avogadro_constant
Definition: The Avogadro constant, commonly denoted NA or L, is the ratio that relates the number of constituent particles (usually molecules, atoms, or ions) in a sample with the amount of substance in that sample. It is an SI defining constant with an exact value of 6.02214076×1023 reciprocal moles. It was named after the Italian scientist Amedeo Avogadro by Jean Perrin in 1909. It was popularized by Stanislao Cannizzaro who advocated Avogadro's work at the Karlsruhe Congress in 1860, four years after his death.The numeric value of the Avogadro constant expressed in reciprocal moles, a dimensionless number, is called the Avogadro number. In older literature, the Avogadro number is denoted N or N0, which is the number of particles that are contained in one mole, exactly 6.02214076×1023.The Avogadro number is the approximate number of nucleons (protons and neutrons) in one gram of ordinary matter. The value of the Avogadro constant was chosen so that the mass of one mole of a chemical compound, expressed in grams, is approximately the number of nucleons in one constituent particle of the substance. It is numerically equal (for all practical purposes) to the average mass of one molecule (or atom) of a compound in daltons (unified atomic mass units); one dalton is 1/12 of the mass of one carbon-12 atom. For example, the average mass of one molecule of water is about 18.0153 daltons, and one mole of water (N molecules) is about 18.0153 grams. Thus, the Avogadro constant NA is the proportionality factor that relates the molar mass of a substance to the average mass of one molecule.The Avogadro constant also relates the molar volume 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 18/6.022×10−23 mL, or about 30 Å3 (cubic angstroms). For a crystalline substance, it similarly relates its molar volume (in mol/mL), the volume of the repeating unit cell of the crystals (in mL), to the number of molecules in that cell.
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