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# Which of the following pairs have the same electronic configuration?

$(1) Mn^{3+}, Fe^{3+} \quad (2) Mn^{2+}, Fe^{3+} \quad (3) Mn^{2+}, Fe^{2+} \quad (4)Mn^{2+}, Fe^{2+}$

Answer: $Mn^{2+}, Fe^{3+}$
The elements are in the first series so the general configuration is $ns^2(n-1)d^x$
The electron configuration of Mn is $4s^23d^5$ and Fe is $4s^23d^6$ Any configuration of $X^{2+}$ loses two 4s and any configuration of $X^{3+}$ loses two 4s and one 3d.
$\Rightarrow Mn^{2+} \rightarrow 3d^5$ and $Mn^{3+} \rightarrow 3d^4$
$\Rightarrow Fe^{2+} \rightarrow 3d^6$ and $Fe^{3+} \rightarrow 3d^5$