Question

Which of the following has zero dipole moment ?

• A. NH\(_3\)
• B. NF\(_3\)
• C. BF\(_3\)
• D. None of these

Hint:

Central atoms with no lone pairs surrounded by identical atoms (e.g., linear, trigonal planar, tetrahedral, octahedral geometries) will inherently have a zero dipole moment due to symmetric vector cancellation.

Answer 1

The Correct Option is C

Step 1: Understanding the Question:
A molecule has a zero dipole moment if its individual bond dipoles perfectly cancel each other out. This happens in highly symmetric molecular geometries without lone pairs on the central atom (or highly specific symmetric lone pair arrangements).
Step 2: Detailed Explanation:
Let's evaluate the structure of each molecule:
(A) NH\(_3\) (Ammonia): The Nitrogen atom is $sp^3$ hybridized and possesses one lone pair. The geometry is trigonal pyramidal. The individual N-H bond dipoles add up, and the lone pair also contributes, resulting in a net non-zero dipole moment ($\approx 1.47 \text{ D}$).
(B) NF\(_3\) (Nitrogen trifluoride): Similar to ammonia, it has a trigonal pyramidal shape due to the lone pair on Nitrogen. The N-F bond dipoles oppose the lone pair dipole, leading to a small but strictly non-zero net dipole moment ($\approx 0.24 \text{ D}$).
(C) BF\(_3\) (Boron trifluoride): The Boron atom is $sp^2$ hybridized and has no lone pairs. The molecule adopts a perfect trigonal planar geometry with bond angles of exactly $120^\circ$. Due to this perfect symmetry, the three B-F bond dipoles pull equally in opposite directions, entirely canceling each other out via vector addition. Thus, the net dipole moment is absolutely zero.
Step 3: Final Answer:
BF\(_3\) has a zero dipole moment.