Abstract

AbstractThere is a common misconception among educators, and even some metrologists, that the Avogadro constant _N_~A~ 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 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.

Highlights

  • The Avogadro constant NA is fundamental for connecting measurements made on macroscopic systems with those made on the atomic scale [1]

  • The Avogadro constant has always been defined as the number of molecules per unit of amount of substance, whether that unit of was called the gram-molecule, the gram-atom or the mole

  • 11 Luminous intensity is the physical quantity measured by the candela

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Summary

Introduction

The Avogadro constant NA is fundamental for connecting measurements made on macroscopic systems with those made on the atomic scale [1]. 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:. “This invariable number N” soon became known as “Avogadro’s number” [3] and treated as a quantity of dimension one, i.e. a pure number The error of incorrectly treating a physical constant as a pure number is hardly unprecedented in metrology It occurred in the conception of the electrical and magnetic units in the CGS system, where the electric constant ε0 and the magnetic constant μ0 were treated as dimensionless and equal to unity. It would be most unwise to consider the redefinition of the mole without first addressing the misconceptions surrounding both the current definition and the very concept of amount of substance

Amount of substance
Historical unit of amount of substance
Quantization of amount of substance
Counting
NA as a fundamental constant
Relative atomic mass and Mu
Relative atomic mass
Criteria for a unit of amount of substance
Redefinition of SI base units
The current link between the mole and the kilogram
Quantum metrology and fundamental constants
Atomic mass constant
Ease of comprehension
Alternative wordings
Conclusions
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