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{{short description|Relative atomic mass as defined by IUPAC (CIAAW)}}
{{distinguish|text=[[Atomic mass]]}}
[[File:CIAAW 2013 - Standard atomic weight for cupper (29, Cu).svg|thumb|right
|Example: copper in terrestrial sources. Two isotopes are present: copper-63 (62.9) and copper-65 (64.9), in  abundances 69%&nbsp;+ 31%. The ''standard atomic weight'' {{nowrap|1=(''A''<sub>r</sub>°(Cu))}} for copper is the average, weighted by their natural abundance, and then divided by the [[atomic mass constant]] ''m''<sub>u</sub>.{{CIAAW2016}}]]

The '''standard atomic weight''' of a [[chemical element]] (symbol '''''A''<sub>r</sub>°(E)''' for element "E") is the [[weighted arithmetic mean]] of the [[relative isotopic mass]]es of all [[isotope]]s of that element [[weighted arithmetic mean|weighted]] by each isotope's abundance on [[Earth]].  For example, isotope <sup>63</sup>Cu (''A''<sub>r</sub> = 62.929) constitutes 69% of the [[copper]] on Earth, the rest being <sup>65</sup>Cu (''A''<sub>r</sub> = 64.927), so
:<math>A_\text{r}\text{°}(_\text{29}\text{Cu})=0.69\times62.929+0.31\times64.927=63.55.</math>
Relative isotopic mass is [[dimensionless quantities|dimensionless]], and so is the weighted average. It can be converted into a measure of mass (with [[Dimensional analysis|dimension]] {{dimanalysis|mass=1}}) by multiplying it with the atomic mass constant [[dalton (unit)|dalton]].

Among various variants of the notion of [[relative atomic mass|atomic weight]] (''A''<sub>r</sub>, also known as ''[[relative atomic mass]]'') used by scientists, the standard atomic weight {{nowrap|1=(''A''<sub>r</sub>°)}} is the most common and practical. The standard atomic weight of each chemical element is determined and published by the [[Commission on Isotopic Abundances and Atomic Weights]] (CIAAW) of the [[International Union of Pure and Applied Chemistry]] (IUPAC) based on natural, stable, [[Wiktionary:terrestrial|terrestrial]] sources of the element. The definition specifies the use of samples from many representative sources from the Earth, so that the value can widely be used as the atomic weight for substances as they are encountered in reality—for example, in pharmaceuticals and scientific research. Non-standardized atomic weights of an element are specific to sources and samples, such as the atomic weight of carbon in a particular bone from a particular archaeological site. Standard atomic weight averages such values to the ''range of atomic weights'' that a chemist might expect to derive from many random samples from Earth. This range is the rationale for the ''interval notation'' given for some standard atomic weight values.

Of the 118 known chemical elements, 80 have stable isotopes and 84 have this Earth-environment based value. Typically, such a value is, for example helium: {{nowrap|1=''A''<sub>r</sub>°(He) =}} {{val|4.002602|(2)}}. The "(2)" indicates the uncertainty in the last digit shown, to read {{val|4.002602|0.000002}}. IUPAC also publishes ''abridged values'', rounded to five significant figures. For helium, {{nowrap|1=''A''<sub>r, abridged</sub>°(He) =}} {{val|4.0026}}.

For fourteen elements the samples diverge on this value, because their sample sources have had a different decay history. For example, thallium (Tl) in sedimentary rocks has a different isotopic composition than in igneous rocks and volcanic gases. For these elements, the standard atomic weight is noted as an interval: {{nowrap|1=''A''<sub>r</sub>°(Tl) =}} {{nowrap|1= [204.38, 204.39]}}. With such an interval, for less demanding situations, IUPAC also publishes a ''conventional value''. For thallium, {{nowrap|1=''A''<sub>r, conventional</sub>°(Tl) =}} {{val|204.38}}.

