The Mole and the New System of Units (SI)

Abstract

The mole (symbol: mol) – the unit of the amount of substance – was first realized and redefined with the Avogadro constant in the context of the revision of the base units of the International System of Units (SI) in 2019. The number of entities in 1 mol is equal to the numerical value of the fixed Avogadro constant: 1 mol = 6.02214076 × 1023 specified elementary entities. Starting with a historical survey, the realization and dissemination after the revision of the SI is explained with the X-ray crystal density (XRCD) method of the Avogadro project: “counting” silicon atoms in a 1 kg Si sphere. As a key experiment, the determination of the molar mass of silicon – for this purpose highly enriched in the isotope 28Si – with its successive improvements is reported. The aim of this chapter is to provide a state-of-the art and compact description of the mole, its history, changes, and the new definition. Links of the amount-of-substance to other relevant amount- and mass-related quantities are outlined, and examples of use for the practical scientist are given.KeywordsInternational System of Units SIRevisionMoleAmount of substanceXRCD methodAvogadro projectSilicon