Mole

The mole (symbol mol) is the SI base unit of amount of substance. One mole contains exactly 6.02214076 × 10²³ entities — atoms, molecules, ions, electrons, photons, or any other elementary entity. That number is Avogadro’s constant, fixed by definition since the 2019 SI revision.

The mole isn’t a measurement of mass or volume; it’s a count, just one expressed in a particular very large unit. The motivation: at the scale of chemistry, individual molecules count, and there are far too many of them to track in normal numbers. The mass of one water molecule is about 3 × 10⁻²³ grams; the mass of one mole of water (18 g) is conveniently bench-scale.

Practical use: a chemist who needs 1 mole of NaCl weighs out 58.44 g (the molar mass). 1 mole of dissolved sodium chloride in 1 litre of water gives a 1 M (1 molar) solution. Reaction stoichiometry — “2 H₂ + O₂ → 2 H₂O” — counts in moles.

Historical note: until 2019, the mole was defined as the number of atoms in 12 grams of carbon-12 — an indirect definition that depended on the kilogram’s artifact-based definition. The 2019 SI revision made Avogadro’s constant a defined value, decoupling the mole from the kilogram.