Question

Pick out the molecule that has a zero dipole moment.
(A) $N{{H}_{3}}$
(B) ${{H}_{2}}O$
(C) $BC{{l}_{3}}$
(D) $S{{O}_{2}}$

Answer 1

Hint: A dipole moment arises due to the electronegativity difference between the two atoms of a bond. Both the ionic and polar covalent bonds can have a dipole moment. It is a vector quantity. The symbols used to represent the charges on the two elements are $\delta +$and$\delta -$, where more electronegative elements get $\delta -$ charge and less electronegative elements get $\delta +$ charge. When all the dipoles in a molecule cancel out each other, it gives a net zero dipole moment.

Complete Step by Step Answer:
(A) $N{{H}_{3}}$- It contains three $N-H$bonds and one lone pair of electrons. It has a trigonal bipyramidal shape. Due to the electronegativity difference between nitrogen and hydrogen, the dipole moment is from hydrogen to nitrogen. It will add up to give a net dipole moment of 1.4 D.

(B) ${{H}_{2}}O$- It contains two $O-H$ bonds and two lone pairs of electrons. It has a bent shape. It forms two dipoles from hydrogen to oxygen. It gives a net dipole moment of 1.84 D.

(C) $BC{{l}_{3}}$- It contains three $B-Cl$ bonds and no lone pair of electrons. It has trigonal planar geometry. The three $B-Cl$dipoles are at an angle of ${{120}^{o}}$to each other. These dipoles cancel out the effect of each other and, thus, give zero dipole moment. Hence, $BC{{l}_{3}}$is a nonpolar molecule.

(D) $S{{O}_{2}}$- It contains two $S-O$double bonds and one lone pair of electrons. The two bonds are at an angle of ${{119}^{o}}$. It gives a dipole moment of 1.6 D.
Correct Option: (C) $BC{{l}_{3}}$.

Note: All the other molecules except $BC{{l}_{3}}$ have a non-zero dipole moment. A molecule with a zero dipole moment is non-polar, whereas a molecule with a non-zero dipole moment is polar.