Arrange the following metals in increasing order of their standard el…

The correct answer is Option (1) → (C), (A), (D), (B)

Across the first transition series the E°(M²⁺/M) values become less negative, because the sum of the first and second ionisation enthalpies generally increases. A less negative value means a smaller tendency to form the M²⁺ ion.

(C) Manganese — First (lowest), −1.18 V. Its value is more negative than the trend predicts, because Mn²⁺ has the stable half-filled d⁵ configuration.

(A) Iron — Second, −0.44 V.

(D) Nickel — Third, −0.25 V. Its value is more negative than the general trend predicts, but it is still less negative than that of iron (−0.44 V).

(B) Copper — Fourth (highest), +0.34 V. Copper is the only metal of the series with a positive E°, so it cannot liberate H₂ from acids.

NCERT (2026-27 edition):
4.3.8 Chemical Reactivity and E° Values – “…The E^o values for M²⁺/M (Table 4.2) indicate a decreasing tendency to form divalent cations across the series. This general trend towards less negative E^o values is related to the increase in the sum of the first and second ionisation enthalpies. It is interesting to note that the E^o values for Mn, Ni and Zn are more negative than expected from the general trend. Whereas the stabilities of half-filled d subshell (d⁵) in Mn²⁺ and completely filled d subshell (d¹⁰) in zinc are related to their E^o values; for nickel, E^o value is related to the highest negative enthalpy of hydration.”

4.3.5 Trends in the M²⁺/M Standard Electrode Potentials – “The unique behaviour of Cu, having a positive E^o, accounts for its inability to liberate H₂ from acids.”

Table 4.2: Electronic Configurations and some other Properties of the First Series of Transition Elements