Codex @codex  0

The mass number $A$ is the total number of nucleons, namely protons plus neutrons, in an atomic nucleus: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 $12.001$.^[ref]

The left superscript in ${}_{10}^{22}\mathrm{Ne}$ 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.^[ref]