(i) Which transition element in 3d series has positive `E_(M^(2+//)M)^(@)` value and why ?
(ii) Name a member of lanthanoid series which is well known to exhibit to exhibit +4 oxidation state and why?
(b) Account for the following :
(i) The highest oxidation state is exhibited in oxoanions of transition metals.
(ii) HCl is not used to acidify `KMnO_(4)` solution.
(iii) Transition metals have high enthalpy of atomisation.

Explanation

(i) Copper is the only metal in 3d series having positive `E_(M^(2+//)M)^(@)` value because the sum of enthalpies of sublimation and ionization is not balanced by hydration enthalpy.
(ii) Cerium is a member of lanthanoid series which exhibit +4 oxidation state as it is having configuration `[Xe]4f^(2)5d^(@)6s^(2)`. Configuration of `Ce^(4+)` is [Xe] `4f^(@)` which is a stable configuration. Hence it exhibits + 4 Oxidation state.
(b) (i) Oxometal anions have highest oxidation state e.g., Cr in `Cr_(2)O_(7)^(2-)` or `CrO_(4)^(2-)` has an oxidation state of +6 whereas Mn in `MnO_(4)^(-)` has an Oxidation state of + 7. This is due to the combination of metal with oxygen, which is highly electronegative and oxidizing element.
(ii) HCl is not used to acidify `KMnO_(4)` solution because oxygen produced from `KMnO_(4)` is partly used up to oxidise HCl to chlroine but when dil `H_(2)SO_(4)` is used, Oxygen produced is used only for oxidizing the reducing agent.
(iii) Strong metallic bond between the atoms of transition elements are responsible for high enthalpies of atomization. Greater the number of unpaired electrons, stronger will be the metallic bonding, greater will be the enthalpies of atomization.