== Definition ==
[[File:IUPAC Periodic Table of the Elements 2011.jpg|thumb|450px|Excerpt of an IUPAC periodic table showing the '''interval notation''' of the '''standard atomic weights''' of boron, carbon, and nitrogen (Chemistry International, IUPAC). Example: the pie chart for boron shows it to be composed of about 20% <sup>10</sup>B and 80% <sup>11</sup>B. This isotope mix causes the atomic weight of ordinary Earthly boron samples to be expected to fall within the interval '''10.806 to 10.821.''' and this interval is the ''standard'' atomic weight. Boron samples from unusual sources, particularly non-terrestrial sources, might have measured atomic weights that fall outside this range. Atomic weight and relative atomic mass are synonyms.]]
The ''standard'' atomic weight is a special value of the relative atomic mass. It is defined as the "recommended values" of relative atomic masses of sources ''in the local environment of the [[Structure of the Earth#Crust|Earth's crust]] and [[Earth's atmosphere|atmosphere]] as determined by the [[IUPAC]] Commission on Atomic Weights and Isotopic Abundances'' (CIAAW).<ref name="goldbook-s05907" /> In general, values from different sources are subject to natural variation due to a different radioactive history of sources. Thus, standard atomic weights are an expectation range of atomic weights from a range of samples or sources. By limiting the sources to terrestrial origin only, the CIAAW-determined values have less variance, and are a more precise value for relative atomic masses (atomic weights) actually found and used in worldly materials.

The [[List of elements by atomic mass|CIAAW-published values]] are used and sometimes lawfully required in mass calculations. The values have an uncertainty (noted in brackets), or are an expectation interval (see example in illustration immediately above). This uncertainty reflects natural variability in isotopic distribution for an element, rather than uncertainty in measurement (which is much smaller with quality instruments).<ref>{{Cite journal|url=https://www.iupac.org/publications/pac/2006/pdf/7811x2051.pdf|doi=10.1351/pac200678112051|title=Atomic weights of the elements 2005 (IUPAC Technical Report)|journal=Pure and Applied Chemistry|volume=78|issue=11|pages=2051–2066|year=2006|last1=Wieser|first1=M. E|s2cid=94552853}}</ref>

Although there is an attempt to cover the range of variability on Earth with standard atomic weight figures, there are known cases of mineral samples which contain elements with atomic weights that are outliers from the standard atomic weight range.<ref name="goldbook-s05907">{{cite journal |url=http://goldbook.iupac.org/terms/view/S05907 |title=IUPAC Goldbook |quote=standard atomic weights: Recommended values of relative atomic masses of the elements revised biennially by the IUPAC Commission on Atomic Weights and Isotopic Abundances and applicable to elements in any normal sample with a high level of confidence. A normal sample is any reasonably possible source of the element or its compounds in commerce for industry and science and has not been subject to significant modification of isotopic composition within a geologically brief period. |journal=[[Compendium of Chemical Terminology]] |doi=10.1351/goldbook.S05907 |access-date=12 July 2019 |doi-access=free }}</ref>

For [[synthetic element]]s the isotope formed depends on the means of synthesis, so the concept of natural isotope abundance has no meaning. Therefore, for synthetic elements the [[mass number|total nucleon count]] of the most stable isotope (i.e., the isotope with the longest half-life) is listed in brackets, in place of the standard atomic weight.{{paragraph break}} When the term "atomic weight" is used in chemistry, usually it is the more specific standard atomic weight that is implied. It is standard atomic weights that are used in periodic tables and many standard references in ordinary terrestrial chemistry.

