In case of heteroatomic molecules there is possibility of difference in electro negativity of the two hetero atoms bonded, this results in development of charges on the two atoms
more electronegative element will have a negative charge
less electronegative element will have a positive charge
Case
a) if the difference is less than 0.4 the molecule is nonpolar.
b) if the difference is from 0.4 to 1.7 the molecule is polar covalent.
c) if the difference is more than than 1.7 the molecule ionic (polar).