The New Definitions of the Mole and Kilogram: How and Why These SI Units Took their New Status

Important changes were recently made to the International System of Units. The readers involved in chemistry and physics, either teachers or advanced students, should be aware of the new definitions of the kilogram, the mole, and more generally of the changes in International System of Units (SI, “Système International”) recently approved at the 26th General Conference of Weights and Measures (26ème Conférence Générale des Poids et Mesures, Versailles, 13-16 November 2018). The general methodology for introducing the changes, along with the historical context, was recently described. The resolutions adopted during this conference, written in parallel in English and French, redefine the four fundamental SI units: kilogram (kg), ampere (A), kelvin (K), and mole (mol). The historical definition of kg followed the 1st General Conference of Weights and Measurements in 1889, and was based on a physical object (Fig. 1), as had been the meter until 1960.