The electronic configuration of an ion${\mathrm{M}}^{2+}$ is 1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁶ and the atomic weight of M is 56.$1{\mathrm{s}}^2 2{\mathrm{s}}^2 2{\mathrm{p}}^6 3{\mathrm{s}}^2 3{\mathrm{p}}^6 3{\mathrm{d}}^6$ The number of neutrons in the nucleus of ${\mathrm{M}}^{2+}$ is