Avogadro constant

Avogadro constant
Avogadro constant
	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 $N A$ ^[1] or $L$ ,^[2] is an SI defining constant with an exact value of 6.02214076×10²³ mol⁻¹ (reciprocal moles).^[3]^[4] It defines the number of constituent particles in one mole, where the particles in question can be either molecules, atoms, ions, ion pairs, or any other elementary entities. The number 6.02214076×10²³ is a dimensionless physical constant known as the Avogadro number, commonly denoted $N 0$ ^[5]^[6] and often confused with the constant. Both are named after the Italian physicist and chemist Amedeo Avogadro.

The Avogadro constant is used as a normalization factor in relating the amount of substance ${\textstyle n(\mathrm {X} )}$ , in a sample of a substance $\mathrm {X}$ , to the corresponding number of elementary entities $N(\mathrm {X} )$ :

$n(\mathrm {X} )={\frac {N(\mathrm {X} )}{N_{\mathrm {A} }}}=N(\mathrm {X} )\,\left({\frac {1\;\mathrm {mol} }{N_{0}}}\right)$

The Avogadro constant $N A$ is also the factor that converts the average mass ( $m$ ) of one particle, in grams, to the molar mass ( $M$ ) of the substance, in grams per mole (g/mol).^[7] That is, $M=m\cdot N_{\text{A}}$ . Historically, this was precisely true, but since the 2019 revision of the SI, the relation is now merely approximate.

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/(6.022×10²³) mL, or about 0.030 nm³ (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).

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^ Newell, David B.; Tiesinga, Eite (2019). The International System of Units (SI). NIST Special Publication 330. Gaithersburg, Maryland: National Institute of Standards and Technology. doi:10.6028/nist.sp.330-2019. S2CID 242934226.
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[SI2019-3] Newell, David B.; Tiesinga, Eite (2019). The International System of Units (SI). NIST Special Publication 330. Gaithersburg, Maryland: National Institute of Standards and Technology. doi:10.6028/nist.sp.330-2019. S2CID 242934226.

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Avogadro constant
Amedeo Avogadro, the constant's namesake
Common symbols	$N .mw-parser-output .noitalic{font-style:normal} A$ , $L$
SI unit	mol⁻¹
Exact value
reciprocal mole	6.02214076×10²³