Avogadro’s Number

Avogadro’s Number, the number of molecules that exist in one mole, or gram molecular weight, of any substance. One gram molecular weight is the weight of a substance, in grams, that is numerically equivalent to the dimensionless molecular weight of that substance (see Periodic Law). The number of molecules in one gram molecular weight has been determined to be approximately 6.0221367 × 10²³ molecules, as established by various methods currently available to physical chemists.

The Avogadro number is named in honor of the Italian physicist Amedeo Avogadro, who postulated in 1811 that equal volumes of gases, at equivalent temperatures and pressures, contain the same number of molecules (see Avogadro's Law). The theory was significant in the development of chemistry, but the number itself was not calculated until the later 19th century, when the concept was extended to include not only gases but all chemicals (see History of Chemistry: Molecular Theory). Volume considerations do not apply to liquids or solids, but Avogadro's number itself holds true for all substances, whatever their state.