(a) Why do actinoids show a wide range of oxidation states? Write one similarity between the chemistry of lanthanoids and actinoids.

(b) Second I.E. of chromium and copper are exceptionally higher than that of their neighbouring elements. Explain.

(c) Zinc has lowest energy of atomization in 3d-series. Why?

OR

(a) Give reasons for the following:

(i) Transition metals are paramagnetic.

(ii) Transition metals show variable oxidation states.

(iii) Zr and Hf have nearly similar atomic radii.

(b) Write the preparation of KMnO₄ from pyrolusite ore(MnO₂).

(a) The valance shell of actinoids consists of 5f, 6d and 7s. Although the principle quantum numbers are different their energy levels are comparable and the electrons are held loosely (as they are bigger in size and nuclear attraction is shielded by inner electrons) so electrons can be easily removed from the orbitals, allowing them to exhibit variable oxidation states.

Similarity between the chemistry of lanthanoids and actinoids.

• Both are highly electropositive

• Highly reactive

• Electrons fill up in (n-2)f

(b) Electronic configuration of Cr: [Ar]3d⁵ 4s¹

Electronic configuration of Cu: [Ar]3d¹⁰ 4s¹

First electron is lost from 4s. In case of chromium the 2^nd electron has to be pulled out from half- filled d-orbital while in copper the 2^nd electron has to be pulled from fully filled d-orbital. Half-filled and fully filled orbitals are more stable due to symmetry hence require more energy to be pulled out.

(c) Electronic configuration of zinc is [Ar]3d¹⁰ 4s² i.e., it has completely filled d and s orbitals. While in rest of the transition metals in that series has one or more number of unpaired electrons which form metallic bonds. Due to absence of this metallic bonding in zinc its atomization energy is least in 3d-series.

OR

(a) (i) Paramagnetism is caused by unpaired electrons. Transition metals have partially filled (n-1)d orbitals, hence paramagnetic in nature.

(ii) The valence shell of transition metals consists of (n-1)d and ns orbitals and there is not much energy difference between orbitals. So both (n-1)d and ns participate in bonding by losing electrons,thereby exhibiting variable oxidation states.

(iii) Zr and Hf have nearly similar atomic radii due to lanthanide contraction. This is caused due to poor shielding of 4f-orbital, leading to stronger attraction of 5d electrons, causing decreasing in size.

(b) Finely powdered pyrolusite(MnO₂) is fused with alkali metal hydroxides in presence of air or oxygen or oxidizing agent like KNO₃ to give potassium manganate(K₂MnO₄ dark green).

This potassium manganate undergoes disproportionation in neutral or acidic medium to give potassium permanganate(KMnO₄).