The SI base unit for the amount of a substance is the mole, with the symbol mol. It provides a way to count an enormous number of atoms, molecules, ions, or other specified elementary entities.
One mole contains exactly 6.02214076 × 10^23 entities, a number called the Avogadro constant when expressed per mole. This large counting unit is useful because ordinary samples contain vast numbers of microscopic particles. For example, one mole of water molecules has a mass of about 18.015 grams.
The mole became an SI base unit in 1971 and received its modern definition during the 2019 revision of the International System of Units. It is now defined by fixing the numerical value of the Avogadro constant, rather than by referring to a physical sample of carbon-12.
A common mistake is choosing gram or liter. A gram measures mass, while a liter measures volume; neither measures the number of chemical entities. The mole is also different from molarity, which describes concentration and is commonly expressed in moles per liter.