In chemistry, the Avogadro constant, commonly denoted NA, is a conversion constant or ratio between an amount of substance and the number of particles that it contains. The particles in question are any designated elementary entity, such as molecules, atoms, ions, or ion pairs. It is an SI defining constant with the exact value 6.02214076×1023 mol−1 (reciprocal mole). The numerical value of this constant when expressed in terms of the mole is known as the Avogadro number, commonly denoted N0. The Avogadro number is an exact number equal to the number of constituent particles in one mole of any substance (by definition of the mole), historically derived from the experimental determination of the number of atoms in 12 grams of carbon-12 (12C) before the 2019 revision of the SI, i.e. the gram-to-dalton ratio, g/Da. Both the constant and the number are named after the Italian physicist and chemist Amedeo Avogadro.