Which has the smallest dipole-dipole forces?
A. CH3 CI
B. HCI
C. AsH3
D. CO
The correct answer and explanation is :
The correct answer is: C. AsH₃
Explanation:
Dipole-dipole forces are a type of intermolecular force that arise between polar molecules—those that have permanent dipoles due to differences in electronegativity between atoms and the molecular geometry. The strength of dipole-dipole interactions depends on two main factors:
- Polarity of the molecule (i.e., the magnitude of the dipole moment).
- Molecular structure and shape, which influence how effectively molecules align to maximize attractions.
Let’s analyze the given options:
A. CH₃Cl (Chloromethane)
- Structure: A tetrahedral molecule with a significant dipole from C to Cl due to Cl’s high electronegativity.
- Polarity: Strongly polar due to the C–Cl bond.
- Dipole moment: ~1.9 D (Debye units)
- Dipole-dipole forces: Moderate to strong
B. HCl (Hydrogen chloride)
- A linear molecule with a highly polar H–Cl bond due to the large electronegativity difference between H and Cl.
- Dipole moment: ~1.1 D
- Dipole-dipole forces: Moderate
C. AsH₃ (Arsine)
- Structure: Trigonal pyramidal, like NH₃, but the electronegativity difference between As (2.0) and H (2.1) is negligible.
- Polarity: Very weak—close to nonpolar despite geometry.
- Dipole moment: ~0.2 D
- Dipole-dipole forces: Very weak
D. CO (Carbon monoxide)
- Structure: Linear and highly polar molecule, with a significant dipole pointing from C to O.
- Dipole moment: ~0.1–0.2 D, but has significant polarity due to uneven electron distribution.
- Dipole-dipole forces: Moderate
Conclusion:
Among the choices, AsH₃ has the weakest dipole-dipole interactions due to:
- Very small electronegativity difference between As and H.
- Low dipole moment.
- Resulting in very weak molecular polarity, and hence, the smallest dipole-dipole forces.