The **Avogadro constant** (NA[1] or L[2]) is the proportionality factor that relates the number of constituent particles (usually molecules, atoms or ions) in a sample with the amount of substance in that sample. Its SI unit is the reciprocal mole, and it is defined as NA = 6.02214076×1023 mol−1.[3][1][4][5][6] It is named after the Italian scientist Amedeo Avogadro.[7]

The numeric value of the Avogadro constant expressed in reciprocal mole, a dimensionless number, is called the **Avogadro number**, sometimes denoted N[8][9] or N0,[10][11] which is thus the number of particles that are contained in one mole, exactly 6.02214076×1023.[4][1]

The value of the Avogadro constant was chosen so that the mass of one mole of a chemical compound, in grams, is numerically equal (for all practical purposes) to the average mass of one molecule of the compound, in daltons (universal atomic mass units); one dalton being 1/12 of the mass of one carbon-12 atom, which is approximately the mass of one nucleon (proton or neutron). Twelve grams of carbon contains one mole of carbon atoms.

For example, the average mass of one molecule of water is about 18.0153 daltons, and one mole of water (N molecules) is about 18.0153 grams. Thus, the Avogadro constant NA is the proportionality factor that relates the molar mass of a substance to the average mass of one molecule, and the Avogadro number is also the approximate number of nucleons in one gram of ordinary matter.[12]

The Avogadro constant also relates the molar volume of a substance to the average volume nominally occupied by one of its particles, when both are expressed in the same units of volume. For example, since the molar volume of water in ordinary conditions is about 18 mL/mol, the volume occupied by one molecule of water is about 18/6.022×10−23 mL, or about 30 Å3 (cubic angstroms). For a crystalline substance, it similarly relates its molar volume (in mL/mol), the volume of the repeating unit cell of the crystals (in mL), and the number of molecules in that cell.

The Avogadro number (or constant) has been defined in many different ways through its long history. Its approximate value was first determined, indirectly, by Josef Loschmidt in 1865.[13] (Avogadro's number is closely related to the Loschmidt constant, and the two concepts are sometimes confused.) It was initially defined by Jean Perrin as the number of atoms in 16 grams of oxygen.[7] It was later redefined in the 14th conference of the International Bureau of Weights and Measures (BIPM) as the number of atoms in 12 grams of the isotope carbon-12 (12C).[14] In each case, the mole was defined as the quantity of a substance that contained the same number of atoms as those reference samples. In particular, when carbon-12 was the reference, one mole of carbon-12 was exactly 12 grams of the element.