Editing Relative atomic mass (section)

{{Short description|Type of atomic measurement}}
{{distinguish|atomic mass}}
{{see also|standard atomic weight}}

'''Relative atomic mass''' (symbol: '''''A''{{sub|r}}'''; sometimes abbreviated '''RAM''' or '''r.a.m.'''), also known by the [[deprecation|deprecated]] synonym '''atomic weight''', is a [[Dimensionless quantity|dimensionless]] [[physical quantity]] defined as the ratio of the average [[mass]] of [[atom]]s of a [[chemical element]] in a given sample to the [[atomic mass constant]]. The atomic mass constant (symbol: ''m''{{sub|u}}) is defined as being {{sfrac|1|12}} of the mass of a [[carbon-12]] atom.<ref name="IUPAC1979">{{cite journal | author = International Union of Pure and Applied Chemistry | title = Atomic Weights of the Elements 1979 | url = http://publications.iupac.org/pac-2007/1980/pdf/5210x2349.pdf | doi = 10.1351/pac198052102349 | journal = [[Pure and Applied Chemistry|Pure Appl. Chem.]] | year = 1980 | volume = 52 | pages = 2349–84 | issue = 10 | author-link = International Union of Pure and Applied Chemistry | doi-access = free }}</ref><ref name="GreenBook">{{GreenBookRef2nd|page=41}}</ref> Since both quantities in the ratio are masses, the resulting value is dimensionless. These definitions remain valid<ref name="SIBrochure9thEd">{{citation |title=The International System of Units (SI) |author=International Bureau of Weights and Measures |author-link=New SI |date=20 May 2019  |edition=9th |isbn=978-92-822-2272-0 |url=https://www.bipm.org/utils/common/pdf/si-brochure/SI-Brochure-9.pdf| archive-url = https://web.archive.org/web/20211018184555/https://www.bipm.org/documents/20126/41483022/SI-Brochure-9.pdf/fcf090b2-04e6-88cc-1149-c3e029ad8232 |archive-date=18 October 2021 |url-status=live}}</ref>{{rp|page=134}} even after the [[2019 revision of the SI]].{{efn|There are only two consequences of the revision that are relevant to the present article. First, the [[molar mass]] of carbon-12, ''M''(<sup>12</sup>C), is no longer defined as exactly equal to 12&nbsp;g/mol, but instead has to be determined experimentally and thus has an uncertainty. Its current best value{{physconst|MC12|ref=only}}<ref name="CODATA 2018">{{cite journal |last1=Tiesinga |first1=Eite |last2=Mohr |first2=Peter J. |last3=Newell |first3=David B. |last4=Taylor |first4=Barry N. |title=CODATA recommended values of the fundamental physical constants: 2018 |journal=Reviews of Modern Physics |date=30 June 2021 |volume=93 |issue=2 |doi=10.1103/RevModPhys.93.025010|pmc=9890581 }}</ref>{{rp|page=49}} is {{val|12.0000000126|(37)|u=g/mol}}. Here the "(37)" is a measure of the uncertainty; basically, the "26" (the last two digits in {{val|12.0000000126}}) should be understood as "26&nbsp;±&nbsp;37", as explained at ''{{slink|Uncertainty#In_measurements}}''. However, this is so close to the old value of 12 g/mol (the relative difference is {{val|1.05|e=-9}}) that, in a vast majority of applications, ''M''(<sup>12</sup>C) may still be taken to be exactly 12&nbsp;g/mol; this is of course so by design. Second, the [[Avogadro constant]] ''N''<sub>A</sub> is now exactly equal to {{val|6.02214076|e=23|u=[[reciprocal mole]]s}} by definition, whereas previously it had to be determined experimentally and thus had an uncertainty.<ref name="SIBrochure9thEd"/>{{rp|page=134}}}}{{efn|Immediately following the 2019 revision, ''M''(<sup>12</sup>C) was equal to {{val|12.0000000000|(54)|u=g/mol}}, corresponding to a relative standard uncertainty<ref>{{cite web|url=http://physics.nist.gov/cgi-bin/cuu/Info/Constants/definitions.html|title=Standard Uncertainty and Relative Standard Uncertainty|work=[[CODATA]] reference|publisher=[[NIST]]|access-date=30 August 2023|url-status=live|archive-url=https://web.archive.org/web/20230724175742/https://physics.nist.gov/cgi-bin/cuu/Info/Constants/definitions.html|archive-date=24 July 2023}}</ref> of {{val|4.5|e=-10}}. This uncertainty was "inherited" from the relative standard uncertainty that the product ''hN''<sub>A</sub> had immediately prior to the revision: also {{val|4.5|e=-10}}. (Here ''h'' is the [[Planck constant]]. Following the revisition, the product ''hN''<sub>A</sub> has an exact value by definition.)<ref name="CODATA 2017 Special Adjustment">{{cite journal |last1=Mohr |first1=Peter J |last2=Newell |first2=David B |last3=Taylor |first3=Barry N |last4=Tiesinga |first4=Eite |title=Data and analysis for the CODATA 2017 special fundamental constants adjustment |journal=Metrologia |date=1 February 2018 |volume=55 |issue=1 |pages=125–146 |doi=10.1088/1681-7575/aa99bc |doi-access=free}}</ref>{{rp|page=143}} Conversely, immediately prior to the revision, the Avogadro constant ''N''<sub>A</sub> had a measured value of {{val|6.022140758|(62)|e=23|u=reciprocal moles}}, corresponding to a relative standard uncertainty of {{val|1.0|e=-8}}.
Note that immediately prior to the revision, the product ''hN''<sub>A</sub> was known far more precisely than either ''h'' or ''N''<sub>A</sub> individually<ref name="CODATA 2017 Special Adjustment"/>{{rp|page=139}}.}}

For a single given sample, the relative atomic mass of a given element is the [[weighted arithmetic mean]] of the masses of the individual atoms (including all its [[isotope]]s) that are present in the sample. This quantity can vary significantly between samples because the sample's origin (and therefore its [[radioactivity|radioactive]] history or diffusion history) may have produced combinations of isotopic [[natural abundance|abundances]] in varying ratios. For example, due to a different mixture of stable carbon-12 and [[carbon-13]] isotopes, a sample of elemental carbon from volcanic [[methane]] will have a different relative atomic mass than one collected from plant or animal tissues.

The more common, and more specific quantity known as [[standard atomic weight]] (''A''{{sub|r,standard}}) is an application of the relative atomic mass values obtained from many different samples. It is sometimes interpreted as the ''expected range'' of the relative atomic mass values for the atoms of a given element from all terrestrial sources, with the various sources being [[isotope geochemistry|taken from Earth]].<ref>[http://goldbook.iupac.org/S05907.html Definition of element sample]</ref> "Atomic weight" is often loosely and incorrectly used as a synonym for standard atomic weight (incorrectly because standard atomic weights are not from a single sample). Standard atomic weight is nevertheless the most widely published variant of relative atomic mass.

Additionally, the continued use of the term "atomic weight" (for any element) as opposed to "relative atomic mass" has attracted considerable controversy since at least the 1960s, mainly due to the technical difference between [[weight]] and mass in physics.<ref name="IUPAChist">{{AtomicWeightHistory}}</ref> Still, both terms are officially sanctioned by the [[IUPAC]]. The term "relative atomic mass" now seems to be replacing "atomic weight" as the preferred term, although the term "''standard'' atomic weight" (as opposed to the more correct "''standard'' relative atomic mass") continues to be used.