Editing Mole (unit) (section)

=== Standardization ===
Developments in [[mass spectrometry]] led to the adoption of [[oxygen-16]] as the standard substance, in lieu of natural oxygen.<ref>{{cite journal |last1=Busch |first1=Kenneth |title=Units in Mass Spectrometry |journal=Current Trends in Mass Spectrometry |date=May 2, 2003 |volume=18 |issue=5S |pages=S32-S34 [S33] |url=https://cdn.sanity.io/files/0vv8moc6/spectroscopy/cfb6f4cb3d02243b516bce3b11dc3584733be2b1.pdf/article-55961.pdf |access-date=29 April 2023}}</ref>

The oxygen-16 definition was replaced with one based on carbon-12 during the 1960s. The International Bureau of Weights and Measures defined the mole as "the amount of substance of a system which contains as many elementary entities as there are atoms in 0.012&nbsp;kilograms of carbon-12." Thus, by that definition, one mole of pure <sup>12</sup>C had a mass of ''exactly'' 12&nbsp;[[Gram|g]].<ref name="SI114-15" /><ref name="IUPAChist">{{AtomicWeightHistory}}</ref> The four different definitions were equivalent to within 1%.

{|class="wikitable" align="center" style="margin:.5em;"
! Scale basis
! Scale basis<br />relative to {{sup|12}}C = 12
! Relative deviation<br />from the {{sup|12}}C = 12 scale
|-
| Atomic mass of hydrogen = 1
| 1.00794(7)
| align="center" | −0.788%
|-
| Atomic mass of oxygen = 16
| {{val|15.9994|(3)}}
| align="center" | +0.00375%
|-
| Relative atomic mass of {{sup|16}}O = 16
| {{val|15.9949146221|(15)}}
| align="center" | +0.0318%
|-
|}

Because a [[Dalton (unit)|dalton]], a unit commonly used to measure [[atomic mass]], is exactly 1/12 of the mass of a carbon-12 atom, this definition of the mole entailed that the mass of one mole of a compound or element in grams was numerically equal to the average mass of one molecule or atom of the substance in daltons, and that the number of daltons in a gram was equal to the number of elementary entities in a mole. Because the mass of a [[nucleon]] (i.e. a [[proton]] or [[neutron]]) is approximately 1 dalton and the nucleons in an atom's nucleus make up the overwhelming majority of its mass, this definition also entailed that the mass of one mole of a substance was roughly equivalent to the number of nucleons in one atom or molecule of that substance.

Since the definition of the gram was not mathematically tied to that of the dalton, the number of molecules per mole ''N''<sub>A</sub> (the Avogadro constant) had to be determined experimentally. The experimental value adopted by [[Committee on Data for Science and Technology|CODATA]] in 2010 is {{nowrap|1=''N''<sub>A</sub> = {{val|6.02214129|(27)|e=23|u=mol-1}}}}.<ref>[http://physics.nist.gov/cgi-bin/cuu/Value?na physics.nist.gov/] {{webarchive|url=https://web.archive.org/web/20150629063615/http://physics.nist.gov/cgi-bin/cuu/Value?na |date=2015-06-29 }} Fundamental Physical Constants: Avogadro Constant</ref>
In 2011 the measurement was refined to {{val|6.02214078|(18)|e=23|u=mol-1}}.<ref>{{cite journal | first = Birk | last = Andreas | title = Determination of the Avogadro Constant by Counting the Atoms in a <sup>28</sup>Si Crystal | journal=Physical Review Letters | volume = 106 | issue = 3 | year=2011 | pages = 30801 | doi=10.1103/PhysRevLett.106.030801 | pmid = 21405263 | bibcode=2011PhRvL.106c0801A|arxiv = 1010.2317 | s2cid = 18291648 |display-authors=etal}}</ref>

The mole was made the seventh [[SI base unit]] in 1971 by the 14th CGPM.<ref>{{cite web|url=http://www.bipm.org/en/CGPM/db/14/3/|title=BIPM – Resolution 3 of the 14th CGPM|website=www.bipm.org|access-date=1 May 2018|url-status=dead|archive-url=https://web.archive.org/web/20171009112117/http://www.bipm.org/en/CGPM/db/14/3|archive-date=9 October 2017}}</ref>