Question
Discuss the relative stability in aqueous solutions of $+2$ oxidation state among the elements: $\text{Cr, Mn, Fe}$ and Co. How would you justify this situation? $($At. Nos. $Cr = 24, Mn = 25, Fe = 26, Co = 27)$

Answer

On the basis of electrochemical series, the standard electrode potential shows the following order:
$\ce{E^\circ Mn^{2+}/Mn < E^\circ Cr^{2+}/Cr < E^\circ Fe^{2+}/Fe < E^\circ CO^{2+}/CO}$
Therefore,$Co^{2+}$ gets easily reduced to metallic cobalt while it is difficult to reduce $Mn^{2+}.$
Hence, $Mn^{2+}$ will be the most stable and the increasing stability order will be
$\ce{Co^{2+} < Fe^{2+} < Cr^{2+} < Mn^{2+}}$

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