Editing Avogadro constant (section)

== Definition ==
[[Image:Mole carbon-12 diagram.svg|thumb|400px|Approximate definition of a mole based on 12 grams of carbon-12]]

The Avogadro constant was historically derived from the old definition of the mole as the amount of substance in 12&nbsp;[[gram|grams]] of [[carbon-12]] (<sup>12</sup>C); or, equivalently, the number of [[dalton (unit)|daltons]] in a gram, where the dalton is defined as {{sfrac|1|12}} of the mass of a <sup>12</sup>C atom.<ref name="bipm8th" /> By this old definition, the numerical value of the Avogadro constant in mol<sup>−1</sup> (the Avogadro number) was a physical constant that had to be determined experimentally.

The redefinition of the mole in 2019, as being the amount of substance containing exactly {{val|6.02214076|e=23}} particles,<ref name=NIST2019/> meant that the mass of 1 mole of a substance is now exactly the product of the Avogadro number and the average mass of its particles. The dalton, however, is still defined as {{sfrac|1|12}} of the mass of a <sup>12</sup>C atom, which must be determined experimentally and is known only with finite [[accuracy and precision|accuracy]]. The prior experiments that aimed to determine the Avogadro constant are now re-interpreted as measurements of the value in grams of the dalton.

By the old definition of mole, the numerical value of the mass of one mole of a substance, expressed in grams, was precisely equal to the average mass of one particle in daltons. With the new definition, this numerical equivalence is no longer exact, as it is affected by the uncertainty of the value of the dalton in SI units. However, it is still applicable for all practical purposes. For example, the average mass of one molecule of [[water]] is about 18.0153&nbsp;daltons, and of one mole of water is about 18.0153&nbsp;grams. Also, the Avogadro number is the approximate number of [[nucleon]]s ([[proton]]s and [[neutron]]s) in one gram of ordinary [[matter]]. 

In older literature, the Avogadro number was also denoted {{mvar|N}},<ref name=pauling/><ref name=mcgraw/> although that conflicts with the symbol for [[number of particles]] in [[statistical mechanics]].