Study Sheet: Predicting Molecular Polarity
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
- 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
- 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.
here to see an example of this task.
here to see an exercise that will allow you to try this task yourself.
the Molecular Polarity Page.