Question

Among \( \mathrm{H_2S}, \mathrm{H_2O}, \mathrm{NF_3}, \mathrm{NH_3} \) and \( \mathrm{CHCl_3} \), identify the molecule \( (X) \) with lowest dipole moment value. The number of lone pairs of electrons present on the central atom of the molecule \( (X) \) is

• A. 1
• B. 0
• C. 3
• D. 2

Hint:

In molecules like \( \mathrm{NF_3} \), bond dipoles can oppose the lone pair dipole, resulting in an unusually low net dipole moment.

Answer 1

The Correct Option is A

Step 1: Compare dipole moments of the given molecules.
Approximate dipole moments are: \[ \mathrm{H_2O} \approx 1.85\,D,\quad \mathrm{H_2S} \approx 0.97\,D, \] \[ \mathrm{NH_3} \approx 1.47\,D,\quad \mathrm{NF_3} \approx 0.24\,D, \] \[ \mathrm{CHCl_3} \approx 1.04\,D. \] Among these, \( \mathrm{NF_3} \) has the lowest dipole moment.

Step 2: Identify the central atom and lone pairs in \( \mathrm{NF_3} \).
In \( \mathrm{NF_3} \): - Central atom is nitrogen. - Nitrogen has 5 valence electrons. - It forms three \( \mathrm{N-F} \) bonds and retains one lone pair.

Step 3: Final conclusion.
The molecule with the lowest dipole moment is \( \mathrm{NF_3} \), and the number of lone pairs on its central atom is \(1\).

Final Answer: \[ \boxed{1} \]