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'''Atomic weight''' (symbol: '''''A''<sub>r</sub>''') is a [[Dimensionless quantity|dimensionless]] [[physical quantity]], the ratio of the average [[mass]] of [[atom]]s of an [[chemical element|element]] (from a given source) to 1/12 of the mass of an atom of [[carbon-12]].<ref name="IUPAC1979">{{IUPAC atomic weights 1979}}.</ref><ref name="GreenBook">{{GreenBook2nd|page=41}}.</ref> The term is usually used, without further qualification, to refer to the '''standard atomic weights''' published at regular intervals by the [[International Union of Pure and Applied Chemistry]] (IUPAC)<ref>The latest edition is {{IUPAC atomic weights 2007}}.</ref> and which are intended to be applicable to normal laboratory materials. These standard atomic weights are reprinted in a wide variety of textbooks, commercial catalogues, wallcharts etc, and in the table below. The fact "[[relative atomic mass]]" may also be used to describe this physical quantity, and indeed the continued use of the term "atomic weight" has attracted considerable controversy since at least the 1960s<ref name="IUPAChist">{{IUPAC atomic weight history}}.</ref> (see below).

Atomic weights, unlike atomic masses (the masses of individual atoms), are not [[physical constant]]s and vary from sample to sample. Nevertheless, they are sufficiently constant in "normal" samples to be of fundamental importance in [[chemistry]].

== Definition ==
The IUPAC definition<ref name="IUPAC1979" /> of atomic weight is:
<blockquote>
An atomic weight (relative atomic mass) of an element from a specified source is the ratio of the average mass per atom of the element to 1/12 of the mass of an atom of <sup>12</sup>C.
</blockquote>

The definition deliberately specifies "'''''An''''' atomic weight…", as an element will have different atomic weights depending on the source. For example, [[boron]] from [[Turkey]] has a lower atomic weight than boron from [[California]], because of its different [[Isotope|isotopic composition]].<ref>{{Greenwood&Earnshaw1st|pages=21, 160}}.</ref><ref name="Rev2000">{{IUPAC atomic weight review 2000}}.</ref> Nevertheless, given the cost and difficulty of [[isotope analysis]], it is usual to use the tabulated values of standard atomic weights which are ubiquitous in chemical laboratories.

== Naming controversy ==
The use of the name "atomic weight" has attracted a great deal of controversy among scientists.<ref name="IUPAChist" /> 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]].

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 | authorlink = John Dalton | title = A New System of Chemical Philosophy | url = http://www.archive.org/details/newsystemofchemi01daltuoft | location = Manchester | date = 1808}}</ref>
*for most of that time, atomic weights really were measured by weighing (that is by [[gravimetric analysis]]) and that the name of a physical quantity shouldn't 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
**[[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 doesn't correspond to the property of any individual atom. The same argument could be made against "relative atomic mass" used in this sense.

== Determination of atomic weight ==
Modern atomic weights are calculated from measured values of atomic mass (for each nuclide) and [[isotopic composition]]. Highly accurate atomic masses are available<ref name="NIST">{{citation | publisher = National Institute of Standards and Technology | url = http://physics.nist.gov/cgi-bin/Compositions/stand_alone.pl?ele=&ascii=html&isotype=some | title = Atomic Weights and Isotopic Compositions for All Elements | accessdate = 2010-03-20}}.</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="TICE2001">{{TICE 2001}}.</ref><ref name="IAvar">{{IUPAC IAV 2002}}.</ref> For this reason, the atomic weights of the twenty-two [[mononuclidic element]]s are known to especially high accuracy – an uncertainty of only one part in 38&nbsp;million in the case of [[fluorine]], a precision which is greater than the current best value for the [[Avogadro constant]] (one part in 20&nbsp;million).

{| class="wikitable" align="right" style="margin:0 0 0 0.5em;"
|-
! rowspan=2 | Isotope
! rowspan=2 | Relative<br/>atomic mass<ref name="AME2003" />
! colspan=2 | Abundance<ref name="TICE2001" />
|-
! Standard
! Range
|-
| align="center" | <sup>28</sup>Si
| 27.976 926 532 46(194)
| 92.223(19)%
| 92.205–92.241%
|-
| align="center" | <sup>29</sup>Si
| 28.976 494 700(22)
| 4.685(8)%
| 4.678–4.692%
|-
| align="center" | <sup>30</sup>Si
| 29.973 770 171(32)
| 3.092(11)%
| 3.082–3.102%
|-
|}
The calculation is exemplified for [[silicon]], whose atomic weight is especially important in [[metrology]]. Silicon exists in nature as a mixture of three isotopes: <sup>28</sup>Si, <sup>29</sup>Si and <sup>30</sup>Si. The atomic masses of these nuclides are known to a precision of one part in 14&nbsp;billion for <sup>28</sup>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</sub>(Si) = (27.976 93 × 0.922 23) + (28.976 49 × 0.046 85) + (29.973 77 × 0.030 92) = 28.0854
The estimation of the [[Measurement uncertainty|uncertainty]] is complicated<ref name="Meija">{{citation | last = Meija | first = Juris | journal = Metrologia | year = 2008 | title = Uncertainty propagation of atomic weight measurement results | volume = 45 | pages = 53–62 | doi = 10.1088/0026-1394/45/1/008 | last2 = Mester | first2 = Zoltán}}.</ref>, especially as the [[sample distribution]] is not necessarily symmetrical: the [[IUPAC]] standard atomic weights are quoted with estimated symmetrical uncertainties,<ref name="Holden">{{citation | last = Holden | first = Norman E. | url = http://www.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.

==See also==
*[[List of standard atomic weights]]

==References==
{{reflist|2}}

== External links ==
*[http://www.ciaaw.org IUPAC Commission on Isotopic Abundances and Atomic Weights]
*[http://physics.nist.gov/cgi-bin/Compositions/stand_alone.pl?ele=&ascii=html&isotype=some NIST relative atomic masses of all isotopes and the standard atomic weights of the elements]
*[http://www.iupac.org/publications/ci/2004/2601/1_holden.html Atomic Weights and the International Committee — A Historical Review]

[[Category:Atomic weight| ]]
[[Category:Amount of substance]]
[[Category:Stoichiometry]]

{{Imported from Wikipedia|title=Atomic weight|id=343389311}}