$B - Cl$ आबंध द्विध्रुव आघूर्ण रखता है, किन्तु $BCl$ अणु का द्विधुव आधूर्ण शून्य होता है। क्यों ?

As a result of the difference in the electronegativities of $B$ and $C l$, the $B-Cl$ bond is polar in nature. However, the $BCl_3$ molecule is non-polar. This is because $BCl_3$ is trigonal planar in shape. It is a symmetrical molecule. Hence, the respective dipole-moments of the $B-Cl$ bond cancel each other, thereby causing a zero-dipole moment.