Other meanings of Avogadro constant
Physical chemistry
The Avogadro constant is the exact number of specified constituent particles in one mole: 6.02214076 × 1023 mol−1. It connects the microscopic scale of atoms, molecules, ions, and other entities with measurable macroscopic amounts of substance.
The Avogadro constant gives the number of specified entities corresponding to one mole. Its exact value is 6.02214076 × 1023 mol−1, where the entities may be atoms, molecules, ions, electrons, other particles, or specified groups of particles.1 The numerical value became exact when the revised International System of Units defined the mole by fixing the numerical value of the constant in 2019.2
The unit mol−1 indicates that the constant converts an amount of substance, measured in moles, into a number of entities. Thus, a sample containing 0.250 mol of water molecules contains 0.250NA molecules. The constant is not itself a count of atoms in every sample: the relevant entity must always be identified, and the amount of substance may be any value, including a fraction of a mole.
The constant provides the bridge between microscopic particle counts and laboratory measurements. In chemistry, the relation N = nNA converts amount of substance n into entity count N, while molar mass connects moles to a sample’s mass; analogous relations connect moles with concentration, gas volume, and charge.3
Before the 2019 SI revision, the mole was defined through the mass of carbon-12: one mole contained as many entities as there are atoms in 0.012 kg of carbon-12. That wording made the numerical value of the constant experimentally determined and linked it to the kilogram. The modern definition separates the mole from any particular substance and makes the constant exact, while measured molar masses retain the effects of isotopic composition and atomic-weight variation.1
The constant’s name honors Amedeo Avogadro, whose 1811 molecular hypothesis distinguished the number of particles in equal volumes of gases under the same conditions from the gases’ chemical identities. Avogadro did not determine the modern numerical value, and the term “Avogadro number” was introduced later in the development of molecular theory.4
Its value was progressively constrained by independent measurements, including crystallographic measurements of silicon, electrical methods involving the Josephson effect and quantum Hall effect, and measurements of the kilogram and Planck constant. Such methods test relationships among macroscopic constants and microscopic entities rather than literally counting every particle in a sample.5 Modern recommended values are maintained through international evaluations of fundamental constants.6
The entity named after “one mole” need not be a chemically elementary particle. A mole of electrons, formula units of sodium chloride, photons, or specified atoms of one isotope is valid if the counting entity is stated precisely. This makes the constant useful in nuclear chemistry, materials science, electrochemistry, and biochemistry as well as conventional stoichiometry.
The exact value does not make every related quantity exact. Molar mass, atomic weight, concentration, and the measured number of particles in a physical sample can still carry uncertainty; natural materials also differ in isotopic composition. A further subtlety is that Avogadro constant and “Avogadro number” are often used interchangeably in informal writing, although the former is the formally defined physical constant with unit mol−1, whereas a pure particle count is dimensionless. The constant also links to the former carbon-12 definition without requiring carbon-12 to serve as the modern reference substance.1
The symbol Nₐ is commonly rendered as N_A in plain text; the exact SI value is written here with mol⁻¹ units.
Help improve the encyclopedia. Reports go straight to the site manager.