Submission to Metrologia On the dimensionality of the Avogadro constant and the definition of the mole Nigel Wheatley Consultant chemist, E-08786 Capellades, Spain E-mail:
[email protected] Abstract There is a common misconception among educators, and even some metrologists, that the –1 Avogadro constant NA is (or should be) a pure number, and not a constant of dimension N (where N is the dimension amount of substance). Amount of substance is measured, and has always been measured, as ratios 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 would cause additional conceptual complexity for no metrological benefit. PACS numbers: 06.20.-f, 06.20.Jr, 01.65.+g, 01.40.Ha Keywords: Avogadro constant, molar mass constant, relative atomic mass, atomic mass constant, amount of substance, mole, New SI 1. Introduction The Avogadro constant NA is fundamental for connecting measurements made on macroscopic systems with those made on the atomic scale [1]. Jean-Baptiste Perrin was the first to coin the name “Avogadro’s constant” in his monumental 1909 paper Brownian Motion and Molecular Reality [2]. Although Perrin was not the first to estimate the size of molecules, the precision of his results effectively silenced the remaining sceptics of atomic theory and earned him the 1926 Nobel Prize in Physics [3]. It is worth briefly quoting from Perrin’s paper:1 “It has become customary to name as the gram-molecule of a substance, the mass of the substance which in the gaseous state occupies the same volume as 2 grams of hydrogen measured at the same temperature and pressure.