Carbon-12 is the more abundant of the two stable isotopes of carbon (Carbon-13 being the other), amounting to 98.93% of the element carbon;[1] its abundance is due to the triple-alpha process by which it is created in stars. Carbon-12 is of particular importance in its use as the standard from which atomic masses of all nuclides are measured. Its mass number is 12 by definition and contains 6 protons, 6 neutrons and 6 electrons.


Before 1959 both the IUPAP and IUPAC used oxygen to define the mole; the chemists defining the mole as the number of atoms of oxygen which had mass 16 g, the physicists using a similar definition but with the oxygen-16 isotope only. The two organizations agreed in 1959/60 to define the mole as follows.

The mole is the amount of substance of a system which contains as many elementary entities as there are atoms in 12 gram of carbon 12; its symbol is "mol".

This was adopted by the CIPM (International Committee for Weights and Measures) in 1967, and in 1971 it was adopted by the 14th CGPM (General Conference on Weights and Measures).

In 1961 the isotope carbon-12 was selected to replace oxygen as the standard relative to which the atomic weights of all the other elements are measured.[2]

In 1980 the CIPM clarified the above definition, defining that the carbon-12 atoms are unbound and in their ground state.

Hoyle state

The Hoyle state is an excited, spinless, resonant state of carbon-12. It is produced via the triple-alpha process, and was predicted to exist by Fred Hoyle in 1954.[3] The existence of the Hoyle state is essential for the nucleosynthesis of carbon in helium-burning red giant stars, and predicts an amount of carbon production in a stellar environment which matches observations. The existence of the Hoyle state has been confirmed experimentally, but its precise properties are still being investigated.[4] In 2011, an ab initio calculation of the low-lying states of carbon-12 found (in addition to the ground and excited spin-2 state) a resonance with all of the properties of the Hoyle state.[5][6]

Isotopic purification

The isotopes of carbon can be separated in the form of carbon dioxide gas by cascaded chemical exchange reactions with amine carbamate.[7]

See also


  1. ^ "Table of Isotopic Masses and Natural Abundances" (PDF). 1999. 
  2. ^ "Atomic Weights and the International Committee — A Historical Review". 2004-01-26. 
  3. ^ Hoyle, F. (1954). "On Nuclear Reactions Occurring in Very Hot Stars. I. the Synthesis of Elements from Carbon to Nickel.". The Astrophysical Journal Supplement Series. 1: 121. Bibcode:1954ApJS....1..121H. doi:10.1086/190005. ISSN 0067-0049. 
  4. ^ Chernykh, M.; Feldmeier, H.; Neff, T.; Von Neumann-Cosel, P.; Richter, A. (2007). "Structure of the Hoyle State in C12" (PDF). Physical Review Letters. 98 (3): 032501. Bibcode:2007PhRvL..98c2501C. doi:10.1103/PhysRevLett.98.032501. PMID 17358679. 
  5. ^ Epelbaum, E.; Krebs, H.; Lee, D.; Meißner, U.-G. (2011). "Ab Initio Calculation of the Hoyle State" (PDF). Physical Review Letters. 106 (19): 192501. arXiv:1101.2547Freely accessible. Bibcode:2011PhRvL.106s2501E. doi:10.1103/PhysRevLett.106.192501. PMID 21668146. [permanent dead link]
  6. ^ Hjorth-Jensen, M. (2011). "Viewpoint: The carbon challenge". Physics. 4: 38. Bibcode:2011PhyOJ...4...38H. doi:10.1103/Physics.4.38. 
  7. ^ Kenji Takeshita and Masaru Ishidaa (December 2006). "Optimum design of multi-stage isotope separation process by exergy analysis". ECOS 2004 - 17th International Conference on Efficiency, Costs, Optimization, Simulation, and Environmental Impact of Energy on Process Systems. 31 (15): 3097–3107. doi:10.1016/j.energy.2006.04.002. 

Carbon-12 is an
isotope of carbon
Decay product of:
boron-12, nitrogen-12
Decay chain
of carbon-12
Decays to: