Conventional atomic weight
The standard atomic weight (Ar, standard) is a physical quantity for a chemical element, expressed as relative atomic mass (Ar). It is specified by (restricted to) the IUPAC (CIAAW) definition of natural, stable, terrestrial sources. Because of this practical definition, the value is widely used as 'the' atomic weight for real life substances. For example, in pharmaceuticals and scientific research.
Out of 118 chemical elements, 84 are stable and have this Earth-environment based value. Typically, such a value is, for example helium: Ar, standard(He) = 4.002602(2). The "(2)" indicates the uncertainty in the last digit shown, or 4.002602 ±0.000002. For twelve elements various terrestrial sources diverge on this value, because these sources have a different decay history. For example, thallium in sedimentary rocks has a different isotopic composition than when in igneous rocks and volcanic gases. For these elements, the standard atomic weight is noted as an interval: Ar, standard(Tl) = [204.38, 204.39].
CIAAW also publishes abridged values (rounded to five significant figures), and simple conventional values for interval values.
The standard atomic weight is a more specific value of a relative atomic mass. It is defined as the relative atomic mass of a source in the local environment of the Earth's crust and atmosphere as determined by the IUPAC Commission on Atomic Weights and Isotopic Abundances. (CIAAW) In general, values from different sources are subject to natural variation due to a different radioactive history of sources. By limiting the sources to terrestrial origin only, the CIAAW determined values have less variance, and are a more precise value for atomic masses actually found and used in worldly materials.
The 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).
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