In chemistry, the concept of polarity refers to how some chemical bonds result in unequal sharing of electrons. This means shared electrons will be closer to one atom in a bond than another, which creates areas of positive and negative charge. You can use the difference in electronegativity of two atoms to predict whether they make polar, nonpolar or ionic bonds. Water is a well-known example of a polar molecule. According to the University of Arizona, "Water has a partial negative charge near the oxygen atom -- and partial positive charges near the hydrogen atoms."
Draw the Lewis structure of the molecule. Note each element in the molecule.
Look up each element in the molecule in an electronegativity table and note their electronegativity.
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Subtract the electronegativity of one atom in a bond from the other. Take the absolute value. A difference from 0.0 to 1.2 is nonpolar. A difference of 1.2 to 1.8 is polar. A difference of 1.8 and above is ionic.
Repeat Step 3 for each bond in the molecule.
Draw an arrow near each polar bond. Point the tip toward the more electronegative atom. If all the arrows point to a common center, the molecule is nonpolar. If they do not, the molecule is polar.
Some elements, such as fluorine, change electronegativity based on what elements they bond with.