Unit of the month – the mole

In preparation for the SI changes that are coming in on 20 May 2019, we are looking at a unit a month

Following the recent decision, taken by measurement scientists from around the world, to revise the International System of Units (SI), on the 20 of each month we will be looking at one of the seven SI base units. You'll be able to find out where it's used in everyday life, how it's defined now, and the changes that will come into force on 20 May 2019.

20 November 2018 the metre (m)
20 December 2018 the candela (cd)
20 January 2019 the ampere (A)
20 February 2019 the kelvin (K)
20 March 2019 the second (s)
20 April 2019 the mole (mol)
20 May 2019 the kilogram (kg)









The International System of Units (SI) will be revised on 20 May 2019. As part of this process four of the SI’s base units – the kilogram, ampere, kelvin and mole will acquire new definitions. The mole is perhaps the least well understood of the SI base units and the differences between its current and future definitions the most subtle.

The current definition refers to the mole containing as many elementary entities as there are atoms in 0.012 kilogram of carbon 12. Therefore currently we only know implicitly how many elementary entities this is by using a measure of an exact mass of a known material as a surrogate. The new definition removes this ambiguity, and reliance of the mole on the kilogram, by stating an exact number of elementary entities explicitly. From 20 May 2019 one mole contains exactly 6.022 140 76  x 1023 elementary entities.

One of the benefits of the new definition is that it reflects the way most chemists already consider and teach the mole. Indeed, one study in the years of stakeholder consultation leading up to redefinition (Sci. Educ. 2008, 17, 403) reviewed 30 university level general chemistry textbooks and discovered that 28 out of 30 did not use the current definition of the mole appropriately – instead they described a mole as being equivalent to an ‘Avogadro number’ of entities: much closer to the new definition!

To provide a clearer explanation of the redefinition of the mole and its implications Dr Richard Brown, NPL’s Head of Metrology, has recently written a background paper on the topic for the Royal Society of Chemistry’s Analytical Methods Committee, now published in Analytical Methods.

Find out more about Redefining the SI units

20 Apr 2019