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.

[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.