Question

Account for the following :

(i) Ti³⁺ is coloured whereas Sc³⁺ is colourless in aqueous solution.

(ii) Cr²⁺ is a strong reducing agent.

Answer 1

(i) Ti³⁺ (3d¹) has one unpaired electron in 3d sub-shell.

Sc³⁺ (3d⁰) does not have any unpaired electron.

Ti³⁺ in aqueous solution is coloured due to d-d transition but Sc³⁺ is colourless.

(ii) Cr²⁺ (3d⁴) is a strong reducing agent as it is easily oxidised to Cr³⁺ (3d³) which has a stable configuration due to half-filled t_2g.

\(\underset{t_{2g}^{3}e_{g}^{1}} {Cr^{2+}} \rightarrow \underset{t_{2g}^{3}}{Cr^{3+}} +e\)