mole
Amount of substanceexact by definition
The mole is the SI base unit of amount of substance, defined since 2019 as exactly 6.02214076e23 elementary entities. That number, the Avogadro number, is now fixed by definition rather than measured, so the mole no longer depends on any physical sample. The entity has to be stated: a mole of oxygen atoms and a mole of oxygen molecules are different amounts of matter.
Watch out: The mole counts entities, not mass and not volume. One mole of hydrogen gas weighs 2 g and one mole of mercury weighs 200 g, and both contain the same number of molecules or atoms. Chemistry is written in moles precisely because reactions balance by count, never by weight.
| 1 mol | 1 mol |
The name comes from the German Mol, a shortening of Molekül, in use by the end of the nineteenth century. From 1971 until 2019 the mole was defined as the number of atoms in 12 grams of carbon-12, which tied it to a mass standard. The 2019 redefinition cut that link by fixing the Avogadro number outright.
About the mole
A mole is a count, in the same way a dozen is a count. The only difference is scale: one mole is \(6.02214076 \times 10^{23}\) of something, a number chosen so that a mole of atoms of an element weighs, in grams, very nearly the element's atomic mass. That is the entire purpose of the unit. Atoms react in whole-number ratios, so the useful bookkeeping unit is a count, but the only thing a laboratory can measure directly is mass. The mole is the bridge between the two.
Concretely: two hydrogen molecules react with one oxygen molecule to make two waters. Weighed out, that is 4 g of hydrogen with 32 g of oxygen making 36 g of water, and the 2:1 ratio is invisible in those numbers. Counted in moles it is 2 mol + 1 mol → 2 mol, and the equation says what it means. Every stoichiometric calculation is the same three steps: convert mass to moles by dividing by molar mass, apply the whole-number ratio from the balanced equation, convert back to mass.
Since 20 May 2019 the mole has been defined by fixing the Avogadro number exactly, rather than by counting the atoms in 12 g of carbon-12. The practical consequence is subtle but real: the molar mass constant, the thing that lets you read an atomic mass off the periodic table and call it grams per mole, is no longer exactly 1 g/mol. It is now a measured quantity, \(0.99999999965\) g/mol, uncertain in the tenth decimal place. Nobody outside metrology will ever notice, and the periodic table works exactly as before.