Editing Avogadro constant (section)

== History ==

=== Origin of the concept ===
[[File:Jean Perrin 1926.jpg|right|thumb|Jean Perrin in 1926]]
The Avogadro constant is named after the Italian scientist [[Amedeo Avogadro]] (1776–1856), who, in 1811, first proposed that the volume of a gas (at a given pressure and temperature) is proportional to the number of [[atom]]s or [[molecule]]s regardless of the nature of the gas.<ref name=avog1811/>

Avogadro's hypothesis was popularized four years after his death by [[Stanislao Cannizzaro]], who advocated Avogadro's work at the [[Karlsruhe Congress]] in 1860.<ref>{{Cite web |date=June 2016 |title=Stanislao Cannizzaro {{!}} Science History Institute |url=https://www.sciencehistory.org/historical-profile/stanislao-cannizzaro |access-date=June 2, 2022 |website=Science History Institute}}</ref>

The name ''Avogadro's number'' was coined in 1909 by the physicist [[Jean Baptiste Perrin|Jean Perrin]], who defined it as the number of molecules in exactly 32 grams of [[oxygen]] gas.<ref name="perrin1909" /> The goal of this definition was to make the mass of a mole of a substance, in grams, be numerically equal to the mass of one molecule relative to the mass of the hydrogen atom; which, because of the [[law of definite proportions]], was the natural unit of atomic mass, and was assumed to be {{sfrac|1|16}} of the atomic mass of oxygen.

=== First measurements ===
[[File:Johann Josef Loschmidt portrait plaque.jpg|right|thumb|Josef Loschmidt]]
The value of Avogadro's number (not yet known by that name) was first obtained indirectly by [[Johann Josef Loschmidt|Josef Loschmidt]] in 1865, by estimating the number of particles in a given volume of gas.<ref name=losch1865/> This value, the [[number density]] {{math|''n''{{sub|0}}}} of particles in an [[ideal gas]], is now called the [[Loschmidt constant]] in his honor, and is related to the Avogadro constant, {{math|''N''{{sub|A}}}}, by
: <math>n_0 = \frac{p_0N_{\rm A}}{R\,T_0},</math>
where {{math|''p''{{sub|0}}}} is the [[pressure]], {{math|''R''}} is the [[gas constant]], and {{math|''T''{{sub|0}}}} is the [[absolute temperature]]. Because of this work, the symbol {{math|''L''}} is sometimes used for the Avogadro constant,<ref name=bipm1971/> and, in [[German language|German]] literature, that name may be used for both constants, distinguished only by the [[units of measurement]].<ref name=virgo1933/> (However, {{math|''N''{{sub|A}}}} should not be confused with the entirely different [[Loschmidt constant]] in English-language literature.)

Perrin himself determined the Avogadro number, which he called "Avogadro's constant" (constante d'Avogadro), by several different experimental methods. He was awarded the 1926 [[Nobel Prize in Physics]], largely for this work.<ref name=oseen1926/>

The electric charge per [[Mole (unit)|mole]] of electrons is a constant called the [[Faraday constant]] and has been known since 1834, when [[Michael Faraday]] published [[Faraday's laws of electrolysis|his works on electrolysis]]. In 1910, [[Robert Millikan]] with the help of [[Harvey Fletcher]] obtained the first measurement of the [[elementary charge|charge on an electron]]. Dividing the charge on a mole of electrons by the charge on a single electron provided a more accurate estimate of the Avogadro number.<ref name=ebrit1974/>

=== 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>).

=== SI redefinition of 2019 ===
{{main|2019 revision of the SI}}
In its 26th Conference, the BIPM adopted a different approach: effective 20 May 2019, it defined the Avogadro constant {{math|''N''{{sub|A}}}} as the exact value {{val|6.02214076|e=23|u=mol-1}}, thus redefining the mole as exactly {{val|6.02214076|e=23}} constituent particles of the substance under consideration.<ref name=cipm106/><ref name=NIST2019/> One consequence of this change is that the mass of a mole of <sup>12</sup>C atoms is no longer exactly 0.012&nbsp;kg. On the other hand, the dalton ({{aka}} universal atomic mass unit) remains unchanged as {{sfrac|1|12}} of the mass of <sup>12</sup>C.<ref name=pave2018/><ref name=IUPAC/> Thus, the [[molar mass constant]] remains very close to but no longer exactly equal to 1&nbsp;g/mol, although the difference ({{val|4.5|e=-10}} in relative terms, as of March 2019) is insignificant for all practical purposes.<ref name=NIST2019/><ref name=bipm9th/>