Editing Relative atomic mass (section)

== Definition ==

Relative atomic mass is determined by the average atomic mass, or the [[weighted mean]] of the atomic masses of all the atoms of a particular chemical element found in a particular sample, which is then compared to the atomic mass of carbon-12.<ref>{{GoldBookRef|file=R05258|title=relative atomic mass}}</ref> This comparison is the quotient of the two weights, which makes the value dimensionless (having no unit). This quotient also explains the word ''relative'': the sample mass value is considered relative to that of carbon-12.

It is a synonym for atomic weight, though it is not to be confused with [[relative isotopic mass]]. Relative atomic mass is also frequently used as a synonym for [[standard atomic weight]] and these quantities may have overlapping values if the relative atomic mass used is that for an element from Earth under defined conditions. However, relative atomic mass (atomic weight) is still technically distinct from standard atomic weight because of its application only to the atoms obtained from a single sample; it is also not restricted to terrestrial samples, whereas standard atomic weight averages multiple samples but only from terrestrial sources. Relative atomic mass is therefore a more general term that can more broadly refer to samples taken from non-terrestrial environments or highly specific terrestrial environments which may differ substantially from Earth-average or reflect different degrees of [[measurement uncertainty|certainty]] (e.g., in number of [[significant figures]]) than those reflected in standard atomic weights.

=== Current definition ===

The prevailing IUPAC definitions (as taken from the "[[Gold Book]]") are:
: ''atomic weight'' – See: relative atomic mass<ref>[http://goldbook.iupac.org/A00505.html IUPAC Gold Book – atomic weight]</ref>
and
: ''relative atomic mass (atomic weight)'' – The ratio of the average mass of the atom to the unified atomic mass unit.<ref>[http://goldbook.iupac.org/R05258.html IUPAC Gold Book – relative atomic mass (atomic weight), A r]</ref>
Here the "unified atomic mass unit" refers to 1/12 of the mass of an atom of {{sup|12}}C ''in its ground state''.<ref>[http://goldbook.iupac.org/U06554.html IUPAC Gold Book – unified atomic mass unit]</ref>

The IUPAC definition<ref name="IUPAC1979"/> of relative atomic mass is:
: 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}}C.

The definition deliberately specifies "'''''An''''' atomic weight&nbsp;...", as an element will have different relative atomic masses depending on the source. For example, [[boron]] from [[Turkey]] has a lower relative atomic mass than boron from [[California]], because of its different [[Isotope|isotopic composition]].<ref>{{Greenwood&Earnshaw1st|pages=21, 160}}</ref><ref name="Rev2000">{{cite journal | author = International Union of Pure and Applied Chemistry | title = Atomic Weights of the Elements: Review 2000 | url = https://media.iupac.org/publications/pac/2003/pdf/7506x0683.pdf | journal = [[Pure and Applied Chemistry|Pure Appl. Chem.]] | volume = 75 | issue = 6 | pages = 683–800 | year = 2003 | doi = 10.1351/pac200375060683 | s2cid = 96800435 | author-link = International Union of Pure and Applied Chemistry }}</ref> Nevertheless, given the cost and difficulty of [[isotope analysis]], it is common practice to instead substitute the tabulated values of [[Standard atomic weight|standard atomic weights]], which are ubiquitous in chemical laboratories and which are revised biennially by the IUPAC's [[Commission on Isotopic Abundances and Atomic Weights]] (CIAAW).<ref>[http://goldbook.iupac.org/html/S/S05907.html IUPAC Gold Book – standard atomic weights]</ref>

=== Historical usage ===

Older (pre-1961) historical relative scales based on the atomic mass unit (symbol: ''a.m.u.'' or ''amu'') used either the [[oxygen-16]] [[relative isotopic mass]] or else the oxygen relative atomic mass (i.e., atomic weight) for reference. See the article on the history of the modern [[unified atomic mass unit]] for the resolution of these problems.