Question: why bcl3 has zero dipole moment?
Answer: B-Cl has a dipole due to the difference in the electronegativity 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.