Which of the following ions cannot be detected by the borax bead or microcosmic bead test?

Options

A. `Cu^(2+)`
B. `Cr^(3+)`
C. `Fe^(3+)`
D. `Zn^(2+)`

Answer

To determine which of the given ions cannot be detected by the borax bead or microcosmic bead test, we need to analyze the color of each cation based on its electronic configuration. The borax bead test is designed to identify colored cations, as color is indicative of unpaired electrons and electronic transitions.

### Step-by-Step Solution:

1. **Identify the Ions**: The ions given are:
- Copper (Cu²⁺)
- Chromium (Cr³⁺)
- Iron (Fe³⁺)
- Zinc (Zn²⁺)

2. **Analyze Copper (Cu²⁺)**:
- Electronic configuration of Copper: [Ar] 3d¹⁰ 4s¹
- For Cu²⁺: Remove one electron from 4s and one from 3d, resulting in 3d⁹.
- Cu²⁺ has one unpaired electron (3d⁹), making it paramagnetic and colored.
- **Conclusion**: Cu²⁺ can be detected by the borax bead test.

3. **Analyze Chromium (Cr³⁺)**:
- Electronic configuration of Chromium: [Ar] 3d⁵ 4s¹
- For Cr³⁺: Remove one electron from 4s and two from 3d, resulting in 3d³.
- Cr³⁺ has three unpaired electrons (3d³), making it paramagnetic and colored.
- **Conclusion**: Cr³⁺ can be detected by the borax bead test.

4. **Analyze Iron (Fe³⁺)**:
- Electronic configuration of Iron: [Ar] 4s² 3d⁶
- For Fe³⁺: Remove two electrons from 4s and one from 3d, resulting in 3d⁵.
- Fe³⁺ has five unpaired electrons (3d⁵), making it paramagnetic and colored.
- **Conclusion**: Fe³⁺ can be detected by the borax bead test.

5. **Analyze Zinc (Zn²⁺)**:
- Electronic configuration of Zinc: [Ar] 4s² 3d¹⁰
- For Zn²⁺: Remove two electrons from 4s, resulting in 3d¹⁰.
- Zn²⁺ has a completely filled d-subshell (3d¹⁰) and no unpaired electrons, making it colorless.
- **Conclusion**: Zn²⁺ cannot be detected by the borax bead test.

6. **Final Conclusion**:
- The ion that cannot be detected by the borax bead test is **Zinc (Zn²⁺)**.

### Answer:
**Zinc (Zn²⁺)** cannot be detected by the borax bead or microcosmic bead test.