Question

A positive dipole moment is present in:
(A) $ CC{{l}_{4}}$
(B) $ {{C}_{6}}{{H}_{6}}$
(C) $ B{{F}_{3}}$
(D) $ HF$

Answer 1

Hint: A dipole moment arises due to the electronegativity difference between the two elements of a bond. This means one of the atoms pulls the electron pair more toward itself than the other. Thus, it measures the polarity of a bond. It is a vector quantity. The greater the electronegativity difference between the two atoms, the greater the dipole moment of their bond.

Complete Step by Step Answer:
(A) $ CC{{l}_{4}}$ - Carbon tetrachloride ($ CC{{l}_{4}}$ ) contains 4 chlorine atoms, each 2 in opposite directions. This will cancel the dipole moments in the opposite direction. Hence, it will have a zero dipole moment.

(B) $ {{C}_{6}}{{H}_{6}}$ - In benzene, there are six $ C-H$ bonds, and due to the electronegativity difference between carbon and hydrogen, there are six dipoles formed in the molecule. But all the six dipoles cancel out the effect of each other. Hence, the dipole moment of $ {{C}_{6}}{{H}_{6}}$ is zero.

(C) $ B{{F}_{3}}$ - Due to the electronegativity difference between boron and fluorine, there are three dipoles in this molecule at an angle of $ {{120}^{o}}$ from boron to fluorine. But they cancel out each other’s effects. Hence, it results in a zero dipole moment.

(D) $ HF$ - As there is a large difference in electronegativity between hydrogen and fluorine, it gives a net positive dipole moment. Hence, $ HF$ is a polar molecule.
Correct Option: (D) $ HF$ .

Note: Dipole moments can be present in both ionic and covalent bonds. The only requirement for a non-zero dipole moment is the electronegativity difference. Also, the distance between the two charges helps in deciding the dipole moment.