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In chemistry and physics, the Avogadro constant (symbols: L, N_{A}) is defined as the ratio of the number of entities (usually atoms or molecules) N in a sample to the amount of substance n (unit mole) through the relationship N_{A} = N/n.^{[1]} Thus it is the proportionality factor that relates the molar mass of an entity, i.e. the mass per amount of substance, to the mass of said entity.^{[2]} The Avogadro constant expresses the number of elementary entities per mole of substance and it has the value 6.02214179(30)×1023 mol^{1}.^{[2]}^{[3]}^{[4]}
Previous definitions of chemical quantity involved Avogadro's number, a historical term closely related to the Avogadro constant. Revisions in the base set of units of the International System of Units (SI) necessitated redefinitions of the concepts of chemical quantity. Avogadro's number was defined by Perrin as the number of molecules in one grammolecule of oxygen and it is also the count of elementary entities as there are atoms in 0.012kg of the isotope carbon12^{[5]} Thus, Avogadro's number is a dimensionless quantity and has the numerical value of the Avogadro constant given in base units.
Contents
History
The Avogadro constant is named after the early nineteenth century Italian scientist Amedeo Avogadro, who, in 1811, first proposed that the volume of a gas (at a given pressure and temperature) is proportional to the number of atoms or molecules regardless of the nature of the gas.^{[7]} The French physicist Jean Perrin in 1909 proposed naming the constant in honor of Avogadro.^{[8]} Perrin won the 1926 Nobel Prize in Physics, in a large part for his work in determining the Avogadro constant by several different methods.^{[9]}
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