Give reasons:

(a) d- block elements exhibit more oxidation states than f-block elements.

(b) Orange solution of potassium dichromate turns yellow on adding sodium hydroxide to it.

(c) Zirconium (Z= 40) and Hafnium (Z = 72) have almost similar atomic radii.

(a) In the d-block element the energy difference or gap between d orbital and the s orbital is very small and the electrons cane be removed easily and hence the d-block element can exhibit variable oxidation state.

In the f block the energy gap is very large and thus these orbitals are not available for bond formation and hence f block elements do not exhibit variable oxidation state.

(b) When the orange coloured solution of potassium dichromate is reacted with sodium hydroxide, the potassium dichromate gets converted into potassium chromate which is faint or pale yellow coloured compound. The reaction is as follows:

(c) Zirconium (Z= 40) and Hafnium (Z = 72) have almost similar atomic radii because the 4f electrons provide a poor shielding and thus the 6s electrons are pulled towards the nucleus with stronger nuclear attraction reducing the size of the atom. This phenomenon of reduction in size of atoms due to poor shielding by 4f electrons is called as Lanthanide contraction.