If B–Cl bond has a dipole moment, explain why `BCl_(3)` molecule has zero dipole moment.

Explanation

The electronegativity of boron is 2.0, while that of CI is 3.0. Due to the difference in electronegativity, B - CI bond is polar and has a definite dipole moment. `BCl_3` is a trigonal planar molecule due to `sp^2` hybridisation state of B in it. In `BCl_3`, the three B - Cl bonds lie in the same plane at an angle of `120^@`. Therefore, the resultant dipole moment of two B - Cl bonds is equal in magnitude and opposite in direction to the dipole moment of the third B-Cl bond. As a result, the net dipole moment of `BCl_3` molecule is zero as shown below.