Avogadro Constant

In chemistry and physics, the Avogadro constant (symbols: L, N_A) is defined as the number of constituent particles (usually atoms or molecules) in a mole of the substance. It is a dimensionless number and has the value 6.02214129(27)×1023 mol−1. Changes in the SI units are proposed that will change the constant to exactly 6.02214X×1023 when it is expressed in the unit mol−1 (see New SI definitions).

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 gram-molecule of hydrogen. It was later redefined as the number of atoms in 12g of the isotope carbon-12. Thus, Avogadro's number is a dimensionless quantity and has the numerical value of the Avogadro constant given in base units.