Lesson 8.7 Dipole Moments
Suggested Reading

Zumdahl Chapter 8 Section 8.3
Essential Question

What is a dipole moment and what is it used for?
Learning Objectives


Define dipole moment.
Predict whether or not a molecule is polar from its molecular shape
and bond polarities.
Introduction
In the previous lesson you learned how to make predictions about
molecular geometry using VSEPR. Such predictions must be verified by
experiment. Measuring an experimental quantity called dipole moment,
which is related to the polarity of the bonds in the molecule, can sometimes
help us verify our predictions.
Dipole Moment
Dipole moment is a measure of net molecular polarity for molecules, which
result from polar bonds between atoms. The larger the difference in
electronegativities of bonded atoms, the larger the dipole moment. Dipole
moments are usually measured in units of debyes (D). In SI units, dipole
moments are usually measured in coulomb-meters (C ∙ m), where 1 D =
3.34 x 10-30 C ∙ m. You will not be asked to calculate dipole moments, but
you should be able to predict whether or not a molecule is polar from it
molecular shape and bond polarities.
For example, in the chloromethane (CH3Cl) molecule, chlorine is more
electronegative than carbon. Therefore, it will attract the electrons in the
C—Cl bond toward itself. This gives chlorine a slightly negative charge and
carbon a slightly positive charge in the C—Cl bond. Since one end of C-Cl
is positive and the other end is negative, it is described as a polar bond. To
indicate the increase in electron density, the dipole is represented by an
arrow with a cross at one end. The cross end of the arrow represents the
positive end and the point of the arrow represents the negative end of the
dipole. The symbol δ (delta) indicates the partial charge of an individual
atom.
Table 1: Dipole Moments of Some Compounds
Compound
Dipole Moment (Debyes)
NaCl
9.0 (measured in the gas phase)
CH3Cl
1.87
H2 O
1.85
NH3
1.47
CO2
0
CCl4
0
Polar and Non-polar Molecules
You can see from the data above that molecules can be either polar of nonpolar depending on whether or not there is a net dipole moment. Any
diatomic molecule that has a polar bond will also show a molecular dipole
moment. Polyatomic molecules can also exhibit polar behavior. Water is a
familiar example of this. Similarly, the NH3 molecule has a dipole moment
since the N is more polar than H. Some molecules with polar bonds do not
have a dipole moment. This occurs when the individual bond polarities
cancel each other out. The CO2 molecule is the most common example of
this.
Dipole Moment and Molecular Geometry
You can sometimes relate the presence or absence of a dipole moment in
a molecule to its molecular geometry. Consider the carbon dioxide
molecule pictured above. Each C-O bond has a polarity in which the more
electronegative oxygen atom has a partial negative charge. However, two
bond dipoles of equal magnitude but opposite direction cancel each other.
Think of a tug of war. As long as each group pulls on the rope with the
same force but in the opposite direction, there is no movement and the net
force is zero. This is true for linear molecules of the form AX2. In water,
which is also AX2, the bond dipoles do not point directly toward or away
from one another. As a result, they add together to give a nonzero dipole
moment as shown above. The fact that water has a dipole moment is
strong experimental evidence for the bent geometry.
The analysis we have just made of the two different geometries of AX2
molecules can be extended to other AXn molecules (in which all X atoms
are identical). Molecules in which the A-X bonds are symmetrical about the
central atom (such as the linear, trigonal planar, and tetrahedral
geometries) have a zero dipole moment, and are nonpolar. Molecules
where the X atoms are not symmetrical (such as the bent and trigonal
pyramidal geometries) can have a dipole moment. Thus, they can be polar
molecules.
The following video does a great job of showing dipole moments in both
polar and nonpolar molecules (dipole = 0)
https://www.youtube.com/watch?v=i4ikq5opgPg
HOMEWORK: Refer to lesson 8.6 (same questions)