A${\mathrm{M}}^{+2}$ ion derived from a metal in the first transition metal series bas four electrons in $3\mathrm{d}$ subshell. What element may M be?

Electronic configuration of $M^{2+}$ion $=\left[Ar\right]{48}^{\circ }3d^4\left(e^{-}=22\right)$

Electronic configuration of M  atom $=\left[Ar\right]4s^{1}3d^5\mathit{ }\mathit{ }\mathit{ }\left(e^{-}=24\right)$

atomic no. of $\mathrm{Cr}=24$

Electronic configuration of $Cr=\left[Ar\right]4s^{1}3d^5$

Therefore the M atom is chromium $\left(\mathrm{Cr}\right)$