In an ionic bond, atoms A with free places for electrons on their outer shell (valence electrons) form a bond with other atoms B which have just the number of valence electrons matching the free places on their outer shell. The "supernumerary" electrons of atoms B thus move to the free places of atoms A. As a result, however, there are now too many electrons at atoms A and too few at atoms B. In the molecule A=B, there is thus a negative centre at A and a positive centre at B. Accordingly, the molecule has a positive and a negative pole and is thus a dipole element. This type of molecule is polar. Despite its ionic bonds, a molecule can be free of dipole moments (be non-polar) if it has several bonds whose dipole moments cancel each other. Methane is an example for this type of molecule; it has a fully symmetric structure so that the dipole moments of the 4 C-H-bonds cancel each other. 
Methane and all its polymers are non-polar.

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