Question

Why is Sc³⁺ colourless while Ti³⁺ coloured? (Atomic number Sc = 21, Ti = 22)

Answer 1

a. Electronic configuration of: 

Sc³⁺ : [Ar] 3d⁰ 

Ti³⁺: [Ar] 3d¹ 

b. Sc³⁺ ion has completely empty 3d-orbital, i.e., no unpaired electrons are present. As d-d transitions are not possible, Sc³⁺ ion is colourless.

Ti³⁺ ion has one unpaired electron in 3d-orbital. As d-d transitions are possible, Ti³⁺ ion is coloured.