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 1980 the CIPM clarified the above definition, defining that the carbon-12 atoms are unbound and in their ground state.