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

Metal ions which have valence electrons in d-orbital and in which d-d transition can take place will be coloured and the metal ions which have completely filled orbital or have d-orbital will be colourless as no d-d transition is possible in those configurations.

ElementAtomic NumberIonic StateElectronic configuration in ionic state

Ti

V23V3+[Ar] 3d2

Cu< Cu+[Ar] 3d10

Sc Sc3+[Ar]

Mn25Mn2+[Ar] 3d5

Fe26Fe3+[Ar] 3d5

Co27Co2+[Ar] 3d7

From the above table, it can be observed that only Sc3+ and Cu+ have either completely filled d-orbital or empty d-orbital. So, all other ions except Sc3+ and Cu+ will be coloured in aqueous solution because of absorption of radiations which fall in the visible region as on obtaining energy electrons jump from one d-orbital to another.