Question 18

Predict which of the following will be coloured in aqueous solution?

Ti3+, V3+, Cu+, Sc3+, Mn2+, Fe3+ and Co2+.

Give reasons for each.

Answer

Only the ions that have electrons in d-orbital and in which d-d transition is possible will be coloured. The ions in which d-orbitals are empty or completely filled will be colourless as no d-d transition is possible in those configurations. 

Element Atomic Number Ionic State Electronic configuration in ionic state
Ti 22 Ti3+ [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 and Cu+ has completely filled d-orbitals. All other ions, except Sc3+ and Cu+, will be coloured in aqueous solution because of d - d transitions.

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