N and Cl have same electronegatively. Whereas hydrogen bonding is present in the molecules of `NH_(3),` it is absent at the sometime in HCl molecules. Explain.

Answer

To explain why hydrogen bonding is present in NH₃ but absent in HCl, despite nitrogen and chlorine having similar electronegativities, we can analyze the factors that contribute to hydrogen bonding.

### Step-by-Step Solution:

1. **Understanding Hydrogen Bonding**:
Hydrogen bonding occurs when a hydrogen atom covalently bonded to a highly electronegative atom (like N, O, or F) interacts with another electronegative atom. This interaction is influenced by the size of the atoms and their electronegativity.

2. **Electronegativity of Nitrogen and Chlorine**:
Both nitrogen (N) and chlorine (Cl) have similar electronegativities, which means they can both attract electrons effectively. However, the ability to form hydrogen bonds also depends on other factors, particularly atomic size.

3. **Atomic Size**:
- **Nitrogen (N)**: Nitrogen has a small atomic size. This small size allows for a closer approach between the hydrogen atom and the nitrogen atom, facilitating the formation of hydrogen bonds.
- **Chlorine (Cl)**: Chlorine has a larger atomic size compared to nitrogen. The larger size means that the distance between the hydrogen atom and the chlorine atom is greater, which weakens the interaction necessary for hydrogen bonding.

4. **Electron Density**:
The electron density around the nitrogen atom in NH₃ is higher due to its smaller size. This high electron density allows for stronger dipole-dipole interactions, which are essential for hydrogen bonding. In contrast, the larger size of chlorine results in a lower electron density that is insufficient to form significant hydrogen bonds.

5. **Conclusion**:
Therefore, while both NH₃ and HCl involve hydrogen and an electronegative atom, the small size and high electron density of nitrogen in NH₃ allow for hydrogen bonding, whereas the larger size and lower effective electron density of chlorine in HCl prevent hydrogen bonding.

### Final Explanation:
In summary, hydrogen bonding is present in NH₃ due to the small size and high electron density of nitrogen, facilitating strong interactions with hydrogen. In contrast, HCl lacks hydrogen bonding because chlorine's larger size and lower effective electron density do not support such interactions.