measure for measure Whack a mole October 23 is (unofcially) known by some chemists as Mole Day. Andrea Taroni attempts to get to grips with the concept of the mole itself, and the imminent change to its defnition. hile it may not be entirely obvious of the mole, the proposed to those of us sitting on this side definition is4: of the Atlantic, the American W “The mole, symbol mol, style of writing dates has allowed a number is the SI unit of amount of chemistry enthusiasts to declare the of substance. One mole period between 6:02 a.m. and 6:02 p.m. on contains exactly October 23 Mole Day (https://www.moleday. 6.02214076 × 1023 org) — a reference to an approximate value elementary entities. for Avogadro’s constant, 6.02 × 1023, which This number is the fixed defines the number of particles in one mole numerical value of the (mol) of substance1. Avogadro constant, NA, For most young scientists encountering when expressed in the unit mol–1 and is called the chemistry for the first time, moles come Avogadro number.” pretty much straight after being introduced to the principles of stoichiometry and the A formal vote law of conservation of mass, where the total scheduled to be held mass of the reactants equals the total mass of on 16 November 2018 the products. For those of us that did not go at the 26th General on to become accomplished chemists, this Conference on Weights probably stirs traumatic memories of endless and Measures (CGPM) exercises attempting to convert grams to in Versailles, France, is moles, while keeping in mind the conceptual expected to approve this difference between the mass of a substance proposal and, alongside and the amount of substance (the ratio of the revised definitions which is the molar mass, of course) — and for the ampere, kilogram somehow not losing track of stoichiometric and kelvin, it is likely to ratios in the chemical reactions themselves. come into force on An impending change to the definition of 20 May 2019 — an event the mole might help us clear up this molar that will be marked as confusion. It is perhaps surprising to recall World Metrology Day3. that the mole is an SI base unit, in the same One consequence category as the second, metre, ampere, of this change is that it Credit: Andrew Lambert Photography/Science Photo Library kelvin, kilogram and candela. Currently, its severs the link with the definition links it to the kilogram, by defining dalton (Da, also known it as the amount of substance of a system as the unified atomic mass unit, u), which carbon-12 reference mass. We must await that contains as many elementary entities is currently also defined in terms of the the outcome of the discussions at the CGPM (atoms, molecules, ions, electrons… ) as there mass of the 12C atom (so that the mass of to find out how this will get resolved. ❐ are atoms in 0.012 kilograms of carbon-12 a 12C atom is exactly 12 dalton). Though (pictured). This number is expressed by the not categorized as an SI unit, the dalton is Andrea Taroni Avogadro constant, which has a (for now accepted within the SI system and used fairly Chief Editor at Nature Physics. still experimentally determined) value of widely — for example, when comparing e-mail: [email protected] 6.02214076 × 1023 mol–1 (refs 1,2). mass on an atomic or molecular scale. As part of the proposed redefinition Whereas the current definition of the Published online: 3 October 2018 of the SI units in terms of fundamental mole also means that the number of daltons https://doi.org/10.1038/s41567-018-0306-6 constants3, the ampere, kilogram, kelvin in a gram is exactly the numerical value and mole will be redefined by choosing of the Avogadro number (that is, N References A 1. Massa, E. & Mana, G. Nat. Phys. 12, 522 (2016). –1 exact numerical values for the elementary u = 0.001 kg mol ), its new definition 2. Fujii, K., Massa, E., Bettin, H., Kuramoto, N. & Mana, G. electric charge, and the Planck, Boltzmann will result in this no longer being true. Metrologia 55, L1–L4 (2018). and Avogadro constants, respectively (the Proposals to address this discrepancy 3. Fischer, J. & Ullrich, J. Nat. Phys. 12, 4–7 (2016). 5 4. BIPM Draf of the SI Brochure 9th edn (2018); https://www.bipm. second, metre and candela are already have been put forward , but they would org/utils/en/pdf/si-revised-brochure/Draf-SI-Brochure-2018.pdf defined by physical constants). In the case necessarily decouple the dalton from the 5. Leonard, B. P. Metrologia 49, 487–491 (2012). 1068 NATURE PHYSICS | VOL 14 | OCTOBER 2018 | 1068 | www.nature.com/naturephysics.