Give reasons for the following:
(i) Transition elements and their compounds act as catalysts.
(ii) E° value for (Mn²⁺|Mn) is negative whereas for (Cu²⁺|Cu) is positive.
(iii) Actinoids show irregularities in their electronic configuration.

(i)  Transition metals and their compounds function as catalysts because of their ability to exhibit variable oxidation states and also their tendency to adsorb other substances on to their surface and activate them in the process.
(ii) The hydration enthalpy of Cu²⁺ ion is not sufficient to compensate the sum of sublimation as well as first and second ionization enthalpy of copper therefore, E° value for (Mn²⁺|Mn) is negative whereas for (Cu²⁺|Cu) is positive.
(iii) The irregularities in the electronic configurations of the actinoids are related to the stabilities of the f⁰, f⁷and f¹⁴ occupancies of the 5f orbitals.