What’s in a number?

The mole is all set to be redefined. The International Union of Pure and Applied Chemistry (Iupac) has decreed that in future the unit no longer needs to be based upon the number of elemental entities in 12g of carbon-12 – a definition many of us had drilled into us at school. In a bid to uncouple the unit from another SI unit – the kilogram – the new definition relies on fixing Avogadro’s constant at precisely 6.02214076 × 1023mol−1. But it’s such a miniscule tweak (see p9), you might wonder why anyone aside from a few metrologists should care. In a nutshell, it’s because the mole is central to the story of chemistry.