Mass Of Carbon 12

Once upon a time, way back before 1961, there actually were two sets of atomic masses (though everybody called them atomic weights then). One scale was used by physicists; the other by chemists. Both were based on weights compared to Oxygen, rather than Hydrogen. Oxygen was used because it combines with a lot of things to form oxides. This made it a better choice as a standard because of the ease of chemical analysis. Oxygen was set to have an atomic mass of 16, which was just about 16 times as heavy as Hydrogen being 1. Unfortunately, Chemists picked naturally occurring Oxygen, which is a mixture of isotopes of Oxygen-16, Oxygen-17, and Oxygen-18. After all when you made an oxide of an element you would do so in naturally occurring oxygen. Physicists picked the pure isotope Oxygen-16, because they tended to make their measurements on the basis of mass spectrometry.

Though the ratio of any two atom’s masses was the same on either scale, it was horribly confusing, so in 1961, a compromise was reached. Instead of using either Hydrogen, or Oxygen as the standard, the isotope of Carbon with 6 protons and 6 neutrons in its nucleus (Carbon-12) was given a mass of exactly 12. It was a good choice, since it was in between the two previously used standards, and meant that nothing had to change too much. Plus, Carbon-12’s atomic mass could be measured particularly accurately compared to the other elements on the periodic table.


The term 'atomic mass' refers to the mass of a single atom.The mass of a single atom of carbon-12 is defined as exactly 12 u. The term atomic mass is also often used (though technically, incorrectly) to refer to the average atomic mass of all of the isotopes of an element.

  • 1 mol of '^12C atoms has a mass of 12.00.g precisely. The mole is the link between the micro world of atoms and molecules to the macro world of grams and kilograms. By definition, NA '^12C atoms, i.e. 'Avogadro's number, '6.022xx10^23 '^12C atoms has a mass of 12.00.g. But wait a minute!
  • Mass of one atom = Avogadro′s numberM ass of one mole of atom = 6.02×102312 Mass of one carbon atom = 1.994×10−23g.

Why do we use carbon 12 as the basis for the relative atomic mass of elements in the periodic table? Here is a fun, light-hearted take on the story.Sourcesht. Atomic Mass of Carbon Atomic mass of Carbon is 12.0107 u.

So the Atomic Mass of Carbon-12 is defined to be 12 exactly and all other atomic, molecular and formula masses are referred to this standard.

Mass of carbon 12 atom
Carbon-12, 12C
Namescarbon-12, C-12
Nuclide data
Natural abundance98.93%
Parent isotopes12N
Isotope mass12 u
Excess energy0± 0 keV
Binding energy92161.753± 0.014 keV
Isotopes of carbon
Complete table of nuclides

Carbon-12 (12C) is the more abundant of the two stableisotopes of carbon (carbon-13 being the other), amounting to 98.93% of the elementcarbon;[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.


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.

In 2018, IUPAC specified the mole as exactly 6.022 140 76 × 1023 'elementary entities'. The number of moles in 12 grams of carbon-12 became a matter of experimental determination.

Hoyle state[edit]

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 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]

The Hoyle state is populated when a helium-4 nucleus fuses with a beryllium-8 nucleus in a high-temperature (108K) 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 seconds; it primarily decays back into its three constituent alpha particles, though 0.0413(11)% of decays 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][8]

Isotopic purification[edit]

The isotopes of carbon can be separated in the form of carbon dioxide gas by cascaded chemical exchange reactions with amine carbamate.[9]

See also[edit]


  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. ISSN0067-0049.
  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. PMID17358679.
  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. doi:10.1051/epjconf/20136301022.
  6. ^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:1101.2547. Bibcode:2011PhRvL.106s2501E. doi:10.1103/PhysRevLett.106.192501. PMID21668146.[permanent dead link]
  7. ^Hjorth-Jensen, M. (2011). 'Viewpoint: The carbon challenge'. Physics. 4: 38. Bibcode:2011PhyOJ..4..38H. doi:10.1103/Physics.4.38.
  8. ^News, Natalie Wolchover, Simons Science. 'The Hoyle State: A Primordial Nucleus behind the Elements of Life'. Scientific American. Retrieved 2020-12-06.
  9. ^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/

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

Mass Of Carbon 12 Atom

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