Give reasons for the following:
(i) Transition elements and their compounds act as catalysts
(ii) `E^(@)` value for `(Mn^(2+) | Mn)` is negative whereas for `(Cu^(2+) | Cu)` is positive.
(iii) Actinoids show irregularities in their electronic configuration.

Explanation

(i) Refer Q. 25 (i), Paper-2009, Outside Delhi, Set-1, [Page 54].
(i) `E_(M^(2+)//M)^(@)` for any metal is related to the sum of enthalpy of atomisation, ionization and hydration Copper has high enthalpy of atomisation and low enthalpy of hydration. Hence `E_(Cu^(++)//Cu)^(@)` is positive.
(iii) This happens because the energy difference between 5f, 6d and 7s subshell of the actinides is very small and hence, electrons can be accommodated in any of them.