On the dimensionality of the Avogadro constant and the definition of the mole

Abstract

There is a common misconception among educators, and even some metrologists, that the Avogadro constant NA is (or should be) a pure number, and not a constant of dimension N−1 (where N is the dimension amount of substance). Amount of substance is (and always has been) measured as a ratio of other physical quantities, and not in terms of a specified pure number of elementary entities. Hence the Avogadro constant has always been defined in terms of the unit of amount of substance, and not vice versa. The proposed redefinition of the mole in terms of a fixed value of the Avogadro constant is examined, and it is shown that such a redefinition would not bring any significant metrological benefit. It is contended that such a redefinition would only add to the confusion in this field, and so should be rejected.