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. Thus, the Avogadro constant N A is the proportionality factor that relates the molar mass of a substance to the average mass of one molecule. 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. 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 being 1 / 12 of the mass of one carbon-12 atom. 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. The Avogadro number is the approximate number of nucleons ( protons and neutrons) in one gram of ordinary matter. In older literature, the Avogadro number is denoted N or N 0, which is the number of particles that are contained in one mole, exactly 6.022 140 76 ×10 23. The numeric value of the Avogadro constant expressed in reciprocal moles, a dimensionless number, is called the Avogadro number. It is named after the Italian scientist Amedeo Avogadro by Stanislao Cannizzaro, who explained this number four years after Avogadro's death while at the Karlsruhe Congress in 1860. It is an SI defining constant with an exact value of 6.022 140 76 ×10 23 reciprocal moles. The Avogadro constant, commonly denoted N A or L, is the proportionality factor that relates the number of constituent particles (usually molecules, atoms or ions) in a sample with the amount of substance in that sample.
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