Answer the question on the basis of passage given below:
All transition elements display typical metallic properties like high melting point, boiling point, tensile strength, high malleability and ductility, lustre, etc. Their metallic character increases down the group and elements of 4d and 5d series exhibit higher metallic characters. They show variable oxidation states and form a number of coloured compounds due to the presence of unpaired electrons which show d-d transitions and make them paramagnetic.
The value of \(E^\circ_{Ni^{2+}/Ni}\) is more negative than the expected value, because
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → Ni is having the highest negative enthalpy of hydration
Across the first transition series the E°(M2+/M) values become less negative because the sum of the first and second ionisation enthalpies increases. Mn, Ni and Zn break this trend: their E° values are more negative than expected. The reasons differ:
Mn and Zn: the extra stability of the half-filled 3d5 (\( \ce{Mn^{2+}} \)) and completely filled 3d10 (\( \ce{Zn^{2+}} \)) configurations.
Ni: the small, doubly charged \( \ce{Ni^{2+}} \) ion has the most negative enthalpy of hydration in the series. The large energy released on hydration makes formation of Ni2+(aq) from Ni easier, so E°(\( \ce{Ni^{2+}} \)/Ni) is more negative.
Why the other options are incorrect
Option (1) Ni is having high \(\Delta_iH_2^\circ\) — Incorrect. A high second ionisation enthalpy makes \( \ce{Ni^{2+}} \) harder to form, which would make E° less negative, not more.
Option (3) Ni is having incompletely filled 3d subshell — Incorrect. Every ion from Sc to Cu has an incomplete 3d subshell, so this cannot explain a value that is special to nickel; \( \ce{Ni^{2+}} \) (3d8) has no extra stability of its own.
Option (4) Sublimation enthalpy of Ni is very low. — Incorrect. Nickel has a high, not a low, enthalpy of atomisation, which works against oxidation.
NCERT (2026-27 edition):
Chemical Reactivity and E° Values – “… The Eo values for M2+/M (Table 4.2) indicate a decreasing tendency to form divalent cations across the series. This general trend towards less negative Eo values is related to the increase in the sum of the first and second ionisation enthalpies. It is interesting to note that the Eo 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 Eo values; for nickel, Eo value is related to the highest negative enthalpy of hydration.”