which of the following hydrogen bond is strongest in vapour phase ?

Options

A. `HF………..Hf`
B. `HF………….HCl`
C. `HCl………….HCl`
D. `Hl…………..HF`

Answer

To determine which hydrogen bond is the strongest in the vapor phase among the given options, we will analyze the electronegativity of the atoms involved in the hydrogen bonding.

### Step-by-Step Solution:

1. **Understanding Hydrogen Bonding**:
- Hydrogen bonding occurs when hydrogen is covalently bonded to highly electronegative atoms such as fluorine (F), oxygen (O), or nitrogen (N). The strength of the hydrogen bond is influenced by the difference in electronegativity between hydrogen and the electronegative atom.

2. **Identifying the Options**:
- The options provided likely include compounds such as HF (hydrogen fluoride), HCl (hydrogen chloride), HI (hydrogen iodide), and H2O (water).

3. **Electronegativity Values**:
- Fluorine (F) has the highest electronegativity (approximately 4.0), followed by oxygen (O) (approximately 3.5), nitrogen (N) (approximately 3.0), chlorine (Cl) (approximately 3.0), bromine (Br) (approximately 2.8), and iodine (I) (approximately 2.5).
- The electronegativity of hydrogen (H) is approximately 2.1.

4. **Calculating Electronegativity Differences**:
- For HF:
- Difference = |4.0 - 2.1| = 1.9
- For HCl:
- Difference = |3.0 - 2.1| = 0.9
- For HI:
- Difference = |2.5 - 2.1| = 0.4
- For H2O:
- Difference = |3.5 - 2.1| = 1.4

5. **Comparing the Differences**:
- The strongest hydrogen bond will be where the electronegativity difference is the greatest. From the calculations:
- HF has the highest difference (1.9), followed by H2O (1.4), HCl (0.9), and HI (0.4).

6. **Conclusion**:
- Therefore, the strongest hydrogen bond in the vapor phase is in HF (hydrogen fluoride).

### Final Answer:
The strongest hydrogen bond in vapor phase is in **HF (hydrogen fluoride)**.