Answer:
\[Mn\,(Z=25)\,\,\,:\,\,{{[Ar]}^{18}}\,4{{s}^{2}}3{{d}^{5}}\]
\[Cr(Z=24):{{[Ar]}^{18}}\,4{{s}^{1}}3{{d}^{5}}\]
\[M{{n}^{2+}}\,:{{[Ar]}^{18}}\,3{{d}^{5}}\] \[C{{r}^{2+}}:\,{{[Ar]}^{18}}3{{d}^{4}}\]
\[M{{n}^{3+}}:\,{{[Ar]}^{18}}\,3{{d}^{4}}\] \[C{{r}^{3+}}:{{[Ar]}^{18}}3{{d}^{3}}\].
The electronic configuration of \[M{{n}^{2+}}\]
ion is more symmetrical as compared to that of \[C{{r}^{2+}}\] ion. Therefore,
third ionisation potential of\[M{{n}^{2+}}\]is much higher. Consequently E°
value for the \[M{{n}^{3+}}/M{{n}^{2+}}\] couple is much more positive than for
\[C{{r}^{3+}}/C{{r}^{2+}}\] couple.
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