[[Lithium]] represents a unique case where the natural abundances of the isotopes have in some cases been found to have been perturbed by human isotopic separation activities to the point of affecting the uncertainty in its standard atomic weight, even in samples obtained from natural sources, such as rivers.{{citation needed|date=January 2018}}{{dubious|date=January 2018}}

=== Terrestrial definition ===
An example of why "conventional terrestrial sources" must be specified in giving standard atomic weight values is the element argon. Between locations in the [[Solar System]], the atomic weight of argon varies as much as 10%, due to extreme variance in isotopic composition. Where the major source of argon is the decay of [[potassium-40|{{chem|40|K}}]] in rocks, {{chem|40|Ar}} will be the dominant isotope. Such locations include the planets Mercury and Mars, and the moon Titan. On Earth, the ratios of the three isotopes <sup>36</sup>Ar&nbsp;:&nbsp;<sup>38</sup>Ar&nbsp;:&nbsp;<sup>40</sup>Ar are approximately 5&nbsp;:&nbsp;1&nbsp;:&nbsp;1600, giving terrestrial argon a standard atomic weight of 39.948(1).

However, such is not the case in the rest of the universe. Argon produced directly, by [[stellar nucleosynthesis]], is dominated by the [[alpha process|alpha-process]] nuclide {{chem|36|Ar}}. Correspondingly, solar argon contains 84.6% {{chem|36|Ar}} (according to [[solar wind]] measurements),<ref>{{cite journal
 |last=Lodders |first=K.|author-link=Katharina Lodders
 |date = 2008
 |title=The solar argon abundance
 |journal=[[Astrophysical Journal]]
 |volume=674 |issue=1
 |pages=607–611
 |arxiv=0710.4523
 |doi=10.1086/524725
|bibcode=2008ApJ...674..607L
|s2cid=59150678
 }}</ref> and the ratio of the three isotopes <sup>36</sup>Ar&nbsp;:&nbsp;<sup>38</sup>Ar&nbsp;:&nbsp;<sup>40</sup>Ar in the atmospheres of the outer planets is 8400&nbsp;:&nbsp;1600&nbsp;:&nbsp;1.<ref>{{Cite journal
| last1 = Cameron | first1 = A. G. W.
| title = Elemental and isotopic abundances of the volatile elements in the outer planets
| doi = 10.1007/BF00214750
| journal = Space Science Reviews
| volume = 14
| issue = 3–4
| pages = 392–400
| year = 1973
|bibcode = 1973SSRv...14..392C | s2cid = 119861943
}}</ref> The atomic weight of argon in the Sun and most of the universe, therefore, would be only approximately 36.3.<ref>This can be determined from the preceding figures per the definition of atomic weight and [[WP:CALC]]</ref>

=== Causes of uncertainty on Earth ===
Famously, the published atomic weight value comes with an uncertainty. This uncertainty (and related: precision) follows from its definition, the source being "terrestrial and stable". Systematic causes for uncertainty are:

# Measurement limits. As always, the physical measurement is never finite. There is always more detail to be found and read. This applies to every ''single'', pure isotope found. For example, today the mass of the main natural fluorine isotope ([[fluorine-19]]) can be measured to the accuracy of eleven decimal places: {{val|18.998403163|(6)}}. But a still more precise measurement system could become available, producing more decimals.
# Imperfect mixtures of isotopes. In the samples taken and measured the ''mix'' (relative abundance) of those isotopes may vary. For example, copper. While ''in general'' its two isotopes make out 69.15% and 30.85% each of all copper found, the natural ''sample'' being measured can have had an incomplete 'stirring' and so the percentages are different. The precision is improved by measuring more samples of course, but there remains this cause of uncertainty. (Example: lead samples vary so much, it can not be noted more precise than four figures: {{val|207.2}})
# Earthly sources with a different history. A ''source'' is the greater area being researched, for example 'ocean water' or 'volcanic rock' (as opposed to a 'sample': the single heap of material being investigated). It appears that some elements have a different ''isotopic mix'' per source. For example, thallium in igneous rock has more lighter isotopes, while in sedimentary rock it has more heavy isotopes. There is no Earthly mean number. These elements show the interval notation: ''A''<sub>r</sub>°(Tl) = [{{val|204.38}},&nbsp;{{val|204.39}}]. For practical reasons, a simplified 'conventional' number is published too (for Tl:&nbsp;204.38).
<!-- todo?: uranium exception; non-stable elements; -->

These three uncertainties are accumulative. The published value is a result of all these.

