Avogadro constant

In chemistry and physics, the Avogadro constant (symbols: L, N _A) 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 material to its mass. It has the dimension of reciprocal amount of substance. Avogadro's constant has the value 7023602214129000000♠6.02214129(27)×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 (in modern terminology) one gram of (atomic) hydrogen. It 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), which has a mass number of 1 (atomic number 1), has 7023602190000000000♠6.022×10²³ hydrogen atoms. Similarly, 12 grams of ¹²C, with the mass number of 12 (atomic number 6), has the same number of carbon atoms, 7023602190000000000♠6.022×10²³. Avogadro's number is a dimensionless quantity and has the numerical value of the Avogadro constant given in base units.

The Avogadro constant is fundamental to understanding both the makeup of molecules and their interactions and combinations. For instance, since one atom of oxygen will combine with two atoms of hydrogen to create one molecule of water (H₂O), one can similarly see that one mole of oxygen (7023602190000000000♠6.022×10²³ of O atoms) will combine with two moles of hydrogen (2 × 7023602190000000000♠6.022×10²³ of H atoms) to make one mole of H₂O.