One mole of any substance contains 6.02214076 times 10 to the 23rd particles. That number is Avogadro's number, named after Amedeo Avogadro, an Italian scientist who proposed in 1811 that equal volumes of gases at the same temperature and pressure contain equal numbers of molecules. He did not calculate the number himself. That came later, through experiments by Jean Perrin and others. In 2019, the mole was redefined as exactly Avogadro's number of entities, fixing the value.

The number is enormous. One mole of water is about 18 milliliters, a little more than a tablespoon. One mole of carbon is 12 grams. One mole of gold is 197 grams. The number is large because atoms and molecules are tiny. It is the bridge between the microscopic world of atoms and the macroscopic world of grams and liters. Chemists use it to count particles by weighing them.

Avogadro's number appears throughout chemistry and physics. It is used to calculate molar mass, reaction stoichiometry, gas volumes, and solution concentrations. It connects the ideal gas law to the kinetic theory of gases. It appears in the definition of the Faraday constant and the Boltzmann constant. It is one of the fundamental constants of nature.

The number is not arbitrary. It is defined so that one mole of carbon-12 has a mass of exactly 12 grams. That definition ties the atomic mass scale to the macroscopic mass scale, making chemistry quantitative.