== Determination of relative atomic mass ==
{{Main|Isotope geochemistry}}
Modern relative atomic masses (a term specific to a given element sample) are calculated from measured values of atomic mass (for each nuclide) and [[isotope|isotopic composition]] of a sample. Highly accurate atomic masses are available<ref name="NIST">{{cite web |work=[[National Institute of Standards and Technology]] |url=https://physics.nist.gov/cgi-bin/Compositions/stand_alone.pl?ele=&ascii=html&isotype=some |title=Atomic Weights and Isotopic Compositions for All Elements }}</ref><ref name="AME2003">{{AME 2003}}</ref> for virtually all non-radioactive nuclides, but isotopic compositions are both harder to measure to high precision and more subject to variation between samples.<ref name="TICE1997">{{AtWt TICE 1997}}</ref><ref name="IAvar">{{AtWt IAV}}</ref> For this reason, the relative atomic masses of the 22 [[mononuclidic element]]s (which are the same as the isotopic masses for each of the single naturally occurring nuclides of these elements) are known to especially high accuracy.

{| class="wikitable" align="right"
|-
! rowspan=2 | Isotope
! rowspan=2 | Atomic mass<ref name="AME2003" />
! colspan=2 | Abundance<ref name="TICE1997" />
|-
! Standard
! Range
|-
| align="center" | {{sup|28}}Si
| 27.976 926 532 46(194)
| 92.2297(7)%
| 92.21–92.25%
|-
| align="center" | {{sup|29}}Si
| 28.976 494 700(22)
| 4.6832(5)%
| 4.67–4.69%
|-
| align="center" | {{sup|30}}Si
| 29.973 770 171(32)
| 3.0872(5)%
| 3.08–3.10%
|-
|}
The calculation is exemplified for [[silicon]], whose relative atomic mass is especially important in [[metrology]]. Silicon exists in nature as a mixture of three isotopes: {{sup|28}}Si, {{sup|29}}Si and {{sup|30}}Si. The atomic masses of these nuclides are known to a precision of one part in 14&nbsp;billion for {{sup|28}}Si and about one part in one billion for the others. However the range of [[natural abundance]] for the isotopes is such that the standard abundance can only be given to about ±0.001% (see table).
The calculation is
:''A''{{sub|r}}(Si) = (27.97693 × 0.922297) + (28.97649 × 0.046832) + (29.97377 × 0.030872) = 28.0854
The estimation of the [[Measurement uncertainty|uncertainty]] is complicated,<ref name="Meija">{{cite journal | last1 = Meija | first1 = Juris| journal = Metrologia | year = 2008 | title = Uncertainty propagation of atomic weight measurement results | volume = 45 | issue = 1 | pages = 53–62 | doi = 10.1088/0026-1394/45/1/008 | last2 = Mester | first2 = Zoltán|bibcode = 2008Metro..45...53M | s2cid = 122229901}}</ref> especially as the [[sample distribution]] is not necessarily symmetrical: the [[IUPAC]] standard relative atomic masses are quoted with estimated symmetrical uncertainties,<ref name="Holden">{{cite journal | last = Holden | first = Norman E. | url = https://media.iupac.org/publications/ci/2004/2601/1_holden.html | journal = Chemistry International | year = 2004 | issue = 1 | title = Atomic Weights and the International Committee—A Historical Review | volume = 26 | pages = 4–7 }}</ref> and the value for silicon is 28.0855(3). The relative standard uncertainty in this value is 1{{e|–5}} or 10&nbsp;ppm. To further reflect this natural variability, in 2010, IUPAC made the decision to list the relative atomic masses of 10 elements as an interval rather than a fixed number.<ref>{{Cite web |url=https://archive.usgs.gov/archive/sites/www.usgs.gov/newsroom/article.asp-ID=2661.html |title=IUPAC – International Union of Pure and Applied Chemistry: Atomic Weights of Ten Chemical Elements About to Change |access-date=2019-07-12 |archive-date=2020-07-28 |archive-url=https://web.archive.org/web/20200728210054/https://archive.usgs.gov/archive/sites/www.usgs.gov/newsroom/article.asp-ID=2661.html |url-status=dead }}</ref>

