What Chemistry Teachers Should Know about the Revised International System of Units (Système International)

Abstract

In November 2018, a revision of the International System of Units (Système International, SI) was approved at the 26th meeting of the General Conference on Weights and Measures (CGPM). What are the implications of the revision of all seven definitions of SI base units for the teaching of chemistry? (1) No names of base units or their corresponding physical quantities have changed. For the precision typically employed in educational settings (say six or fewer significant figures), no values of base units have changed either. (2) The mole is officially defined as a definite number (6.02214076 × 10 23 ) of elementary entities. (3) The kilogram is defined by setting a fixed value for the Planck constant. (4) The revision defines units indirectly by assigning fixed values to key physical constants (explicit-constant definitions rather than explicit-unit definitions). (5) Some quantities previously regarded as exact now have small uncertainties and vice versa.