Avogadro’s number is the number of units in one mole of a substance, or 6.02214076 × 1023. This number is also called the Avogadro constant. It is named after the 19th-century Italian physicist Amedeo Avogadro, who found that under the same temperature and pressure, two gases with the same volume have the same number of molecules. It was the French physicist Jean Perrin who in the early 20th century dubbed the amount of units in a mole as Avogadro’s number.