Editing Avogadro constant (section)

=== SI definition of 1971 ===
In 1971, in its 14th conference, the [[International Bureau of Weights and Measures]] (BIPM) decided to regard the [[amount of substance]] as an independent [[dimensional analysis|dimension of measurement]], with the mole as its [[SI unit|base unit]] in the [[International System of Units]] (SI).<ref name=bipm1971/> Specifically, the mole was defined as the amount of a substance that contains as many elementary entities as there are atoms in {{nowrap|12 grams}} ({{nowrap|0.012 [[kilogram]]s}}) of [[carbon-12]] (<sup>12</sup>C).<ref name=bipm8th/> Thus, in particular, an amount of one mole of carbon 12 had a corresponding mass that was ''exactly'' {{nowrap|12 grams}} of that element.  

By this definition, one mole of any substance contained exactly as many elementary entities as one mole of any other substance. However, this number {{math|''N''{{sub|0}}}} was a physical constant that had to be experimentally determined since it depended on the mass (in grams) of one atom of <sup>12</sup>C, and therefore, it was known only to a limited number of decimal digits.<ref name=bipm1971/> The common rule of thumb that "one gram of matter contains {{math|''N''{{sub|0}}}} nucleons" was exact for carbon-12, but slightly inexact for other elements and isotopes.

In the same conference, the BIPM also named {{math|''N''{{sub|A}}}} (the factor that related the amount of a substance to the corresponding number of particles) the "Avogadro ''constant''". However, the term "Avogadro number" continued to be used, especially in introductory works.<ref name=kotz2008/> As a consequence of this definition, {{math|''N''{{sub|A}}}} was not a pure number, but had the [[dimensional analysis|metric dimension]] of reciprocal of amount of substance (mol<sup>−1</sup>).