Which of the following molecule form hydrogen bond even in the vapour state also 

Options

A. `NO_2`
B. `H_2O`
C. `HF`
D. `C_2H_5NH_2`

Answer

To determine which of the following molecules can form hydrogen bonds even in the vapor state, we will analyze each option: NO2, H2O, HF, and C2H5NS2.

### Step-by-Step Solution:

1. **Understanding Hydrogen Bonds**:
- Hydrogen bonds are formed when hydrogen is covalently bonded to a highly electronegative atom (such as fluorine, oxygen, or nitrogen) and interacts with another electronegative atom.
- The strength of hydrogen bonds can influence whether they exist in different states of matter (solid, liquid, vapor).

2. **Analyzing Each Molecule**:
- **NO2 (Nitrogen Dioxide)**:
- NO2 does not have hydrogen atoms bonded to highly electronegative atoms, so it cannot form hydrogen bonds.
- **H2O (Water)**:
- Water can form hydrogen bonds due to the presence of hydrogen atoms bonded to oxygen. However, in the vapor state, these bonds are not as strong as in the liquid state, and thus, they may not be significant.
- **HF (Hydrogen Fluoride)**:
- HF has a very strong hydrogen bond due to the high electronegativity of fluorine. This bond is strong enough to persist even in the vapor state, allowing HF to exist in a hexameric form (a cluster of six molecules held together by hydrogen bonds).
- **C2H5NS2 (Ethylamine)**:
- Ethylamine can form hydrogen bonds due to the presence of nitrogen, but these bonds are generally weaker than those in HF and may not be significant in the vapor state.

3. **Conclusion**:
- Among the given options, **HF (Hydrogen Fluoride)** is the only molecule that can form hydrogen bonds even in the vapor state due to its strong electronegativity difference and the resulting polarity.

### Final Answer:
**HF (Hydrogen Fluoride)** is the molecule that forms hydrogen bonds even in the vapor state.

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