meaning of a metric prefix, such as "milli-".

The unit for amount of substance, 20 May 2019 – present, a base unit in SI. Symbol, mol.

The mole, symbol mol, is the SI 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 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.

Resolutions Adopted.
Resolution 1, appendix 3.
26 CGPM, Versailles 13-16 November 2018.

When the mole is used, the user must specify what elementary entity is meant.

Previous definition, 1967 – 20 May 2019

One mole was “the amount of substance of a system that contains as many elementary entities as there are atoms in 0.012 kilograms of carbon-12”. It was defined by the CIPM in 1967 and adopted in 1971 by the 14ᵗʰ CGPM (Resolution 3). In 1980, the CIPM added that it is understood the atoms of carbon-12 referred to are unbound, at rest, and in their ground state. This later qualification is an essential part of the definition. B. W. Petley points out:

Thus, as a consequence of the binding energy of 7.425 eV per atom, 0.012 kg of ¹²C graphite contains ~4 × 10¹⁴ more ¹²C atoms than the same mass of gas-phase carbon atoms, while the same mass of diamond at 298 K contains ~1 × 10¹² fewer atoms than the same mass of graphite.¹

Originally, the mole was a unit used by chemists to count the uncountable, and was also known as gram molecular weight or gram molecule. One mole of any element or compound was the atomic weight, molecular weight or formula weight, depending on the substance, of the substance in atomic mass units, expressed in grams. This definition made the number of molecules in the mole 6.02 × 10²³, which is the Avogadro constant.

One mole of a perfect gas at a temperature of 273.16 K (0°C) and a pressure of 101,325 pascals (1 atmosphere) has a volume of 22.414 liters.

Recommendation 1 of the 94th meeting of the CIPM (2005), called for Member States to “give consideration to the possibility of redefining ... the mole in terms of a fixed value of the Avogadro constant.”

At its 24ᵗʰ meeting (Paris, October 2011), the CGPM decided to declare in advance its intention to make the value of the Avogadro constant a matter of definition, rather than something to be determined experimentally. The new value was to be exactly 6.022 14X × 10²³ entities per mole, where X stood for one or more yet to be determined digits. The mole was then be redefined as an Avogadro constant number of entities, removing all reference to atoms of carbon. The new definition will not be adopted before 2014.

1. B. W. Petley.
The mole and the unified atomic mass unit.
Metrologia, volume 33, pages 261-264 (1996).

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