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Concept
Avogadro constant
Summary
The Avogadro constant, commonly denoted N_A or L, is an SI defining constant with an exact value of 6.02214076e23reciprocal moles.
It is used as a normalization factor in the amount of substance in a sample (in units of moles), defined as the number of constituent particles (usually molecules, atoms, or ions) divided by N_A. In practice, its value is often approximated as 6.02×1023 or 6.022×1023 particles per mole. The constant is named after the physicist Amedeo Avogadro (1776–1856).
In the SI dimensional analysis of measurement units, the dimension of the Avogadro constant is the reciprocal of mole (mol-1). The Avogadro number, sometimes denoted N_0, is the numeric value of the Avogadro constant, namely the dimensionless number 6.02214076e23.
The Avogadro constant N_A is also the factor that converts the average mass of one particle, in grams, to the molar mass of the substance, in grams per mole (g/mol).
The constant N_A also relates the molar volume (the volume per mole) of a substance to the average volume nominally occupied by one of its particles, when both are expressed in the same units of volume. For example, since the molar volume of water in ordinary conditions is about 18 mL/mol, the volume occupied by one molecule of water is about 18/6.022 mL, or about 0.030nm3 (cubic nanometres). For a crystalline substance, N_0 relates the volume of a crystal with one mole worth of repeating unit cells, to the volume of a single cell (both in the same units).
The Avogadro constant was historically derived from the old definition of the mole as the amount of substance in 12 grams of carbon-12 (12C); or, equivalently, the number of daltons in a gram, where the dalton is defined as 1/12 of the mass of a 12C atom. By this old definition, the numerical value of the Avogadro constant in mol-1 (the Avogadro number) was a physical constant that had to be determined experimentally.
The redefinition of the mole in 2019, as being exactly 6.02214076e23 particles, meant that the mass of 1 mole of a substance is now exactly 6.
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