= Mass number
{title2=$A=Z+N$}
{wiki}
The <mass number> $A$ is the total number of <nucleons>, namely <protons> plus <neutrons>, in an <atomic nucleus>:
$$
A=Z+N,\qquad N=A-Z.
$$
It is an exact positive <integer> with no units, rather than a measured <mass>. <Carbon-12> has $A=12$ because its <atomic nucleus> contains six <protons> and six <neutrons>. No improvement in a <mass> measurement can turn that count into $11.999$ or $12.001$.<IUPAC Compendium of Chemical Terminology (Gold Book) by International Union of Pure and Applied Chemistry/mass number>{ref}
The left superscript in ${}^{22}_{10}\mathrm{Ne}$ denotes <mass number> $22$, not an <atomic mass> of exactly $22\,\mathrm u$. For <neon-22>, the displayed reference <atomic mass> is about $21.991385114\,\mathrm u$. Even a <weighted mean> of <mass numbers>, such as $20.1877$ for the illustrative <neon> population below, is a population average, not the <mass number> of an individual <atomic nucleus>.<Atomic Weights and Isotopic Compositions for Neon>{ref}
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