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

Explanation

Due to electronegativity difference between `B` (E.N. `= 2.0`) and Cl (E.N. `= 3.0`), the B-Cl bond is polar and hence has a finite dipole moment. The overall dipole moment of a molecule, however, depends upon its geometry . Now `BCl_(3)` is a planar molecule in which the three `B-Cl` bonds are inclined at an angle of `120^(@)` . Therefore , the resultant of two B-Cl bonds is cancelled by equal and opposite dipole moment of the third `B-Cl` bond as shown:
As a result , overall dipole moment of `BCl_(3)` is zero.