Carbon-12

Carbon-12 (12C) is the most abundant of the two stable isotopes of carbon (carbon-13 being the other), amounting to 98.93% of element carbon on Earth;[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, thus, its atomic mass is exactly 12 daltons by definition. Carbon-12 is composed of 6 protons, 6 neutrons, and 6 electrons.

Carbon-12, 12C
General
Symbol12C
Namescarbon-12, 12C, C-12
Protons (Z)6
Neutrons (N)6
Nuclide data
Natural abundance98.93%
Isotope mass12 Da
Spin0
Excess energy0.0 keV
Binding energy92161.753±0.014 keV
Parent isotopes12N
12B
Isotopes of carbon
Complete table of nuclides

History

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.

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.

In 2018, IUPAC specified the mole as exactly 6.02214076×1023 "elementary entities". The number of moles in 12 grams of carbon-12 became a matter of experimental determination.

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 7.7 MeV resonance Hoyle state is essential for the nucleosynthesis of carbon in helium-burning 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]

The Hoyle state is populated when a helium-4 nucleus fuses with a beryllium-8 nucleus in a high-temperature (108 K) environment with densely concentrated (105 g/cm3) helium. This process must occur within 10−16 seconds as a consequence of the short half-life of 8Be. The Hoyle state also is a short-lived resonance with a half-life of 2.4×10−16 s; it primarily decays back into its three constituent alpha particles, though 0.0413% of decays (or 1 in 2421.3) occur by internal conversion into the ground state of 12C.[5]

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.[6][7]

Isotopic purification

The isotopes of carbon can be separated in the form of carbon dioxide gas by cascaded chemical exchange reactions with amine carbamate.[8]

See also

References
  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. Freer, M.; Fynbo, H. O. U. (2014). "The Hoyle state in 12C". Progress in Particle and Nuclear Physics. 78: 1–23. Bibcode:2014PrPNP..78....1F. doi:10.1016/j.ppnp.2014.06.001.
  5. Alshahrani, B.; Kibédi, T.; Stuchberry, A. E.; Williams, E.; Fares, S. (2013). "Measurement of the radiative branching ratio for the Hoyle state using cascade gamma decays". EPJ Web of Conferences. 63: 01022-1–01022-4. Bibcode:2013EPJWC..6301022A. doi:10.1051/epjconf/20136301022. hdl:1885/101943.
  6. Epelbaum, E.; Krebs, H.; Lee, D.; Meißner, U.-G. (2011). "Ab Initio Calculation of the Hoyle State". Physical Review Letters. 106 (19): 192501. arXiv:1101.2547. Bibcode:2011PhRvL.106s2501E. doi:10.1103/PhysRevLett.106.192501. PMID 21668146. S2CID 33827991.
  7. Hjorth-Jensen, M. (2011). "Viewpoint: The carbon challenge". Physics. Vol. 4. p. 38. Bibcode:2011PhyOJ...4...38H. doi:10.1103/Physics.4.38.
  8. Kenji Takeshita and Masaru Ishidaa (December 2006). "Optimum design of multi-stage isotope separation process by exergy analysis". Energy. 31 (15): 3097–3107. doi:10.1016/j.energy.2006.04.002.
  • Jenkins, David; Kirsebom, Oliver (2013-02-07). "The secret of life". Physics World. Retrieved 2021-08-27.
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