EXAMPLE
Predicting Molecular Polarity:
Decide
whether the molecules represented by the following formulas are polar or
nonpolar. (You may need to draw Lewis structures and geometric sketches to do
so.)
a. CO2 b. OF2
c. CCl4 d. CH2Cl2
e. HCN
Solution:
a. The Lewis structure for CO2 is
The electronegativities of carbon and oxygen
are 2.55 and 3.44. The 0.99 difference in electronegativity indicates that the
C-O bonds are polar, but the symmetrical arrangement of these bonds makes
the molecule nonpolar.
If we put arrows into the geometric sketch for CO2, we see that they exactly balance each other, in both direction and
magnitude. This shows the symmetry of the bonds.
b. The Lewis structure for OF2 is
The electronegativities of oxygen and fluorine,
3.44 and 3.98, respectively, produce a 0.54 difference that leads us to predict
that the O-F bonds are polar. The molecular geometry of OF2 is bent. Such
an asymmetrical distribution of polar bonds would produce a polar molecule.
c. The molecular geometry of CCl4 is
tetrahedral. Even though the C-Cl bonds are polar, their symmetrical arrangement
makes the molecule nonpolar.
 
d. The
Lewis structure for CH2Cl2 is
The electronegativities of hydrogen, carbon, and
chlorine are 2.20, 2.55, and 3.16. The 0.35 difference in electronegativity for
the H-C bonds tells us that they are essentially nonpolar. The 0.61
difference in electronegativity for the C-Cl bonds shows that they are
polar. The following geometric sketches show that the polar bonds are
asymmetrically arranged, so the molecule is polar. (Notice that the Lewis
structure above incorrectly suggests that the bonds are symmetrically arranged.
Keep in mind that Lewis structures often give a false impression of the geometry
of the molecules they represent.)
e. The
Lewis structure and geometric sketch for HCN are the same:
The electronegativities of hydrogen,
carbon, and nitrogen are 2.20, 2.55, and 3.04. The 0.35 difference in
electronegativity for the H-C bond shows that it is essentially nonpolar. The
0.49 difference in electronegativity for the C-N bond tells us that it is polar.
Molecules with one polar bond are always polar.
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here to see a Study Sheet for this task.
Click
here to see an exercise that will allow you to try this task yourself.
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Molecular Polarity Page.
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