Question

The correct order of dipole moments for the given species is

• A. \( \mathrm{BF_3 = NH_4^+ < NF_3 < NH_3} \)
• B. \( \mathrm{BF_3 < NH_4^+ < NF_3 < NH_3} \)
• C. \( \mathrm{NH_4^+ < BF_3 < NH_3 < NF_3} \)
• D. \( \mathrm{BF_3 < NH_4^+ < NH_3 < NF_3} \)

Hint:

While comparing dipole moments, always check:
• Molecular geometry
• Symmetry of the molecule
• Direction of bond dipoles
• Effect of lone pairs A very important exception is: \[ \mu(\mathrm{NH_3}) > \mu(\mathrm{NF_3}) \] even though fluorine is more electronegative, because in \( \mathrm{NF_3} \) the lone pair dipole and bond dipoles oppose each other.

Answer 1

The Correct Option is B

Step 1: Understanding the Concept:
Dipole moment (\(\mu\)) is a vector quantity that depends on bond polarity (electronegativity difference) and molecular geometry (symmetry). A perfectly symmetrical molecule has a net dipole moment of zero because the individual bond dipoles cancel out.
Step 3: Detailed Explanation:
1. BF\(_3\): Trigonal planar geometry (sp\(^2\) hybridized B). The three B-F bond dipoles are equal in magnitude and directed at 120\(^{\circ}\) from each other. Vector sum is zero. \(\mu = 0\).
2. NH\(_4^+\): Tetrahedral geometry (sp\(^3\) hybridized N). Due to the perfect tetrahedral symmetry, the four N-H bond dipoles cancel each other out. \(\mu = 0\). Thus, BF\(_3 = \) NH\(_4^+\).
3. NH\(_3\) vs NF\(_3\): Both have pyramidal geometry. - In NH\(_3\), N is more electronegative than H. The bond dipoles point towards N. The lone pair dipole also points away from N in the same direction. They add up, resulting in a large net dipole. - In NF\(_3\), F is more electronegative than N. The bond dipoles point towards F. The lone pair dipole points in the opposite direction. They partially cancel out. Therefore, \(\mu(\text{NH}_3)>\mu(\text{NF}_3)\). Comparing all: \(0 = 0<\mu(\text{NF}_3)<\mu(\text{NH}_3)\).
Step 4: Final Answer:
The symmetrical molecules have zero dipole moment. NH\(_3\) has a higher dipole moment than NF\(_3\) because its bond dipoles and lone pair dipole reinforce each other.