Developments in mass spectrometry led to the adoption of oxygen-16 as the standard substance, in lieu of natural oxygen.
The oxygen-16 definition was replaced with one based on carbon-12 during the 1960s. The mole was defined by International Bureau of Weights and Measures as "the amount of substance of a system which contains as many elementary entities as there are atoms in 0.012 kilogram of carbon-12." Thus, by that definition, one mole of pure C had a mass of exactly 12 g. The four different definitions were equivalent to within 1%.
Because a dalton, a unit commonly used to measure atomic mass, is exactly 1/12 of the mass of a carbon-12 atom, this definition of the mole entailed that the mass of one mole of a compound or element in grams was numerically equal to the average mass of one molecule or atom of the substance in daltons, and that the number of daltons in a gram was equal to the number of elementary entities in a mole. Because the mass of a nucleon (i.e. a proton or neutron) is approximately 1 dalton and the nucleons in an atom's nucleus make up the overwhelming majority of its mass, this definition also entailed that the mass of one mole of a substance was roughly equivalent to the number of nucleons in one atom or molecule of that substance.
Since the definition of the gram was not mathematically tied to that of the dalton, the number of molecules per mole N (the Avogadro constant) had to be determined experimentally. The experimental value adopted by CODATA in 2010 is .
In 2011 the measurement was refined to .
The mole was made the seventh SI base unit in 1971 by the 14th CGPM.