Editing Relative atomic mass (section)

=== 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>