The definition specifies the use of samples from many representative sources from the Earth, so that the value can widely be used as "the" atomic weight for substances as they are encountered in reality-for example, in pharmaceuticals and scientific research. The standard atomic weight of each chemical element is determined and published by the Commission on Isotopic Abundances and Atomic Weights (CIAAW) of the International Union of Pure and Applied Chemistry (IUPAC) based on natural, stable, terrestrial sources of the element. It can be converted into a measure of mass (with dimension M) by multiplying it with the dalton, also known as the atomic mass constant.Īmong various variants of the notion of atomic weight ( A r, also known as relative atomic mass) used by scientists, the standard atomic weight ( A r°) is the most common and practical. īecause relative isotopic masses are dimensionless quantities, this weighted mean is also dimensionless. The standard atomic weight of a chemical element (symbol A r°(E) for element "E") is the weighted arithmetic mean of the relative isotopic masses of all isotopes of that element weighted by each isotope's abundance on Earth. The standard atomic weight ( A r°(Cu)) for copper is the average, weighted by their natural abundance, and then divided by the atomic mass constant m u.
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