A new definition of the mole
The story behind changing the unit at the heart of chemistry
The mole became part of the International System of Units (the SI) in 1971. Currently, it is defined as the amount of substance that contains as many entities as there are in 0.012kg of carbon-12. But the need to redefine the kilogram has been discussed for decades – not least because of the long-term instability of ‘Le Grand K’, the artefact that serves to define the kilogram. Following the success of the 1983 definition of the metre, which is based on the speed of light in vacuum, the International Bureau of Weights and Measures (BIPM) decided that this is also an opportune time to recast the definitions of all seven SI base units in terms of the most stable things known to scientists: physical constants.