Integer numbers and their ratios are key concepts in describing the interactions of atoms and molecules

Abstract

A general discussion of the redefinition of the mole, the SI unit for ‘amount of substance’, is now ongoing in ever broadening circles as part of a general overhaul of the Systeme International d’Unites, SI. Important for chemists are the unit mole (symbol mol) and the unit kilogram (symbol kg). For several thousands of years, ‘mass’ was the property enabling measurement of ‘‘how much is there?’’ by using a comparative balance and the property of masses on the surface of the earth commonly called ‘‘weight’’ (concepts put in single quotes; double quotes used for quotations or to emphasize). Then, Isaac Newton (1642–1727) fundamentally changed our thinking by explaining that ‘mass’ was related to both ‘gravitational attraction’ and ‘inertia’. Einstein (1879–1955) further showed that these are the same quantity. Quite different to this is the modern concept of ‘amount of substance’ mentioned in the SI but basically different from each of these. Equally important is the concept ‘number of particles’ (in generic language: ‘number of entities’; ‘entities’ here must be specified), based on the property of ‘numerosity’ [1]. It is present in the mind of chemists making analytical measurements. ‘Mass’ was included in the International System of Quantities (ISQ) as a base quantity (Sect. 1.2 pp 104–105 in [2] and entry 1.6 in the Vocabulaire International de Metrologie—VIM [3]), and assigned the base unit kilogram. ‘Numerosity’ was not. Neither was ‘number of entities’. This disregarded the discontinuous structure of matter. In 1971, the Conference Generale des Poids et Mesures (CGPM), on the proposal of the Comite International des Poids et Mesures (CIPM) and without a consultative committee on chemical measurements in existence (that Committee started to be operational in 1996 only), decided to define a unit ‘‘mole’’ as follows (see Sect. 2.1.1.6 p 115 in [2]):