Question

Predict which of the following will be coloured in aqueous solution? Ti³⁺, V³⁺, Cu+, Sc³⁺, Mn²⁺, Fe³⁺ and Co²⁺. Give reasons for each.

Answer 1

Only the ions that have electrons in d-orbital will be coloured. The ions in which d-orbital is empty will be colourless.

Element	Atomic Number	Ionic State	Electronic configuration in ionic state
Ti	22	T13+	[Ar] 3d1
v	23	V3+	[Ar] 3d2
Cu	29	Cu+	[Ar] 3d10
Sc	21	Sc3+	[Ar]
Mn	25	Mn2+	[Ar] 3d5
Fe	26	Fe3+	[Ar] 3d5
Co	27	Co2+	[Ar] 3d7

From the above table, it can be easily observed that only Sc³⁺ has an empty d-orbital. All
other ions, except Sc^3+, will be coloured in aqueous solution because of d−d transitions.