Which of the following may be colourless

Options

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

Explanation

To determine which of the following ions may be colorless, we need to analyze the electronic configurations of the given ions and check for the presence of unpaired electrons. Colorless ions typically have no unpaired electrons, as unpaired electrons can lead to d-d transitions that result in color.

### Step-by-Step Solution:

1. **Understanding Color and Electron Configuration:**
- Ions that are colorless have no unpaired electrons. This is because the absence of unpaired electrons means there are no d-d transitions possible, which are responsible for color in transition metal complexes.

2. **Analyzing Chromium Ion (Cr³⁺):**
- Chromium has the electron configuration of [Ar] 3d⁵ 4s¹.
- For Cr³⁺, it loses three electrons: two from 4s and one from 3d.
- The configuration for Cr³⁺ is 3d³.
- In 3d³, there are three unpaired electrons.
- **Conclusion:** Cr³⁺ is not colorless.

3. **Analyzing Copper Ion (Cu⁺):**
- Copper has the electron configuration of [Ar] 3d¹⁰ 4s¹.
- For Cu⁺, it loses one electron from 4s.
- The configuration for Cu⁺ is 3d¹⁰.
- In 3d¹⁰, all electrons are paired.
- **Conclusion:** Cu⁺ is colorless.

4. **Analyzing Iron Ion (Fe²⁺):**
- Iron has the electron configuration of [Ar] 3d⁶ 4s².
- For Fe²⁺, it loses two electrons from 4s.
- The configuration for Fe²⁺ is 3d⁶.
- In 3d⁶, there are four unpaired electrons.
- **Conclusion:** Fe²⁺ is not colorless.

5. **Analyzing Cobalt Ion (Co²⁺):**
- Cobalt has the electron configuration of [Ar] 3d⁷ 4s².
- For Co²⁺, it loses two electrons from 4s.
- The configuration for Co²⁺ is 3d⁷.
- In 3d⁷, there are three unpaired electrons.
- **Conclusion:** Co²⁺ is not colorless.

6. **Final Conclusion:**
- Among the ions analyzed, only Cu⁺ is colorless due to having a completely filled d-orbital with no unpaired electrons.

### Summary:
The colorless ion from the options provided is **Cu⁺**.