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Polarity is induced in the molecules when there are significant electronegativity differences between its atoms. Due to this difference of electronegativity, one atoms pulls shared electrons more towards itself and gets partial negative charge while the other atoms that has less electronegativity gets a partial positive charge.This electronegativity difference creates poles and a dipole moment in the whole molecule. The example of such a polar molecule is H2O.
However, sometimes the molecule is composed of atoms that have substantial electronegativity differences but still the molecule is not polar.
Why ?
Because the polar bonds are organized in a symmetrical way, in a way that the dipole moment of whole molecule becomes zero. This is because bond polarity is cancelled because of a symmetrical shape.
Take the example of carbon tetrachloride or CCL4. No doubt, chlorine and carbon have electronegativity difference and the molecule should be polar, but in actual it is not polar. If you look at the structure of CCl4 in attached figure, you can see that you can see that the shape of molecule is symmetrical. There is one bond on right and left each side of carbon , which cancel the net polarity of each other. One bond is on top and bottom of carbon each, that cancel the dipole moment of each other. Though electronegativity difference still exists but the overall molecule is non-polar. However, when the molecule reacts with some other atom or molecule and replaces chlorine atoms, the symmetry is broken and the resultant molecule becomes polar.
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