Chloride of which element will be coloured ?

Options

A. Ag
B. Hg
C. Zn
D. Co

Explanation

To determine which element's chloride will be colored among the given options (silver, mercury, zinc, and cobalt), we need to analyze the electronic configurations and oxidation states of these elements when they form chlorides.

### Step-by-Step Solution:

1. **Analyze Silver (Ag):**
- The chloride of silver is AgCl.
- Silver has a +1 oxidation state in AgCl.
- Electronic configuration of silver: [Kr] 4d10 5s1.
- In AgCl, silver loses one electron to become Ag⁺: [Kr] 4d10.
- Since Ag⁺ has a completely filled d-orbital (4d10), it has no free electrons to absorb visible light, thus AgCl is colorless.

2. **Analyze Mercury (Hg):**
- The chloride of mercury is HgCl2.
- Mercury has a +2 oxidation state in HgCl2.
- Electronic configuration of mercury: [Xe] 4f14 5d10 6s2.
- In HgCl2, mercury loses two electrons to become Hg²⁺: [Xe] 4f14 5d10.
- Hg²⁺ also has a completely filled d-orbital (5d10), which means it has no free electrons to absorb visible light, so HgCl2 is also colorless.

3. **Analyze Zinc (Zn):**
- The chloride of zinc is ZnCl2.
- Zinc has a +2 oxidation state in ZnCl2.
- Electronic configuration of zinc: [Ar] 3d10 4s2.
- In ZnCl2, zinc loses two electrons to become Zn²⁺: [Ar] 3d10.
- Zn²⁺ has a completely filled d-orbital (3d10), indicating no free electrons to absorb visible light, making ZnCl2 colorless.

4. **Analyze Cobalt (Co):**
- The chloride of cobalt can be CoCl2 or CoCl3.
- Cobalt can have variable oxidation states, but we will consider CoCl3 for this analysis.
- Cobalt has a +3 oxidation state in CoCl3.
- Electronic configuration of cobalt: [Ar] 3d7 4s2.
- In CoCl3, cobalt loses three electrons to become Co³⁺: [Ar] 3d6.
- Co³⁺ has unpaired electrons in its d-orbital (3d6), which allows it to absorb visible light, resulting in a colored compound.

### Conclusion:
The chloride of cobalt (CoCl3) is colored due to the presence of unpaired electrons in the d-orbitals of Co³⁺. Therefore, the correct answer is **cobalt**.