Standard Atomic Weight Is a Measured, Revisable Value
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A standard atomic weight is a measured, weighted-average figure that IUPAC's commission reviews and revises as new abundance data arrive.
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Because natural isotopic abundance varies slightly by geographic source, IUPAC lists some elements' weights as an interval, not one fixed number.
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A tabulated atomic weight is not a fixed constant of nature — it is a measured figure, open to revision.