Question

if B-CI bond has a dipole moment, why does BCI3 has zero dipole moment?

Answer 1

Dipoles are vectors. 3 B-Cl bonds (vectors) in BCl3 act upon one another in such a way that the net vecor resultant is zero. So BCl3 is non polar.

In BCl3, three B-Cl bonds are arranged in planner trigonal geometry, at 120 degree from one another. So 3 vectors of equal magnitude when act upon one another in such arrangement the net resultant becomes zero.

The resultant of any two B-Cl bond is exactly same as that of third B-Cl bond but opposite in direction. So net dipole of BCl3 is zero.

Answer 2

B-Cl has a dipole due to the difference in the elctronegativity of boron and chlorine atom. The overall dipole of a molecule also depends on the geometry. The geometry of BCl3 is planar with a bond angle of 120 degree. The resultant dipole of two B-Cl bonds cancels the third one, resulting in net zero dipole.