In chemistry and physics, the Avogadro constant (named after the scientist Amedeo Avogadro) is the number of constituent particles, usually atoms or molecules, that are contained in the amount of substance given by one mole. Thus, it is the proportionality factor that relates the molar mass of a compound to the mass of a sample. Avogadro's constant, often designated with the symbol NA or L, has the value 6.022140857(74)×1023 mol−1 in the International System of Units (SI). Previous definitions of chemical quantity involved Avogadro's number, a historical term closely related to the Avogadro constant, but defined differently: Avogadro's number was initially defined by Jean Baptiste Perrin as the number of atoms in one gram-molecule of atomic hydrogen, meaning one gram of hydrogen. This number is also known as Loschmidt constant in German literature. The constant was later redefined as the number of atoms in 12 grams of the isotope carbon-12 (12C), and still later generalized to relate amounts of a substance to their molecular weight. For instance, to a first approximation, 1 gram of hydrogen element (H), having the atomic (mass) number 1, has 6.022×1023 hydrogen atoms. Similarly, 12 grams of 12C, with the mass number 12 (atomic number 6), has the same number of carbon atoms, 6.022×1023. Avogadro's number is a dimensionless quantity, and has the same numerical value of the Avogadro constant given in base units. In contrast, the Avogadro constant has the dimension of reciprocal amount of substance. Revisions in the base set of SI units necessitated redefinitions of the concepts of chemical quantity. Avogadro's number, and its definition, was deprecated in favor of the Avogadro constant and its definition. Changes in the SI units are proposed to fix the value of the constant to exactly 6.02214X×1023 when it is expressed in the unit mol−1, in which an "X" at the end of a number means one or more final digits yet to be agreed upon.

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