Molecular polarity depends on two factors: the type of bonds in a molecule and the arrangement of those bonds. In diatomic molecules, bond polarity and molecular polarity are the same. However, in larger molecules the arrangement of bonds must be considered. While a molecule may contain polar bonds, it is only polar at the molecular level if the bonds are arranged asymmetrically. Symmetric molecules with polar bonds can be nonpolar overall if the polar bonds cancel each other out. Molecular shape provides clues about polarity, with linear, tetrahedral and bent molecules often being polar and trigonal pyramids always polar.

1. Molecular Polarity

2. Molecular Polarity
• Depends on 2 factors.
–Type of bonds in the molecule
–Arrangement of bonds or shape of
molecule

3. Diatomic Molecules
• Simplest case:
• Bond Polarity and Molecular Polarity are
identical
• Larger Molecules – Have to look at how
the bonds are arranged.

4. For larger molecules, look at the kind
& arrangement of bonds to determine
overall polarity of molecule.
CH4 has 4 bonds.

5. Polarity of Molecules
• A molecule may contain polar bonds, but
not be polar! Depends on the geometry of
the molecule.
• If molecule is symmetric, the “pull” of one
polar bond is offset by the “pull” of another
polar bond.
– It’s a tug-of-war that no one can win!

6. Symmetric Molecules
• Contain at least one
mirror plane.

7. Symmetric vs. Asymmetric
• CO2 is nonpolar.
• Can’t tell the
ends apart.
• H2O is bent.
• Electron cloud is
lopsided.
• H2O is polar.

8. Polarity of CO2?
Draw an arrow along each bond pointing
to the more electronegative atom.
If the arrows cancel out, the molecule is
NONPOLAR.

9. Polarity of H2O?
The green
arrows do not
cancel out, so
water is polar!

10. Symmetry of Larger Molecules
CF4 is fairly
symmetric.
Overall, it is
nonpolar.
The arrows
cancel out.

11. Use the structural formula to
predict Molecular Polarity!
H
H  C  H
H



12. Ethane = C2H6
Ethene = C2H4
Ethyne = C2H2
These molecules are symmetric and
the electron cloud is the same on both
ends. Overall, they are nonpolar.

13. Molecular Polarity
• If you know the shape, you can
use the arrow technique to
determine the polarity.
• So how do you get the shape?

14. Polarity
– Linear molecules
• Nonpolar if the ends are the same
• Polar if the ends are different
– Tetrahedral
• Nonpolar if all 4 corners are the same
• Polar if even 1 corner is different
– Trigonal Pyramids
• Always polar
– Bent molecules
• Always polar