The mole, symbol mol, is the SI base unit of amount of substance.
One mole contains exactly 6.022 140 76 × 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 definition of the mole implies the exact relation NA = 6.022 140 76 × 1023 mol–1. Inverting this relation gives an exact expression for the mole in terms of the defining constant NA :
The effect of this definition is that the mole is the amount of substance of a system that contains 6.022 140 76 × 1023 specified elementary entities.
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.