== Naming controversy ==
The use of the name "atomic weight" has attracted a great deal of controversy among scientists.<ref name="IUPAChist">{{AtomicWeightHistory}}</ref> Objectors to the name usually prefer the term "[[relative atomic mass]]" (not to be confused with [[atomic mass]]). The basic objection is that atomic weight is not a [[weight]], that is the [[force]] exerted on an object in a [[gravitational field]], measured in units of force such as the [[newton (unit)|newton]] or [[poundal]].<ref>{{Cite report |url=https://www.osti.gov/biblio/5115105 |title=Atomic weight versus atomic mass controversy |last=Holden |first=N. E. |date=1985-01-01 |publisher=Brookhaven National Lab., Upton, NY (US) |issue=BNL-NCS-36952; CONF-8508138-5 |osti=5115105 |language=English}}</ref>

In reply, supporters of the term "atomic weight" point out (among other arguments)<ref name="IUPAChist" /> that:
* the name has been in continuous use for the same quantity since it was first conceptualized in 1808;<ref>{{cite book | first = John | last = Dalton | author-link = John Dalton | title = A New System of Chemical Philosophy | url = https://archive.org/details/newsystemofchemi01daltuoft | location = Manchester | year = 1808}}</ref>
* for most of that time, atomic weights really were measured by weighing (that is by [[gravimetric analysis]]) and the name of a physical quantity should not change simply because the method of its determination has changed;
* the term "relative atomic mass" should be reserved for the mass of a specific [[nuclide]] (or [[isotope]]), while "atomic ''weight''" be used for the [[weighted mean|''weighted'' mean]] of the atomic masses over all the atoms in the sample;
* it is not uncommon to have misleading names of physical quantities which are retained for historical reasons, such as
** [[electromotive force]], which is not a force
** [[Optical resolution|resolving power]], which is not a [[power (physics)|power]] quantity
** [[molar concentration]], which is not a molar quantity (a quantity expressed per unit amount of substance).
It could be added that atomic weight is often not truly "atomic" either, as it does not correspond to the property of any individual atom. The same argument could be made against "relative atomic mass" used in this sense.

== Published values ==
{{update section|reason={{tl|CIAAW2021}} data and value notation to be used|date=July 2022}}
IUPAC publishes one formal value for each stable [[chemical element]], called the ''standard atomic weight''.<ref name="CIAAW2015"/>{{CIAAW2016}}{{Rp|loc=Table 1}} Any updates are published biannually (in uneven years). In 2015, the atomic weight of ytterbium was updated.<ref name="CIAAW2015">{{cite web|url= http://www.ciaaw.org/atomic-weights.htm|title=Standard Atomic Weights 2015|publisher=[[Commission on Isotopic Abundances and Atomic Weights]]|date=12 October 2015|access-date=18 February 2017}}</ref> Per 2017, 14 atomic weights were changed, including argon changing from single number to interval value.<ref>{{cite web|url=https://iupac.org/standard-atomic-weights-of-14-chemical-elements-revised/|title=Standard atomic weights of 14 chemical elements revised|date=2018-06-05|access-date=2019-02-02|publisher=[[CIAAW]]}}</ref><ref>{{Cite journal|date=2018|title=Standard Atomic Weights of 14 Chemical Elements Revised|journal=Chemistry International|volume=40|issue=4|pages=23–24|doi=10.1515/ci-2018-0409|issn=0193-6484|doi-access=free}}</ref>

The value published can have an uncertainty, like for neon: {{val|20.1797|(6)}}, or can be an interval, like for boron: [10.806,&nbsp;10.821].

