Question

Which of the following molecule has zero dipole moment?

• A. \(BF_3\)
• B. \(CH_2Cl_2\)
• C. \(NH_3\)
• D. \(SO_2\)
• E. \(NF_3\)

Hint:

For zero dipole moment, check molecular symmetry. Even if bonds are polar, symmetry can cancel the dipoles completely.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
A molecule has a zero dipole moment if it is nonpolar. This can happen in two ways: either all the bonds in the molecule are nonpolar (which is rare), or the molecule has polar bonds but its geometry is perfectly symmetrical, causing the individual bond dipoles to cancel each other out. We need to analyze the geometry and bond polarities of each molecule.
Step 2: Detailed Explanation:

(A) BF\(_3\) (Boron trifluoride): The central atom, Boron (B), has 3 valence electrons and forms three single bonds with Fluorine (F) atoms. There are no lone pairs on Boron. According to VSEPR theory, the geometry is trigonal planar with bond angles of 120°. Each B-F bond is polar due to the high electronegativity of F. However, the three identical bond dipoles are arranged symmetrically and point towards the vertices of an equilateral triangle. Their vector sum is zero. Thus, BF\(_3\) has a zero dipole moment.

(B) CH\(_2\)Cl\(_2\) (Dichloromethane): The central Carbon (C) atom is bonded to two Hydrogen (H) atoms and two Chlorine (Cl) atoms in a tetrahedral geometry. Both C-H and C-Cl bonds are polar, and their dipoles are of different magnitudes. The geometry is not perfectly symmetrical because of the different atoms attached. The individual bond dipoles do not cancel out, resulting in a net dipole moment.

(C) NH\(_3\) (Ammonia): The central Nitrogen (N) atom forms three single bonds with H atoms and has one lone pair of electrons. The geometry is trigonal pyramidal. The N-H bonds are polar, and the lone pair also contributes to the dipole moment. All dipoles point generally in the same direction, leading to a large net dipole moment.

(D) SO\(_2\) (Sulfur dioxide): The central Sulfur (S) atom forms two double bonds with Oxygen (O) atoms and has one lone pair. The geometry is bent or V-shaped. The S-O bonds are polar, and due to the bent shape, the bond dipoles do not cancel out. This results in a net dipole moment.

(E) NF\(_3\) (Nitrogen trifluoride): Similar to NH\(_3\), the central N atom is bonded to three F atoms and has one lone pair. The geometry is trigonal pyramidal. The N-F bonds are highly polar. Although the lone pair dipole opposes the bond dipoles, they do not cancel out completely, resulting in a net dipole moment.

Step 3: Final Answer:
Among the given options, only BF\(_3\) has a symmetrical geometry (trigonal planar) that allows for the complete cancellation of its polar bond dipoles, resulting in a zero dipole moment.