Polar Molecules
Chemists believe that molecules are made up of charged particles (electrons and nuclei).
A polar molecule is one in which the charge is not distributed symmetrically among
the atoms making up the molecule. The existence of polar molecules can be demonstrated by running a stream of any liquid past a charged rod (Figure 1). When repeated
with a large number of pure liquids, this experiment produces a set of empirical rules for
predicting whether a molecule is polar or not (Table 1).
(a)
(b)
– +
– +– +
– +– +
– +– +
– +– +
– +– +
– +– +
– +– +
– +– +
– +– +
– +– +
– + – +
– + – +
– + – +
+
–
– +
–+ – –
–
random
+ + +
– + + –
orientation
+ ++–
– –+ –
+
+
– –
– –+ –
+
buret
oriented polar
molecule
liquid
stream
negatively charged vinyl strip
4.4
INVESTIGATION 4.4.1
Testing for Polar Molecules
(p. 277)
Test the rules for polar and nonpolar
molecules presented in Table 1
using the effect of electric charges
on a liquid.
Figure 1
(a) Testing a liquid with a charged
strip provides evidence for the
existence of polar molecules in
a substance.
(b) In a liquid, molecules are able
to move in a limited way. Polar
molecules in a liquid become
oriented so that their positive
poles are closer to a negatively
charged material. Near a positively charged material they
become oriented in the opposite direction. Polar molecules
are thus attracted by either
kind of charge.
Table 1 Empirical Rules for Polar and Nonpolar Molecules
Type
Polar
Description of molecule
?
DID YOU
Evidence for Polar Bonds
The energy required to break a
bond can be determined experimentally. The energy required to
break the HF bond is considerably greater than either HH or
FF bonds. Pauling realized that
an unequal sharing of the electron
pair produced a polar bond that
enhanced the bonding between
the atoms.
AB
diatomic with different atoms
HCl (g), CO(g)
NxA y
containing nitrogen and other atoms
NH3(g), NF3(g)
OxAy
containing oxygen and other atoms
H2O(l), OCl2(g)
CxA yBz
containing carbon and two other kinds of atoms
CHCl3(l),
Ax
all elements
Cl2(g), N2(g)
CxA y
containing carbon and only one other kind of atom
CO2(g), CH4(g)
C2H5OH(l)
Nonpolar
KNOW
Examples
Electronegativity and Polarity of Bonds
Linus Pauling realized that while two atoms can share one or more pairs of electrons, there
is nothing that requires them to share those electron pairs equally. He saw the need for
a theory to explain and predict the polarity of molecules and so combined properties such
as bond energies with valence bond theory to create a new property of atoms called
NEL
Chemical Bonding
251
polar bond a polar bond results
from a difference in electronegativity
between the bonding atoms; one
end of the bond is, at least partially,
positive and the other end is equally
negative
Figure 2
Electronegativity of the elements
increases as you move up the periodic table and to the right. Fluorine
has the highest electronegativity of
all atoms.
nonpolar bond a nonpolar bond
results from a zero difference in
electronegativity between the
bonded atoms; a covalent bond with
equal sharing of bonding electrons
ionic bond a bond in which the
bonding pair of electrons is mostly
with one atom/ion
Figure 3
Since the transfer of an electron in a
polar covalent bond is not complete,
only a partially negative (d) and a
partially positive (d) charge
appear on the bond.
electronegativity. Because Pauling’s calculations produced differences in the attraction
of nuclei for a shared pair of electrons in a covalent bond, he arbitrarily assigned values
so that fluorine, the most electronegative atom, had a value of about 4.0. All other atoms
have a lower electronegativity and, therefore, a lower attraction for electrons when
bonded. Electronegativity increases when moving to the right and up the periodic table
toward fluorine (Figure 2).
Pauling used the difference in electronegativity
(i.e., the difference in attraction for the
increase
pair of electrons in a bond) to explain the
F
polarity of a chemical bond. The greater the difference in electronegativity, the more polar the
increase
increase
bond. The smaller the difference, the more nonpolar the bond. A very polar bond is an ionic
bond. A nonpolar bond is a covalent bond. A
somewhat polar bond is a polar covalent bond.
According to Pauling, a polar covalent bond (or simply a polar bond) results when two
different kinds of atoms (usually nonmetals) form a bond. The bond is covalent because
the electrons are being shared. The bond is polar covalent because the sharing of electrons is unequal. This means that the electrons spend more of their time closer to one
atomic nucleus than the other. The end of the bond where the negatively charged electrons
spend more time is labelled as being partially negative (d). The end of the bond that
is partially positive is labelled d (Figure 3).
