Avogadro constant | |
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![]() Amedeo Avogadro, the constant's namesake | |
Common symbols | NA, L |
SI unit | mol−1 |
Exact value | |
reciprocal mole | 6.02214076×1023 |
The Avogadro constant, commonly denoted NA[1] or L,[2] is an SI defining constant with an exact value of 6.02214076×1023 mol−1 (reciprocal moles).[3][4] It is defined as the number of constituent particles (usually molecules, atoms, or ions) per mole (SI unit) and used as a normalization factor in the amount of substance in a sample. The numerical value of the constant was chosen based on the number of atoms in 12 grams of carbon-12 in alignment with the historical definition of a mole.[1] The constant is named after the Italian physicist and chemist Amedeo Avogadro (1776–1856).
The Avogadro constant NA 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).[5]
The constant NA 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/(6.022×1023) mL, or about 0.030 nm3 (cubic nanometres). For a crystalline substance, N0 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).
In the SI dimensional analysis of measurement units, the dimension of the Avogadro constant is the reciprocal of amount of substance, denoted N−1. The Avogadro number, sometimes denoted N0,[6][7] is the numeric value of the Avogadro constant (i.e., without a unit), namely the dimensionless number 6.02214076×1023.[1][8]
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