Which of the following pairs is coloured in aqueous solution?

Options

A. `Sc^(3+),Co^(3+)`
B. `Ni^(2+),Cu^(2+)`
C. `Ni^(2+),Ti^(3+)`
D. `Sc^(3+),Ti^(3+)`

Answer

To determine which pairs of transition metal ions are colored in aqueous solution, we need to analyze the electronic configurations of the ions in each option and check for unpaired electrons, as the color arises from d-d transitions facilitated by these unpaired electrons.

### Step-by-Step Solution:

1. **Understanding Color in Transition Metals**:
- Transition metal ions are colored due to d-d transitions, which occur when electrons move between d-orbitals. For this transition to happen, the metal ion must have unpaired electrons in its d-orbitals.

2. **Analyzing Each Option**:
- **Option 1: Sc³⁺ and Co³⁺**
- Scandium (Sc) has an atomic number of 21. In the +3 oxidation state (Sc³⁺), it loses three electrons, resulting in a d-electron count of 0 (configuration: [Ar]).
- Cobalt (Co) has an atomic number of 27. In the +3 oxidation state (Co³⁺), it has a d-electron count of 6 (configuration: [Ar] 3d⁶).
- Sc³⁺ has no unpaired electrons (colorless), while Co³⁺ has unpaired electrons (colored). However, since Sc³⁺ is colorless, this pair does not show color.
- **Conclusion**: Incorrect.

- **Option 2: Ni²⁺ and Cu²⁺**
- Nickel (Ni) has an atomic number of 28. In the +2 oxidation state (Ni²⁺), it has a d-electron count of 8 (configuration: [Ar] 3d⁸).
- Copper (Cu) has an atomic number of 29. In the +2 oxidation state (Cu²⁺), it has a d-electron count of 9 (configuration: [Ar] 3d⁹).
- Both Ni²⁺ and Cu²⁺ have unpaired electrons (Ni²⁺ has 2 unpaired, Cu²⁺ has 1 unpaired), so both ions will show color in aqueous solution.
- **Conclusion**: Correct.

- **Option 3: Ni²⁺ and Ti³⁺**
- As established, Ni²⁺ has a d-electron count of 8 with 2 unpaired electrons (colored).
- Titanium (Ti) has an atomic number of 22. In the +3 oxidation state (Ti³⁺), it has a d-electron count of 1 (configuration: [Ar] 3d¹).
- Ti³⁺ has 1 unpaired electron (colored). Therefore, both ions in this pair show color.
- **Conclusion**: Correct.

- **Option 4: Sc³⁺ and Ti³⁺**
- Sc³⁺ has no unpaired electrons (colorless).
- Ti³⁺ has 1 unpaired electron (colored).
- Since Sc³⁺ is colorless, this pair does not show color.
- **Conclusion**: Incorrect.

3. **Final Answer**:
- The pairs that are colored in aqueous solution are **Option 2 (Ni²⁺ and Cu²⁺)** and **Option 3 (Ni²⁺ and Ti³⁺)**.