(a) Account for the following,
(i) Transition metals show variable oxidation states.
(ii) Zn, Cd and Hg are soft metals.
(c) `E^(@)` value for the `Mn^(3+)//Mn^(2+)` couple is highly positive (+1.57V) as compared to `Cr^(3+)//Cr^(2+)`
(b) Write one similarity and one difference the chemistry of lanthanoid and actinoid elements.

Explanation

(a) (i) In the transition elements, the energies of (n-1)d orbitals and ns orbitals are very close. Hence, electrons from both can participate in bonding.
(ii) Zn, Cd and Hg, all the `e^(-s)` in d-subshell are paired. Hence, the metallic bonds are present in them are weak. That is why, they are soft metals and have low melting and boiling points.
(iii) the large positive value of `E^(@)` for `Mn^(3+)//Mn^(2+)` shows that `Mn^(2+)` is much more stable than `Mn^(3+)` because `Mn^(2+)` has half-filled configuration whereas, `E^(@)` value for `Cr^(3+)//Cr^(2+)` is negative because `Cr^(3+)` is more stable because of having stable half-filled `t_(2)g^(3)` configuration.
(b) Similarity between lanthanoids and actinoids: Both show mainly an oxidation state of +3 and both are electopositive and very reactive.
difference between Lanthanoids and actinoids:
(i) Because +3, Lanthanoids show +2 and +4 oxidation state in few cases, whereas, besides +3, actinoids show higher oxidation staets of +4,+5,+6++7 also.
(ii) Lanthanoids have less tendency to form complexes whereas, actinoids have greater tendency to form complexes.