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

To determine which molecule among \( \mathrm{H_2S}, \mathrm{H_2O}, \mathrm{NF_3}, \mathrm{NH_3} \), and \( \mathrm{CHCl_3} \) has the lowest dipole moment value, we need to consider the molecular geometry and the electronegativities of the atoms involved.

1. Dipole Moment Concept: A dipole moment arises in a molecule containing atoms of different electronegativities. It depends on both the difference in electronegativity and the geometry of the molecule. Symmetrical molecules often have a lower dipole moment because the polarity cancels out.
2. Analysis of Each Molecule:
   • \(\mathrm{H_2S}\): Bent molecular geometry, moderate dipole moment due to difference in electronegativity between H and S.
   • \(\mathrm{H_2O}\): Bent molecular geometry, significant dipole moment because O is highly electronegative.
   • \(\mathrm{NF_3}\): Pyramidal geometry, less dipole moment compared to \(\mathrm{NH_3}\), but not the lowest because \(\mathrm{N}\) is more electronegative than \(\mathrm{F}\).
   • \(\mathrm{NH_3}\): Pyramidal geometry, higher dipole moment due to high electronegativity of N versus H.
   • \(\mathrm{CHCl_3}\): Tetrahedral geometry, moderate dipole moment due to C-H and C-Cl bonds not cancelling completely.
3. Identifying Lowest Dipole Moment: Among these, \(\mathrm{CHCl_3}\) has the lowest dipole moment. This is because the tetrahedral shape of the molecule partially cancels out the dipole moments of the C-H and C-Cl bonds.
4. Number of Lone Pairs on Central Atom: In \(\mathrm{CHCl_3}\), the central atom is carbon (C), which has no lone pairs of electrons when forming four bonds (tetrahedron structure).

Conclusion: The molecule \(\mathrm{CHCl_3}\) has the lowest dipole moment among the given options, and the number of lone pairs of electrons present on the central atom (carbon) is 0.