Question

Which of the following order of dipole moment for stated molecules is correct?

• A. \(\text{BF}_3 > \text{NF}_3 > \text{NH}_3\)
• B. \(\text{NF}_3 > \text{BF}_3 > \text{NH}_3\)
• C. \(\text{NH}_3 > \text{BF}_3 > \text{NF}_3\)
• D. \(\text{NH}_3 > \text{NF}_3 > \text{BF}_3\)

Hint:

BF3 is planar and symmetric, so its dipole moment is zero. In NF3 the bond dipoles oppose the lone pair, and in NH3 they add to it.

Answer 1

The Correct Option is D

Step 1: Understanding the Concept:
The molecular dipole moment is the vector sum of bond dipoles and the lone pair contribution.

Step 2: Boron trifluoride.
\(\text{BF}_3\) is trigonal planar. The three B-F bond dipoles cancel, so \(\mu = 0\).

Step 3: Ammonia.
\(\text{NH}_3\) is pyramidal. Nitrogen is more electronegative than hydrogen, so each N-H dipole points towards N, in the same direction as the lone pair. The dipoles add up, giving \(\mu \approx 1.47\) D.

Step 4: Nitrogen trifluoride.
\(\text{NF}_3\) is also pyramidal, but fluorine is more electronegative than nitrogen, so each N-F dipole points away from N, opposite to the lone pair. They partly cancel, giving \(\mu \approx 0.24\) D.

Step 5: Order.
\[ \text{NH}_3 > \text{NF}_3 > \text{BF}_3 \]

Final Answer:
The correct order is \(\text{NH}_3 > \text{NF}_3 > \text{BF}_3\), option (D). \[ \boxed{\text{NH}_3 > \text{NF}_3 > \text{BF}_3} \]