Which of the following exhibits the weakest intermolecular forces?

Options

A. He
B. HCl
C. `NH_(3)`
D. `H_(2)O`

Answer

To determine which of the given substances exhibits the weakest intermolecular forces, we will analyze the options provided: helium (He), hydrogen chloride (HCl), ammonia (NH₃), and water (H₂O).

### Step-by-Step Solution:

1. **Identify the Intermolecular Forces in Helium (He)**:
- Helium is a noble gas and exists as individual atoms. The only intermolecular forces present in helium are Van der Waals forces (also known as London dispersion forces), which are very weak.

**Hint**: Noble gases have weak intermolecular forces due to their monoatomic nature.

2. **Identify the Intermolecular Forces in Hydrogen Chloride (HCl)**:
- HCl is a polar molecule due to the difference in electronegativity between hydrogen and chlorine. This leads to dipole-dipole interactions between HCl molecules, which are stronger than Van der Waals forces.

**Hint**: Polar molecules exhibit dipole-dipole interactions due to partial charges.

3. **Identify the Intermolecular Forces in Ammonia (NH₃)**:
- Ammonia has a nitrogen atom bonded to three hydrogen atoms. The presence of a highly electronegative nitrogen atom leads to hydrogen bonding, which is a strong type of intermolecular force.

**Hint**: Hydrogen bonding occurs when hydrogen is bonded to highly electronegative atoms like nitrogen, oxygen, or fluorine.

4. **Identify the Intermolecular Forces in Water (H₂O)**:
- Water is also a polar molecule and exhibits hydrogen bonding due to the presence of lone pairs on the oxygen atom. This results in strong intermolecular forces.

**Hint**: Water's unique properties are largely due to its ability to form hydrogen bonds.

5. **Compare the Strength of Intermolecular Forces**:
- The order of strength of intermolecular forces from weakest to strongest is:
- Van der Waals forces (Helium)
- Dipole-dipole interactions (HCl)
- Hydrogen bonding (Ammonia and Water)

6. **Conclusion**:
- Since Van der Waals forces are the weakest type of intermolecular force, helium exhibits the weakest intermolecular forces among the given options.

### Final Answer:
**Helium (He) exhibits the weakest intermolecular forces.**