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 is this defined number of constituent particles (usually molecules, atoms, ions, or ion pairs—in general, entities) per mole (SI unit) and used as a normalization factor in relating the amount of substance, n(X), in a sample of a substance X to the corresponding number of entities, N(X): n(X) = N(X)(1/N_A), an aggregate of N(X) reciprocal Avogadro constants. By setting N(X) = 1, a reciprocal Avogadro constant is seen to be equal to one entity, which means that n(X) is more easily interpreted as an aggregate of N(X) entities. 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,^[5][6] is the numeric value of the Avogadro constant (i.e., without a unit), namely the dimensionless number 6.02214076×10²³; the value chosen based on the number of atoms in 12 grams of carbon-12 in alignment with the historical definition of a mole.^[1][7] The constant is named after the Italian physicist and chemist Amedeo Avogadro (1776–1856).

The Avogadro constant N_A is also the factor that converts the average mass (${\displaystyle m}$) of one particle, in grams, to the molar mass (${\displaystyle M}$) of the substance, in grams per mole (g/mol).^[8] That is, ${\displaystyle M=m\cdot N_{A}}$.

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, 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).