The Mole

Before atomic weights existed and after chemists realized some of the fundamental laws of chemistry chemists and physicists used different standards as for how to define what was then called an "atom gram". Eventually in 1959/1960 an agreement was reached to standardize the atomic weight of carbon-12 as 12,

It remained to determine how much of a substance was in a certain weight of an atom. By agreement this defined the definition of a mole as the number of atoms present in .012 kilogram of carbon 12.