Which one of the following forms a colourless solution in aqueous medium ?
(Atomic number : `Sc =21, Ti = 22, V = 23`, and `Cr = 24`)

Options

A. `V^(3+)`
B. `Cr^(3+)`
C. `Ti^(3+)`
D. `Sc^(3+)`

Explanation

To determine which of the given transition metal ions forms a colorless solution in aqueous medium, we need to analyze the electronic configurations of each ion and the number of unpaired electrons they possess.

### Step-by-Step Solution:

1. **Identify the Ions:**
- Scandium (Sc) with atomic number 21
- Titanium (Ti) with atomic number 22
- Vanadium (V) with atomic number 23
- Chromium (Cr) with atomic number 24

2. **Write the Electronic Configurations:**
- **Scandium (Sc³⁺):**
- Atomic number = 21
- Electronic configuration of Sc: [Ar] 3d¹ 4s²
- For Sc³⁺, remove 3 electrons (2 from 4s and 1 from 3d):
- Configuration: [Ar] 3d⁰ (0 unpaired electrons)

- **Titanium (Ti³⁺):**
- Atomic number = 22
- Electronic configuration of Ti: [Ar] 3d² 4s²
- For Ti³⁺, remove 3 electrons (2 from 4s and 1 from 3d):
- Configuration: [Ar] 3d¹ (1 unpaired electron)

- **Vanadium (V³⁺):**
- Atomic number = 23
- Electronic configuration of V: [Ar] 3d³ 4s²
- For V³⁺, remove 3 electrons (2 from 4s and 1 from 3d):
- Configuration: [Ar] 3d² (2 unpaired electrons)

- **Chromium (Cr³⁺):**
- Atomic number = 24
- Electronic configuration of Cr: [Ar] 3d⁵ 4s¹
- For Cr³⁺, remove 3 electrons (1 from 4s and 2 from 3d):
- Configuration: [Ar] 3d³ (3 unpaired electrons)

3. **Count the Unpaired Electrons:**
- Sc³⁺: 0 unpaired electrons (colorless)
- Ti³⁺: 1 unpaired electron (colored)
- V³⁺: 2 unpaired electrons (colored)
- Cr³⁺: 3 unpaired electrons (colored)

4. **Conclusion:**
- The only ion that has 0 unpaired electrons is Sc³⁺, which means it will form a colorless solution in aqueous medium.

### Final Answer:
**Scandium (Sc³⁺) forms a colorless solution in aqueous medium.**

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