(a) (i) Transition metals show variable oxidation states because of the small difference in energy of the ns and $(n-1)d$ orbitals. Thus, addition to $ns$ electrons, $(n-1)d$ electrons also participate in bonding.(ii) $\mathrm{Zn}, \mathrm{Cd}$ and $\mathrm{Hg}$ are not transition elements but are regarded as soft metals as they easily oxidize to +2 oxidation state with low melting point. (iii) Because $\mathrm{Mn}^{2+}$ has $d^5$ as a stable configuration whereas $\mathrm{Cr}^{3+}$ is more stable due to stable $t_{2g}^3 .$ (b) Similarity-both are stable in +3 oxidation state/ both show contraction/ irregular electronic configuration (or any other suitable similarity) Difference-actinoids are radioactive and lanthanoids are not / actinoids show wide range of oxidation states but lanthanoids don't (or any other correct difference). OR (a) (i) $\mathrm{Cr}^{3+}$ is the most stable in aqueous solution as the tripositive ions with increasing atomic number. (ii) $\mathrm{Mn}^{3+}$ as they have the highest $E^{\circ}$ $(\mathrm{M}^{3+}/\mathrm{M}^{2+})$. (iii) $\mathrm{Ti}^{4+}$ is colourless. Electronic configuration : $1S^2 2S^2 2p^6 3S^2 3p^6$ $4\mathrm{S}^2 3d^{\circ}$ It has completely empty $d$-orbital and there are no electrons for the $d-d$ transition. (b) (i) $2\mathrm{MnO}_4^{-}+16\mathrm{H}^{+}+5\mathrm{S}^{2-} \rightarrow 5\mathrm{S}$ $+2\mathrm{Mn}^{2+}+$ $8\mathrm{H}_2\mathrm{O}$ (ii) $2\mathrm{KMnO}_4 \rightarrow \mathrm{K}_2\mathrm{MnO}_4+\mathrm{MnO}_2+\mathrm{O}_2$