(i) Transition elements and their compounds act as catalysts.
The primary reason is that, Transition metals exhibit multiple oxidation states so they can have multiple intermediate forms during a chemical reaction.
Generally transition elements are paramagnetic in nature suggesting that they have valence electrons in their outermost shell. These are loosely bound to the metal itself and can be released during a chemical reaction easily by the heat of reaction. Thus metals lend electrons during a chemical reaction which eases the transfer of electrons between reactants.
Metals also have high porosity which is essential during a reaction to occur (especially for gaseous reactions).
Considering all these factors and the ease with which they can be incorporated in a chemical reaction, (e.g. Presence of Pt metal for reaction of H2 and O2 makes it explosive) metal acts as catalyst.
They also have great malleable and grain properties so they can be used under robust conditions and in fine powdered form.
(ii) E° value for (Mn2+|Mn) is negative whereas for (Cu2+|Cu) is positive.
The electronic configuration and nuclear charge density on the electrons of an atom plays an important role in deducing the magnitude and justifying the nature of the energy released during a half-cell reaction.
Mn has electronic configuration = [Ar] 4s23d5
When Mn loses two electrons it achieves half-filled state, which is highly stable due to symmetrical distribution of electrons in the d-orbital. Thus in order to achieve a stable state than its natural ground state, heat is released which in turn implies that Mn will easily undergo oxidation rather than reduction. Thus the reduction potential of (Mn2+|Mn) is negative.
Cu has electronic configuration = [Ar] 4s13d10
And after losing 2 electrons, it tends to attain 1 electron to achieve fully-filled d-orbitals which is a much stable EC as compared to 4s03d9 configuration which is not stable.
Thus the reduction potential of Mn2+ is negative while that of Cu2+ is positive.
(iii) Actinides show irregularities in their electronic configuration.
Actinide is a series of elements after actinium (89) consisting of 14 elements up to Lutetium (71). The irregularity is based on the stability of the 5f orbital configurations
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