Molecular Polarity

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.
• Its 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.
• Cant 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