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### Sample Study Sheet: Predicting Molecular Polarity

Tip-off You are asked to predict whether a molecule is polar or nonpolar; or you are asked a question that cannot be answered unless you know whether a molecule is polar or nonpolar. (For example, you are asked to predict the type of attraction holding the particles together in a given liquid or solid.)

General Steps -

Step 1: Draw a reasonable Lewis structure for the substance.

Step 2: Identify each bond as either polar or nonpolar. (If the difference in electronegativity for the atoms in a bond is greater than 0.4, we consider the bond polar. If the difference in electronegativity is less than 0.4, the bond is essentially nonpolar.)

• If there are no polar bonds, the molecule is nonpolar.
• If the molecule has polar bonds, move on to Step #3.

Step 3: If there is only one central atom, examine the electron groups around it.

• If there are no lone pairs on the central atom, and if all the bonds to the central atom are the same, the molecule is nonpolar. (This shortcut is described more fully in the Example that follows.)
• If the central atom has at least one polar bond and if the groups bonded to the central atom are not all identical, the molecule is probably polar. Move on to Step #4.

Step 4: Draw a geometric sketch of the molecule.

Step 5: Determine the symmetry of the molecule using the following steps.

• Describe the polar bonds with arrows pointing toward the more electronegative element. Use the length of the arrow to show the relative polarities of the different bonds. (A greater difference in electronegativity suggests a more polar bond, which is described with a longer arrow.)
• Decide whether the arrangement of arrows is symmetrical or asymmetrical
• If the arrangement is symmetrical and the arrows are of equal length, the molecule is nonpolar.
• If the arrows are of different lengths, and if they do not balance each other, the molecule is polar.
• If the arrangement is asymmetrical, the molecule is polar.
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