[3][4] It is this defined number of constituent particles (usually molecules, atoms, ions, or ion pairs—in general, entities) per mole (SI unit) and used as a normalization factor in relating the amount of substance, n(X), in a sample of a substance X to the corresponding number of entities, N(X): n(X) = N(X)(1/NA), an aggregate of N(X) reciprocal Avogadro constants.

In the SI dimensional analysis of measurement units, the dimension of the Avogadro constant is the reciprocal of amount of substance, denoted N−1.

The redefinition of the mole in 2019, as being the amount of substance containing exactly 6.02214076×1023 particles,[7] 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 prior experiments that aimed to determine the Avogadro constant are now re-interpreted as measurements of the value in grams of the dalton.

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.

In 1910, Robert Millikan with the help of Harvey Fletcher obtained the first measurement of the charge on an electron.

[19] In 1971, in its 14th conference, the International Bureau of Weights and Measures (BIPM) decided to regard the amount of substance as an independent dimension of measurement, with the mole as its base unit in the International System of Units (SI).

[16] Specifically, the mole was defined as the amount of a substance that contains as many elementary entities as there are atoms in 12 grams (0.012 kilograms) of carbon-12 (12C).

[16] The common rule of thumb that "one gram of matter contains N0 nucleons" was exact for carbon-12, but slightly inexact for other elements and isotopes.

In the same conference, the BIPM also named NA (the factor that related the amount of a substance to the corresponding number of particles) the "Avogadro constant".

[20] As a consequence of this definition, NA was not a pure number, but had the metric dimension of reciprocal of amount of substance (mol−1).

[22][23] Thus, the molar mass constant remains very close to but no longer exactly equal to 1 g/mol, although the difference (4.5×10−10 in relative terms, as of March 2019) is insignificant for all practical purposes.