= Relative atomic mass
{title2=$A_{\rm r}=\bar m/\mathrm u$}
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= Atomic weight
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For a <chemical element> in a specified population, its <relative atomic mass> or <atomic weight> is the <dimensionless> ratio $A_{\rm r}=\bar m/\mathrm u$, with $\bar m$ the mean neutral <atomic mass> weighted by <isotopic abundance>. For the illustrative <neon> population, $A_{\rm r}\approx20.1800$, whereas $\bar m\approx20.1800\,\mathrm u$ has <mass> units.<Column Descriptions for Atomic Weights and Isotopic Compositions>{ref}
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