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Q.
The correct order of $E^0_{M^{2+}/M}$ values with negative sign for the four successive elements $Cr, Mn, Fe$ and $Co$ is
AIEEEAIEEE 2010The d-and f-Block Elements
Solution:
Usually across the first transition series, the negative values for standard electrode potential decrease except for Mn due to stable $d 5$ configuration.
The electronic configuration of various elements in $Mn ^{2+}$ state is: $Cr -[ Ar ] 3 d ^{4}$
$Mn - Cr -[ Ar ] 3 d ^{5}$ -half filled- most stable.
$Fe - Cr -[ Ar ] 3 d ^{6}$
$Co - Cr -[ Ar ] 3 d ^{7}$
So, based on above facts the correct order is:
$Mn > Cr > Fe > Co .$