Question

Consider the given species: H\(_2\)S, H\(_2\)O, NH\(_3\), NF\(_3\), CHCl\(_3\) The number of lone pairs on the central atom which has the lowest dipole moment is:

• A. 0
• B. 1
• C. 2
• D. 3

Hint:

Symmetry plays a crucial role in determining dipole moments. A lone pair on the central atom may have little or no effect on the dipole moment if the geometry is symmetric enough.

Answer 1

The Correct Option is B

To determine the number of lone pairs on the central atom of the species with the lowest dipole moment among H₂S, H₂O, NH₃, NF₃, and CHCl₃, we start by examining the molecular structure and dipole moment of each of these compounds:

1. **H₂S**: The central atom is sulfur. It has two hydrogen atoms bonded to it and two lone pairs. The shape is bent or V-shaped, which creates a dipole moment due to the difference in electronegativity between sulfur and hydrogen.
2. **H₂O**: The central atom is oxygen. It has two hydrogen atoms bonded to it and two lone pairs. The shape is bent (similar to H₂S), leading to a significant dipole moment due to the highly polar OH bonds.
3. **NH₃**: The central atom is nitrogen, bonded to three hydrogen atoms and possessing one lone pair. The molecular geometry is trigonal pyramidal, resulting in a substantial dipole moment due to the lone pair and the polar N-H bonds.
4. **NF₃**: The central atom is nitrogen, bonded to three fluorine atoms with one lone pair. Despite being similar in structure to NH₃, the higher electronegativity of fluorine reduces the net dipole moment, as the bond polarities tend to cancel each other out to some extent.
5. **CHCl₃**: The central atom is carbon, and it is surrounded by three chlorine atoms and one hydrogen atom. There are no lone pairs on carbon, and the molecule has a net dipole moment due to the different polarities of the C-H and C-Cl bonds.

Among the molecules, NF₃ exhibits the lowest dipole moment due to the cancellation effects of the polar N-F bonds. The central atom, nitrogen, has one lone pair. Therefore, the number of lone pairs on the central atom with the lowest dipole moment is 1.

Based on this analysis, the correct answer is 1.