Carbon (
C) has 15 known
isotopes, from
8C to
22C, 2 of which (
12C and
13C) are
stable. The longest-lived radioisotope is
14C, with a
half-life of 5,700 years. This is also the only carbon radioisotope found in nature—trace quantities are formed
cosmogenically by the reaction
14N +
1n →
14C +
1H. The most stable artificial radioisotope is
11C, which has a half-life of 20.334 minutes. All other radioisotopes have half-lives under 20 seconds, most less than 200 milliseconds. The least stable isotope is
8C, with a half-life of 2.0 x 10
−21 s. Averaging over natural abundances, the
relative atomic mass for carbon is 12.0107(8).