Question

Give reasons for the following:

(a) Copper (I) has d10 configuration,while copper (II) has d9

configuration, still copper (II) is more stable in

aqueous solution than copper (I).

(b) The second and third transition series elements have almost similar atomic radii.

Answer 1

a) In aqueous solution, stability of ions depends on the hydration energy of the ions. Hydration energy is directly proportional to the charge density. Cu²⁺ has more charge density than Cu⁺ ion, so Cu²⁺ has more negative hydration energy which compensate for the second ionization enthalpy of Cu. Therefore, Cu⁺ compounds are unstable in aqueous medium and under disproportionation 2Cu⁺ $\to$ Cu²⁺ + Cu

b) The second and third transition series elements have almost similar atomic radii.

This is due to intervention of 4f-orbitals which must be filled before 5d-orbitals of elements. Since 4f-orbitals have poor shielding effect than the 5d-orbitals which results in regular decrease in atomic radii called as Lanthanoid contraction which compensates the increase in atomic size with increase in atomic number. Thus, this contraction leads to similar radii of second and third transition series.