Магний — химический элемент с порядковым номером 12. Значит известны три изотопа ${}_{12}{}^{24} \mathrm{Mg}, {}_{12}{}^{25} \mathrm{Mg}, {}_{12}{}^{26} \mathrm{Mg}$.

${}_{12}{}^{24} \mathrm{Mg}, \left[{\mathrm{p}}^{+}\right] = \mathrm{Z} = 12, \left[{\mathrm{n}}^0\right] = \mathrm{A} — \mathrm{Z} = 24 — 12 = 12.$

${}_{12}{}^{25} \mathrm{Mg}, \left[{\mathrm{p}}^{+}\right] = \mathrm{Z} = 12, \left[{\mathrm{n}}^0\right] = \mathrm{A} — \mathrm{Z} = 25 — 12 = 13.$

${}_{12}{}^{26} \mathrm{Mg}, \left[{\mathrm{p}}^{+}\right] = \mathrm{Z} = 12, \left[{\mathrm{n}}^0\right] = \mathrm{A} — \mathrm{Z} = 26 — 12 = 14.$