= Atomic mass
{title2=$m_{\rm a}$}
{wiki}
The <rest mass> of a <chemical atom> in its <ground state>, a <physical quantity> expressed in <SI> <mass> units or the <unified atomic mass unit>. Here $m_i$ means the <atomic mass> of a neutral <chemical atom> of <isotope> $i$, not the <mass> of its bare <atomic nucleus> and not a population average.<IUPAC Compendium of Chemical Terminology, fifth edition (Gold Book) by International Union of Pure and Applied Chemistry/atomic mass>{ref}
<Atomic mass> includes <electrons> and depends on <nuclear binding energy>; it is not just <mass number> times a universal <nucleon> <mass>. The <atomic mass> of neutral <carbon-12> in its <ground state> is exactly $12\,\mathrm u$ by definition of the <unified atomic mass unit>. That exactness establishes the reference scale, rather than making the <atomic masses> of all <isotopes> exact <integers> in the same unit.
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