Carbon-12 | Wikipedia audio article


Carbon-12 (12C) is the more abundant of the
two stable isotopes of carbon (carbon-13 being the other), amounting to 98.93% of the element
carbon; 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.==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. 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.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==
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. 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.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 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.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.==Isotopic purification==
The isotopes of carbon can be separated in the form of carbon dioxide gas by cascaded
chemical exchange reactions with amine carbamate.==See also==
Avogadro constant Carbon-11
Carbon-13 Carbon-14
Isotopes of carbon Isotopically pure diamond
Mole (unit

Add a Comment

Your email address will not be published. Required fields are marked *