carbon-12

Carbon-12 is the most abundant of the two stable isotopes of the element carbon, accounting for 98.89% of carbon; it contains 6 protons, 6 neutrons, and 6 electrons.

Carbon-12 is of particular importance as it is used as the standard from which atomic masses of all nuclides are measured: its mass number is by definition 12.
History

Prior to 1959 both the IUPAP and IUPAC tended to use 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 1959/1960 to define the mole as:
"The mole is the amount of substance of a system which contains as many elementary entities as there are atoms in 0.012 kilogram 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.

[edit] Hoyle State

The Hoyle State is an excited state of carbon-12 with precisely the properties necessary to allow just the right amount of carbon to be created in a stellar environment. The existence of the Hoyle state is essential for the nucleosynthesis of carbon in helium-burning red giant stars. The resonant state was predicted by Fred Hoyle in the 1950s based on the observed abundances of heavy elements in the universe. The resonant state allows carbon to be produced via the triple-alpha process. The existence of the Hoyle state has been confirmed but its precise properties are still being investigated.