2008}}</ref> This isotope decays by 0.158&nbsp;MeV [[beta decay|β{{sup|−}} emission]]. Because of its relatively short [[half-life]] of {{val|5700|30}}&nbsp;years,{{NUBASE2020|name}} {{sup|14}}C is virtually absent in ancient rocks. The amount of {{sup|14}}C in the [[atmosphere]] and in living organisms is almost constant, but decreases predictably in their bodies after death. This principle is used in [[radiocarbon dating]], invented in 1949, which has been used extensively to determine the age of carbonaceous materials with ages up to about 40,000&nbsp;years.<ref>{{cite book |last=Libby |first=W. F. |date=1952 |title=Radiocarbon dating |publisher=Chicago University Press and references therein}}</ref><ref>{{cite web |last=Westgren |first=A. |date=1960 |url=http://nobelprize.org/nobel_prizes/chemistry/laureates/1960/press.html |title=The Nobel Prize in Chemistry 1960 |publisher=Nobel Foundation |access-date=2007-11-25 |url-status=live |archive-url=https://web.archive.org/web/20071025003508/http://nobelprize.org/nobel_prizes/chemistry/laureates/1960/press.html |archive-date=2007-10-25}}</ref>

There are 15 known isotopes of carbon and the shortest-lived of these is {{sup|8}}C which decays through [[proton emission]] and has a half-life of 3.5{{e|−21}} s.{{NUBASE2020|ref}} The exotic {{sup|19}}C exhibits a [[nuclear halo]], which means its radius is appreciably larger than would be expected if the nucleus were a sphere of constant density.<ref>{{cite journal |title=Beaming Into the Dark Corners of the Nuclear Kitchen |last1=Watson |first1=A. |journal=Science |volume=286 |issue=5437 |pages=28–31 |date=1999 |s2cid=117737493 |doi=10.1126/science.286.5437.28}}</ref>