SI definition of the mole

1. The mole is the amount of substance of a system that contains as many elementary entities as there are atoms in 0.012 kilogram of carbon 12 (12C); its symbol is mol.

2. When the mole is used, the elementary entities must be specified and may be atoms, molecules, ions, electrons, other particles, or specified groups of such particles.

The dimension of amount of substance is [N].

Possible future definition of the mole

Presently under discussion is redefining the unit of amount of substance in the following way:

The mole is an amount of substance such that the Avogadro constant is exactly 6.022 141 5 × 1023 mol-1 (per mole).

Avogadro's constant and atomic masses

In keeping with the proposed new definition of the mole, Avogadro's constant may be defined as the number NA of elementary entities per mole of substance which has the (current) value 6.022 141 79 × 1023 mol-1 (Mohr et al., 2007). So the number of atoms in 0.012 kg of 12C is 6.022 141 79 × 1023.

Note that the dimension of Avogadro's constant is [N]-1 and its symbol mol-1.

Atomic and molar masses

In SI units, the atomic mass unit (amu) is defined to be exactly 1/12 the mass of one atom of the 12C isotope.

In SI units, the molar mass of a substance is defined to be the mass in kg of 1 mol of the elementary entities (e.g., atoms or molecules) composing the substance.