The Avogadro constant was redefined in 2018.

Prior to the 2018 definition of the Avogadro constant, the molar mass of carbon-12 was a constant with the value of exactly 0.012.

With the new definition, it is still with great accuracy 0.012 but not exactly so and is given by the formula (see the article on the ‘Bohr model‘ for derivation):

where

is the Planck constant

is the Rydberg constant

is the speed of light

is the fine-structure constant

is the ‘relative atomic mass’ of an electron

The molar mass of carbon-12 now has a relative uncertainty in the order of 10^{-10 }that is primarily due to the uncertainty in the fine-structure constant. The value of this uncertainty will be refined in the future experiments. Similarly, the molar mass constant *M _{u}*, which was a constant with the value 1

*gmol*, is now described by the formula:

^{-1}The mass of the carbon-12 isotope is still pegged to 12 unified atomic mass unit and the values of all relative masses (isotopic, atomic, molecular) remain unchanged.