Next to these 84 values, IUPAC also publishes ''abridged'' values (up to five digits per number only), and for the twelve interval values, ''conventional'' values (single number values).

Symbol ''A''<sub>r</sub> is a relative atomic mass, for example from a specific sample. To be specific, the standard atomic weight can be noted as {{nowrap|1=''A''<sub>r</sub>°(E)}}, where (E) is the element symbol.

=== Abridged atomic weight ===
The '''abridged atomic weight''',<!-- CIAAW 2016 Section 6 --> also published by CIAAW, is derived from the standard atomic weight, reducing the numbers to five digits (five significant figures). The name does not say 'rounded'.

Interval borders are rounded ''downwards'' for the first (low most) border, and ''upwards'' for the ''upward'' (upmost) border. This way, the more precise original interval is fully covered.{{CIAAW2016}}{{Rp|loc=Table 2}}

Examples:
* Calcium: {{nowrap|1=''A''<sub>r</sub>°(Ca) = 40.078(4)}} → {{nowrap|1=''A''<sub>r, abridged</sub>°(Ca) = 40.078}}
* Helium: {{nowrap|1=''A''<sub>r</sub>°(He) = 4.002602(2)}} → {{nowrap|1=''A''<sub>r, abridged</sub>°(He) = 4.0026}}
* Hydrogen: {{nowrap|1=''A''<sub>r</sub>°(H) = [1.00784, 1.00811]}} → {{nowrap|1=''A''<sub>r, abridged</sub>°(H) = [1.0078, 1.0082]}}

=== Conventional atomic weight ===
Fourteen chemical elements – hydrogen, lithium, boron, carbon, nitrogen, oxygen, magnesium, silicon, sulfur, chlorine, argon, bromine, thallium, and lead – have a standard atomic weight that is defined not as a single number, but as an interval. For example, hydrogen has {{nowrap|1=''A''<sub>r</sub>°(H) = [1.00 784,&nbsp;1.00811]}}. This notation states that the various sources on Earth have substantially different isotopic constitutions, and that the uncertainties in all of them are just covered by the two numbers. For these elements, there is not an 'Earth average' constitution, and the 'right' value is not its middle (which would be 1.007975 for hydrogen, with an uncertainty of (±0.000135) that would make it just cover the interval). However, for situations where a less precise value is acceptable, for example in trade, CIAAW has published a single-number '''conventional atomic weight'''. For hydrogen, {{nowrap|1=''A''<sub>r, conventional</sub>°(H) = 1.008}}.{{CIAAW2016}}{{Rp|loc=Table 3}}

=== A formal short atomic weight ===
By using the abridged value, and the conventional value for the fourteen interval values, a short IUPAC-defined value (5 digits plus uncertainty) can be given for all stable elements. In many situations, and in periodic tables, this may be sufficiently detailed.{{CIAAW2016}}{{Rp|loc=Tables 2 and 3}}

{{Infobox element/standard atomic weight/value illustration}}

== List of atomic weights ==

{{Standard atomic weight of the elements}}

=== In the periodic table ===
{{Periodic table}}

== See also ==
* [[International Union of Pure and Applied Chemistry]] (IUPAC)
* [[Commission on Isotopic Abundances and Atomic Weights]] (CIAAW)

== References ==
{{Reflist|2}}

== External links ==
* [http://www.ciaaw.org/ IUPAC Commission on Isotopic Abundances and Atomic Weights]
* [https://www.iupac.org/publications/pac/85/5/1047/ Atomic Weights of the Elements 2011]

{{Chemical elements data}}
{{navbox periodic table}}

{{Authority control}}

[[Category:Amount of substance]]
[[Category:Chemical properties]]
[[Category:Stoichiometry]]
[[Category:Periodic table]]
[[Category:Chemical quantities]]