Concept: Dipole Moment and Molecular Geometry

The dipole moment of a molecule is a measure of the net polarity of its bonds. It depends on both the polarity of individual bonds and the molecular geometry. If a molecule has polar bonds but a symmetrical geometry such that the individual bond dipoles cancel each other out, the net dipole moment will be zero.

Why (A) BF\u2083 is correct:

BF\u2083 has a trigonal planar geometry. The boron atom is at the center, and three fluorine atoms are symmetrically arranged around it. Although each B-F bond is polar due to the electronegativity difference between boron and fluorine, the three bond dipoles are oriented at 120\u00b0 to each other and cancel each other out perfectly. Therefore, the net dipole moment of BF\u2083 is zero.

Option Analysis:

• A) BF\u2083: Trigonal planar geometry, bond dipoles cancel out. Net dipole moment = 0.
• B) CH\u2082Cl\u2082: Tetrahedral geometry but with different peripheral atoms (H and Cl). The C-Cl bonds are more polar than C-H bonds, and the molecule is not symmetrical enough for the dipoles to cancel. Net dipole moment \u2260 0.
• C) NF\u2083: Pyramidal geometry due to the presence of a lone pair on nitrogen. The N-F bond dipoles do not cancel out, and the lone pair also contributes to the overall dipole moment. Net dipole moment \u2260 0.
• D) SO\u2082: Bent (V-shaped) geometry due to the presence of a lone pair on sulfur. The S-O bond dipoles do not cancel out. Net dipole moment \u2260 0.

Correct Answer: (A)