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Polar molecules occur when two atoms do not share electrons equally in a covalent bond. If the electronegativity difference between the atoms is greater than 2.0, the bond is ionic. Ionic compounds are extremely polar molecules. But if the dipole moments cancel out on there molecule, its symmetrical, then it is nonpolar. Also you can sometimes tell if the molecule will be polar depending on lone pairs. For example in a linear shape with two atoms that are the same, the addition of a lone pair would cause the molecule to become polar.
Essentially, to determine if a molecule is polar, you must first look to see that the bonds are polar and that the molecule is asymmetrical. To tell if the bonds are polar, you look at the difference in electronegativity. On notable nonpolar bond is C-H. Then, look at the shape of the molecule and take note of any asymmetries caused by lone pairs.
It also helps to think of the regions of electron distribution and whether one area is more localized with electrons. If the electron cloud is more localized, it is most definitely polar. If the electron cloud is evenly distributed, it is nonpolar.
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