Module 4 Unit 2
Inferring Charge Distribution
John Pollard
University of Arizona
In our quest to understand why the particles that make up substances behave differently,
we have zoomed in to evaluate the composition, connectivity and geometric locations of
atoms. In order to complete our understanding we must now look at how these factors
affect the distribution of charge in molecules. What we will find is that the charge
distribution is an essential feature that directly relates to the properties of molecular
compounds.
Explaining Attractions
Simple experiments can be done to demonstrate that the distribution of charge is different
in the molecules that make up varying substances. For example, a static charge can be
built up on a balloon (by rubbing it on your hair) and can in turn be used to manipulate
the pathway of a stream of water. This experiment does not work on a hexane. To
explain this behavior we first assume that the water molecules have a “buildup” of charge
on one side as compared to the other. This imparts a partial negative and partial positive
charge to opposing sides of each molecule which causes the molecules to align with any
external electric field (i.e. the charged balloon surface). Therefore, there must not be an
asymmetrical charge distribution in hexane molecules. Why? To answer this we need to
look closely at how electrons are attracted to atoms involved in a covalent bond.
Bond Polarity
Uneven charge distribution in molecules arises from covalently bonded atoms having
different levels of attraction for shared electrons. For example, consider HCl.
+
H
-
Cl
The electrons in the H-Cl bond spend more time closer to the Cl atom than the H atom.
We therefore say that Cl is more electronegative than H which results in a bond where the
charge is asymmetrically distributed (a polar bond).
In general, the electronegativity of atoms increases upon moving up and to the right on
the periodic table. Electronegativity, often represented by χ, is a measure of the attractive
force atoms have for electrons in bonds and is assigned numbers between 0.7-4 (4 being
for fluorine, the most electronegative atom that forms bonds).
General Trend:
 increases up a group and down a period in the
periodic table
The existence of δ+ and δ¯ partial charges creates a “dipole moment” between bonded
atoms. The dipole moment for a bond, represented by μ, is represented by a vector that
points towards the δ¯ partial charge. The magnitude of the dipole moment vector
indicates the extent to which the bond is polar and is related to the difference in
electronegativity between the atoms. The larger the dipole moment, the more polar the
bond or the more asymmetrically charge is distributed between the two atoms.
Molecular Polarity
When molecules contain more than 2 atoms, we have to take into consideration not only
bond polarities, but the molecular geometry. To illustrate, let’s look at CO2.
-
+
-
Carbon dioxide has a linear molecular geometry with an O-C-O bond angle of 180º.
Both C=O bonds are polar as oxygen is more electronegative than carbon. Despite the
presence of polar bonds, the molecule overall has a symmetrical charge distribution and
is classified as non-polar. This is because the two dipole moment vectors cancel each
other out when added which is illustrated by visually adding the vectors aligning them
head to tail.
O
C
+
O
0
=
In the case of H2O, the dipole moment vectors do not cancel out. Oxygen is more
electronegative than hydrogen so both O-H bonds are polar. But as a result of the bent
geometry, the vectors sum to a net dipole moment vector which indicates a δ¯ charge
near the oxygen and a δ+ charge near the hydrogens.

O
O
H
_
+
H
=
H

+
H
It is the symmetry of the charge distribution as determined by the atom positions and
electronegativities that determine whether a molecule is polar or non-polar. Molecules
can contain very polar bonds, but due to their overall symmetry can be non-polar. Here
are a few examples.
Example 1- CCl4
The molecular geometry of CCl4 is tetrahedral. All 4 C-Cl bonds are polar, but due to the
fact that all the Cl’s are equidistant from the carbon and each other, all the dipole moment
vectors will cancel resulting in a symmetrical distribution of charge. In general, any
molecule with a tetrahedral geometry where all 4 outer atoms are the same will be nonpolar. Substitution of any one or more atom results in a polar molecule.
Carbon
Tetrachloride
CCl4
The color image on the right is called an electrostatic potential plot. Red indicates
regions of charge buildup and blue represents regions of charge deficiency. The plots are
often used to further illustrate the charge distribution in molecules.
Example 2- C6F6
This molecule contains 6 C-F bonds, each of which are each very polar. But, due to the
flat hexagonal geometry which is highly symmetrical, the charge is evenly distributed
and the molecule is non-polar.
Hexafluoro
benzene
C6F6
Example 3- Ethane, Propane and Hexane (Hydrocarbons)
The electronegativity difference between C and H is very small (∆χ ~ 0.4). This very low
bond polarity for C-H bonds results in molecules made up of just C and H to be
considered essentially non-polar regardless of the arrangements of the atoms.
Ozone
Ozone (O3) is worthy of particular note when it comes to molecular polarity. The
structure of ozone is:
δ+
O
O
Polar
O
δ¯
Since all the oxygens have the same electronegativities, none of the bonds are formally
polar. But, because of the bent geometry and that the central oxygen must share electrons
with 2 oxygens, a charge deficiency occurs on the central oxygen. If the molecule was
linear, this would not matter as it would be analogous to CO2 in terms of charge
distribution. But, because of the bent geometry, the molecule ends up with an
asymmetric charge distribution and is polar.
Molecular Polarity and the absorption of Infrared Radiation
Whether or not a molecule will absorb light in the infrared region is
dependent on its charge distribution. The absorption of IR radiation stimulates
vibrational motions in molecules. For a molecule to absorb IR, the molecule must exhibit
vibrational modes that induce a dipole moment in the molecule. Diatomic molecules
that are non-polar with non-polar bonds do not absorb IR radiation. Examples of nonabsorbing molecules are N2, O2, and monoatomic gases like Ar and He. Non-polar
molecules can absorb if they possess a vibrational mode that generates a net dipole. As
an example, consider some of the possible stretching modes for CO2.
Both (b) and (c) stretching modes create a temporary dipole moment by disrupting the
symmetry of the molecule and are “IR active mode”. Mode (a) maintains the symmetry
(linear with both oxygens equidistant to the carbon) so it is not an “IR active mode”.
Having at least one IR active mode means that CO2 absorbs IR radiation. The fact that it
has more than one means it will absorb IR radiation of varying wavelengths.
Additional Problems
Molecular Polarity
1) Rank the following molecules from least polar to most polar
CH2Br2, CH2F2, CH2Cl2, CBr4, CF2Cl2, CF2Br2
2) There are 3 different dichloroethylenes (all with the formula C2H2Cl2). Draw all three
and determine which would be polar and which would be non-polar.
3) Dinitrogen difluoride, N2F2, is the only stable, simple inorganic compound with an
N=N bond. It exists as two different isomers (cis and trans), one which is polar and one
which is non-polar. Draw the Lewis structures of each and identify the polar and nonpolar isomers.
Greenhouse Gases
1) Describe why methane, although non-polar, is a greenhouse gas and exhibits an IR
spectrum.
2) Would you suspect CF4 to be a greenhouse gas? Do you think it would exhibit an IR
spectrum? Explain.