What is Lanthanoid contraction?(b) Compounds of ‘d’ block elements are coloured in solid state as well as in an aqueous state. Explain.

Answer:

(a) Lanthanoid contraction: The steady decrease in atomic and ionic (M³⁺) radii of the lanthanoids from lanthanum to lutetium with increasing atomic number is called lanthanoid contraction. It occurs because the added electrons enter the 4f orbitals, which shield one another poorly from the nuclear charge, so the effective nuclear charge increases and the electron cloud is pulled inward. As a result, 4d and 5d elements of the same group (e.g. Zr and Hf) have almost equal radii.

(b) Colour of d-block compounds: Transition metal ions with partly filled d orbitals (d¹ to d⁹) form coloured compounds. In a compound or in aqueous solution the metal ion is surrounded by ligands (anions, or water molecules), which split the five d orbitals into groups of slightly different energy. When visible light falls on the substance, an electron absorbs light of a particular wavelength and jumps from a lower d level to a higher d level (d–d transition). The colour seen is the complementary colour of the light absorbed, e.g. [Cu(H₂O)₄]²⁺ absorbs red-orange light and appears blue. Ions with d⁰ (Sc³⁺, Ti⁴⁺) or d¹⁰ (Zn²⁺, Cu⁺) configuration cannot have d–d transitions and are colourless.