A recommended atomic weight applicable to normal materials, rather than the result for every possible enriched sample. The tabulated neon value and the arithmetic result above need not agree in every last displayed digit. Some chemical elements have intervals of standard atomic weight because their isotopic abundances vary across normal materials.[ref]
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The standard atomic weight, often represented by the symbol \( A_r \), is a dimensionless quantity that expresses the average mass of an element's atoms, weighted according to the natural abundance of its isotopes. This value is typically based on the carbon-12 isotope, which is assigned a mass of exactly 12 atomic mass units (amu). Standard atomic weights are used primarily for elements in their natural isotopic mixtures and are typically listed in the periodic table.