He is most noted for his contribution to molecular theory now known as Avogadro's law, which states that equal volumes of gases under the same conditions of temperature and pressure will contain equal numbers of molecules. In tribute to him, the ratio of the number of elementary entities in a substance to its amount of substance, 6.02214076×10^23 mol−1, is known as the Avogadro constant. via @wikipedia
Don't trust the Wikipedia article. The definition of "mole" was changed in 2010 and some editors apparently haven't realized that yet. Now the Avogadro number is an exact number, essentially adimensional like pi or 360 degrees per turn. No longer a "physical constant".
The mole, symbol mol, is the SI unit of amount of substance. One mole contains exactly 6.022 140 76 × 1023 elementary entities. This number is the fixed numerical value of the Avogadro constant, NA, when expressed in mol−1, and is called the Avogadro number. This new definition is in contrast to the current definition adopted in 1971 which relies on the mass of the kilogram.
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