Question

BF molecule has dipole moment but BR3 molecule has no dipole moment.Why?

Answer 1

The BF3 molecule has a symmetrical trigonal planar geometry, like the SO3 molecule. In such a structure, the resultant moment of any two B-F dipoles is equal in magnitude but opposite in direction to the moment of the third one. So, the net dipole moment of the BF3 molecule is zero, and it is non-polar.