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Question
Predict which of the following will be coloured in the aqueous solution?
Ti3+, V3+, Cu+, Sc3+, Mn2+, Fe3+ and Co2+. Give reasons for each.
Solution 1
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.
Solution 2
Those ions are coloured which have one or more unpaired electrons. Ti3+, V3+, Mn2+, Fe3+ and Co2+ are coloured. Cu+ and Sc3+ are colourless.
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