Definition:Mole/Historical Note

Historical Note on Mole
The definition of the mole was adopted in November $2018$ as one of the seven SI base units.

The previous definition was the number of atoms in $12$ grams of carbon-$12$.

The original definition was as the mass in grams of a substance equal to its molecular weight, with atoms of hydrogen taken to be $1$.

The definition evolved, via a definition based upon oxygen-$16$, to the carbon-$12$ definition, which was the standard until $2018$.

At that point, the mole was defined to be exactly Avogadro's number of particles.

Hence a mole of carbon-$12$ atoms has a mass of almost exactly $12$ grams.