(a) (i) Why do $\text{Zr}$ and $\text{Hf}$ exhibit similar properties ? (ii) "$E^{\circ}$ for $\text{Mn}^{3+}/\text{Mn}^{2+}$ couple is more positive than that for $\text{Fe}^{3+}/\text{Fe}^{2+}$." Justify the statement. [Atomic number : $\text{Mn} = 25$, $\text{Fe} = 26$] OR (b) (i) Which element in $3\text{d}$ series of transition elements does not exhibit variable oxidation states ? Give reason. (ii) Explain why $\text{Cu}^{+}$ is colourless in aqueous solution, whereas $\text{Cu}^{2+}$ is coloured.