Negative values of $\mathrm{Mn}^{2+}$ and $\mathrm{Zn}^{2+}$ are related to stabilities of half-filled and completely filled configuration respectively. But for $\mathrm{Ni}^{2+}, E^{\circ}$ value is related to the highest negative enthalpy of hydration. Hence, $E^{\mathrm{S}}$ values for $\mathrm{Mn}, \mathrm{Ni}$ and Zn are more negative than expected.

Ionisation enthalpy of Cr is less than that of Zn because Cr has stable $d^5$ configuration. In case of zinc, electron comes out from completely filled $4 s$-orbital. So, removal of electron from zinc requires more energy as compared to the chromium.

Transition elements show high melting point, due to involvement of greater number of electrons in the interatomic bonding from $(n-1) d$-orbitals in addition to $n s$ electrons in forming metallic bond. Thus, large number of electrons participate forming large number of metallic bond.

When $\mathrm{Cu}^{2+}$ ion is treated with KI , it produces $\mathrm{Cu}_2 \mathrm{I}_2$ white precipitate in the final product.

$$2 \mathrm{Cu}^{2+}+4 \mathrm{I}^{-} \longrightarrow \underset{\text { (White ppt.) }}{\mathrm{Cu}_2 \mathrm{I}_2}+\mathrm{I}_2$$

(In this reaction, $\mathrm{CuI}_2$ is formed which being unstable, dissociates into $\mathrm{Cu}_2 \mathrm{I}_2$ and $\mathrm{I}_2$ ).

Among $\mathrm{Cu}_2 \mathrm{Cl}_2$ and $\mathrm{CuCl}_2, \mathrm{CuCl}_2$ is more stable. Stability of complex can be explained on the basis of stability of oxidation state of copper. Stability of $\mathrm{Cu}^{2+}(a q)$ rather than $\mathrm{Cu}^{+}(a q)$ is due to much more negative value of $\left[\Delta_{\text {hyd }} H^{\mathrm{S}}\right.$ of $\left.\mathrm{Cu}^{2+}(\mathrm{aq})\right]$ than $\mathrm{Cu}^{+}$which more than compensates for the second ionisation enthalpy of Cu .