National Physical Laboratory

The mole (mol)

The mole is a base unit of the International System of Units (SI).

A mole is the amount of substance containing as many elementary entities as there are atoms in exactly 0.012 kilogram (or 12 grams) of carbon-12, where the carbon-12 atoms are unbound, at rest and in their ground state.

The mole is used to describe a practical quantity of material and is the link between the microscopic and macroscopic worlds, used to scale phenomena from the atomic up to 'relevant' sizes. As a result of the definition, the mole contains a defined number of entities, usually atoms or molecules. This number is the Avogadro constant (NA).

The current value for NA is:

6.022 141 79(30)x1023 mol-1

This number is a dauntingly large figure.

  • This number of sand grains would cover the United Kingdom to a depth of about 40┬ácentimetres.
  • There are about this number of human cells on Earth.
  • It would take you twenty thousand million million years to count this number of coins (counting about one coin per second).

The mole is not realised as a unique physical artefact like the prototype kilogram. However, it can be realised by certain reproducible experimental methods known as 'primary methods'. One example is by weighing a sample of material of known composition.

In common with several other SI units, the definition of the mole is presently being reconsidered. Since it is related to the kilogram, there is a pressure to reconsider its definition and relate it more directly to a specified number of entities. This would lead to a fixed value for the Avogadro constant.

Last Updated: 14 Feb 2014
Created: 14 Oct 2010

SI Base Units

Ampere Candela Kelvin Kilogram Metre Mole Second


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