Mark the correct statements:

Options

A. Hg forms an amalgam with iron
B. Hg vapour is non-poisonous
C. Hg is mono atomic and monovalent in mercurous compound
D. Oxysalts of mercury are thermally unstable

Answer

To solve the question of marking the correct statements regarding mercury, we will analyze each statement one by one.

### Step-by-Step Solution:

1. **Statement 1: Mercury forms an amalgam with iron.**
- **Analysis:** Mercury does form amalgams with most metals; however, it does not form an amalgam with iron and platinum. Therefore, this statement is incorrect.
- **Conclusion:** This statement is **wrong**.

2. **Statement 2: Mercury vapor is non-poisonous.**
- **Analysis:** Mercury is known to be poisonous, and its vapor is particularly toxic. Hence, this statement is also incorrect.
- **Conclusion:** This statement is **wrong**.

3. **Statement 3: Mercury is monatomic and monovalent in mercurous compounds.**
- **Analysis:** In mercurous compounds (like Hg2Cl2), mercury exists as dimers (Hg2^2+), meaning it does not exist as monatomic. Therefore, this statement is incorrect as well.
- **Conclusion:** This statement is **wrong**.

4. **Statement 4: Oxysols of mercury are thermally unstable.**
- **Analysis:** Oxysols of mercury do decompose upon heating to form metallic mercury and oxygen, indicating that they are thermally unstable. This statement is correct.
- **Conclusion:** This statement is **correct**.

### Final Answer:
The only correct statement is **Statement 4**: "Oxysols of mercury are thermally unstable."

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