What is Avogadro's number?

Avogadro’s number is 6.022 × 10^23 elementary entities per mole. It tells us how many particles—such as atoms, molecules, ions, or electrons—are present in one mole of a substance. The more precise modern value is 6.02214076 × 10^23, and since 2019 this value has been exact by definition in the International System of Units.

The number is named after Italian scientist Amedeo Avogadro, who proposed in 1811 that equal volumes of gases at the same temperature and pressure contain equal numbers of molecules. Avogadro himself did not determine the numerical value; later scientists developed experimental methods to estimate it.

A mole is a counting unit, much like a dozen, but enormously larger. One mole of water contains approximately 6.022 × 10^23 water molecules, while one mole of carbon contains the same number of carbon atoms. Do not confuse Avogadro’s number with the speed of light or the elementary charge: those are represented by different constants and have different units.