Molecular Polarity
Molecular Structure
Courtesy Christy Johannesson www.nisd.net/communicationsarts/pages/chem
Dipole Moment
•Direction of the polar bond in a molecule.
•Arrow points toward the moreelectronegative atom.
H Cl
+
-
Courtesy Christy Johannesson www.nisd.net/communicationsarts/pages/chem
Determining Molecular Polarity
•Nonpolar Molecules
–Dipole moments are symmetrical and cancelout.
BF3
F
F
F
B
Courtesy Christy Johannesson www.nisd.net/communicationsarts/pages/chem
Determining Molecular Polarity
•Polar Molecules
–Dipole moments are asymmetrical and don’tcancel .
net
dipole
moment
H2O
H
H
O
Courtesy Christy Johannesson www.nisd.net/communicationsarts/pages/chem
CHCl3
H
Cl
Cl
Cl
Determining Molecular Polarity
•Therefore, polar molecules have...
–asymmetrical shape (lone pairs) or
–asymmetrical atoms
net
dipole
moment
Courtesy Christy Johannesson www.nisd.net/communicationsarts/pages/chem
Dipole Moment
Nonpolar
Polar
..
..
H
H
O
C
O
O
Bond dipoles
Overall dipole moment = 0
Bond dipoles
Overall dipole moment
The overall dipole moment of a molecule
is the sum of its bond dipoles. In CO2 the
bond dipoles are equal in magnitude but
exactly opposite each other. The overall
dipole moment is zero.
In H2O the bond dipoles are also equal in
magnitude but do not exactly oppose each
other. The molecule has a nonzero overall
dipole moment.
Dipole moment,
Brown, LeMay, Bursten, Chemistry The Central Science, 2000, page 315
Determining Molecular Polarity
•Ammonia, NH3
Methane and Chloro-methane
Carbon dioxide and sulfur dioxide
..
..
..
Polar Bonds
H
Cl
Polar
A molecule has a zero dipole moment because their dipoles cancel one another.
H
H
O
Polar
F
F
B
F
Nonpolar
H
H
H
N
Polar
Polar
Nonpolar
F
F
Cl
F
F
F
Xe
F
F
Cl
Cl
C
Cl
Nonpolar
Polar
Cl
H
C
Cl
H
H