Avogadro constant
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The Avogadro constant is a physical constant that gives the number of particles—atoms, molecules or other entities—contained in one mole of substance. It is denoted by the symbol N_A. Its value is 6.02214076 × 10^23 particles per mole.
This constant links the atomic scale to the macroscopic scale of measurement. For example, when measuring mass in grams, the Avogadro constant converts it to the number of molecules participating in a chemical reaction.
Definition and the SI system
Following the redefinition of SI units in 2019, the mole is defined precisely: one mole is 6.02214076 × 10^23 particles. Accordingly, the Avogadro constant is now an exactly defined number, not an experimentally determined value with measurement uncertainty. Before 2019, the mole was tied to the mass of 12 grams of carbon-12, and the Avogadro constant was known through experiment.
Name and history
The name comes from Italian scientist Amedeo Avogadro, who in the 19th century proposed that equal volumes of different gases, at the same temperature and pressure, contain the same number of molecules. Avogadro himself did not calculate the constant; the value was later determined by other scientists and was given his name.
Application and relationship to other constants
The Avogadro constant is used in calculating molar mass, the number of electrons transferred in electrolysis, and conversions between macroscopic and microscopic scales of measurement. It is related to other constants: the gas constant R equals N_A multiplied by the Boltzmann constant k, while the Faraday constant F equals N_A multiplied by the elementary charge e.