Question

Zero dipole moment is present in
(A) $N{{H}_{3}}$
(B) ${{H}_{2}}O$
(C) cis-1,2-dichloroethane
(D) trans-1,2-dichloroethane

Answer 1

Hint: When there is an electronegativity difference between the two atoms of a bond, then the bond is said to be polar and possess a dipole moment. There is an unequal sharing of electron pairs in the bond. However, if the two atoms have the same electronegativity, then the bond does not possess a dipole moment. Also, if a molecule contains more than two atoms, that is, there is more than one dipole in a molecule, then the net dipole moment is the vector sum of all the dipoles in the molecule.

Complete Step by Step Answer:
(A) $N{{H}_{3}}$- This molecule contains three dipoles from hydrogen to nitrogen. This gives the resultant, which is the sum total of all the dipoles. Hence, it possesses a non-zero dipole moment.

(B) ${{H}_{2}}O$- This molecule contains two dipoles from hydrogen to oxygen. This gives the resultant, which is the sum total of all the dipoles. Hence, it possesses a non-zero dipole moment.

(C) cis-1,2-dichloroethane – This molecule contains one $C-C$ double bond and the one chlorine atom is attached to each carbon. In cis-1,2-dichloroethane, both the dipoles from carbon to chlorine are in the same direction. Hence, it gives a non-zero dipole moment.

(D) trans-1,2-dichloroethane - This molecule also contains one $C-C$ double bond, and the one chlorine atom is attached to each carbon. But in trans-1,2-dichloroethane, both the dipoles from carbon to chlorine are in the opposite direction. This is why the two dipoles cancel out the effect of each other. Hence, it gives zero dipole moment.
Correct Option: (D) trans-1,2-dichloroethane.

Note: The dipole moment is a vector quantity that has both magnitude and direction. This is why we should also keep in mind the direction of dipoles to find the dipole moment of a molecule.