The mole, symbol mol, is the SI unit of amount of substance. One mole contains exactly 6.022 140 76 x 1023 elementary entities. This number is the fixed numerical value of the Avogadro constant, NA, when expressed in the unit mol-1 and is called the Avogadro number.
The amount of substance, symbol n
, of a system is a measure of the number of specified elementary entities. An elementary entity may be an atom, a molecule, an ion, an electron, any other particle or specified group of particles.
The formulation of this definition was agreed upon by the 26th CGPM in November 2018 with effect from 20 May 2019.
BIPM, The International System of Units, SI Brochure, 9th ed. (2019), p. 134 (https://www.bipm.org/en/publications/si-brochure/)