Question

Which of the following is more polar?

• A. $H_2S$
• B. $NH_3$
• C. $NF_3$
• D. $CHCl_3$

Hint:

A classic chemistry trap compares $NH_3$ and $NF_3$. Even though fluorine is much more electronegative than hydrogen, $NH_3$ is far more polar because its bond dipoles and orbital (lone pair) dipole act in the exact same direction, while in $NF_3$ they act in opposite directions.

Answer 1

The Correct Option is B

Step 1: Understanding the Question:
The question requires us to identify the most polar molecule among the given choices. The polarity of a molecule is quantified by its net dipole moment ($\mu$), measured in Debye (D).

Step 2: Detailed Explanation:
The net polarity depends on both the individual bond dipoles and the vector geometry of the entire molecule, heavily influenced by lone pairs.
Let's analyze the dipole moments of each option:
- (a) $H_2S$: Has a bent shape. Sulphur is more electronegative than hydrogen, but the electronegativity difference is relatively moderate. Dipole moment $\mu \approx 0.95$ D.
- (c) $NF_3$: Has a pyramidal shape with one lone pair. Fluorine is highly electronegative, pulling electron density away from nitrogen. However, the vector of the $N-F$ bond dipoles points completely downwards, directly opposing and canceling out the upward dipole moment generated by the nitrogen lone pair. This drastic cancellation makes it almost non-polar. Dipole moment $\mu \approx 0.23$ D.
- (d) $CHCl_3$: Has a tetrahedral geometry. The three highly electronegative chlorine atoms pull electron density downward, while the less electronegative hydrogen is at the top. It is moderately polar. Dipole moment $\mu \approx 1.04$ D.
- (b) $NH_3$: Has a pyramidal shape with one lone pair. Nitrogen is highly electronegative compared to hydrogen. The $N-H$ bond dipoles all point upward toward the nitrogen atom. Crucially, the dipole moment created by the lone pair also points upward. Because all vectors align and reinforce each other, $NH_3$ possesses an exceptionally high net polarity. Dipole moment $\mu \approx 1.47$ D.

Step 3: Final Answer:
Ammonia ($NH_3$) has the highest dipole moment, making it the most polar compound. This corresponds to option (b).