Question

Why polar molecules have dipole moment and non-polar molecules do not have dipole moment?

Answer 1

The shape of a molecule and the polarity of its bonds determine the overall polarity of that molecule. A molecule that contains polar bonds might not have any overall polarity, depending upon its shape. The simple definition of whether a complex molecule is polar or not depends upon whether its overall centres of positive and negative charges overlap. If these centres lie at the same point in space, then the molecule has no overall polarity (and is non-polar). If a molecule is completely symmetric, then the dipole moment vectors on each molecule will cancel each other out, making the molecule nonpolar. A molecule can only be polar if the structure of that molecule is not symmetric.