Pauling liked to think of chemical bonds as being
2.1
3.0
H Cl
different in degree rather than different in kind.
According to him, all chemical bonds involved a sharing
of electrons, even ionic bonds (Figure 4). The degree
of sharing depends upon the difference in electronegativities of the bonded atoms.
δ+
δ–
covalent bond or nonpolar bond
a bond in which the bonding electrons are shared equally between
atoms
A general rule is that when the difference in electronegativity exceeds 1.7, the percent ionic character exceeds 50%.
polar covalent bond a bond in
which electrons are shared somewhat unequally
percent ionic character
100
50
ionic
Figure 4
The greater the difference in electronegativity between bonding
atoms, the greater the percent ionic
character of the bond.
252 Chapter 4
3.3
0
polar covalent
1.7
electronegativity difference
covalent
0
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Section 4.4
Classifying Bonds
SAMPLE problem
Label the following atoms and bonds with electronegativity and bond polarity, and
classify the bond:
(a) HH
(b) PCl
(c) NaBr
From the periodic table, assign each atom an electronegativity.
(a) H H
2.1 2.1
The electronegativity difference is 0.0, indicating a nonpolar covalent bond.
(b) P Cl
2.1 3.0
The electronegativity difference is 0.9, indicating a polar covalent bond.
(c) Na Br
0.9
2.8
The electronegativity difference is 1.9, indicating an ionic bond.
DID YOU
KNOW
?
Poles and Polar
In general, the term “pole” refers
to one or the other of two opposite ends of something; e.g., North
and South Poles of Earth or a
magnet, or the positive and negative charges on two ends of an
object such as a molecule. When
we say something is polar we
mean it has two opposite ends.
Practice
Understanding Concepts
1. Draw the following bonds, label the electronegativities, and label the charges (if
any) on the ends of the bond. Classify the bond as ionic, polar covalent, or nonpolar covalent:
(a) HCl
(b) CH
(c) NO
(d) IBr
(e) MgS
(f) PH
2. Using electronegativity as a guide, classify the following bonds as ionic, polar
covalent, or nonpolar covalent:
(a) the bond in HBr(g)
(b) the bond in LiF(s)
(c) the C—C bond in propane, C3H8(g)
3. List and order the bonds in the following substances according to increasing bond
polarity. Provide your reasoning.
(a) H2O(l), H2(g), CH4(g), HF(g), NH3(g), LiH(s), BeH2(s)
(b) PCl3(l), LiI(s), I2(s), ICl(s), RbF(s), AlCl3(s)
(c) CH3OH(l)
(d) CHFCl2(g)
Applying Inquiry Skills
4. Values for the Pauling electronegativities have changed over time. Why would
these values change? Are the new values the “true” values?
Extension
5. There are other electronegativity scales created by different chemists. Research
and compare the scales created by Pauling, Mulliken, and Allred-Rochow. For
example, what properties did they use when calculating their electronegativity
scales?
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Chemical Bonding
253
O
C
O
Figure 5
The central carbon atom has no
lone pairs and two groups or sets of
electrons (remember that multiple
bonds count as one group of electrons). The least repulsion of two
groups of electrons is a linear
arrangement.
bond dipole the electronegativity
difference of two bonded atoms
represented by an arrow pointing
from the lower (d) to the higher
(d) electronegativity
nonpolar molecule a molecule that
has either nonpolar bonds or polar
bonds whose bond dipoles cancel
to zero
polar molecule a molecule that has
polar bonds with dipoles that do not
cancel to zero
δ+
H
C
δ+
δ–
H
H
H
δ+
Polar Molecules
The existence of polar bonds in a molecule does not necessarily mean that you have a polar
molecule. For example, carbon dioxide is considered to be a nonpolar molecule, although
each of the CO bonds is a polar bond. To resolve this apparent contradiction, we need
to look at this molecule more closely. Based on the Lewis structure and the rules of
VSEPR, carbon dioxide is a linear molecule (Figure 5). Using electronegativities, we can
predict the polarity of each of the bonds. It is customary to show the
δ–
δ+
δ–
bond polarity as an arrow, pointing from the positive (d ) to the O C O
3.5
2.5
3.5
negative (d) end of the bond. This arrow represents the bond dipole.
These arrows are vectors and when added together produce a zero total. In other
words, the bond dipoles cancel to produce no polarity for the complete molecule, or a
nonpolar molecule.
Let’s try this procedure again with another small molecule. As you
know, water is a polar substance and the OH bonds in water are
O
polar bonds. The Lewis structure and VSEPR rules predict a V-shaped H
H
molecule, shown here with its bond dipoles.
In this case, the bond dipoles (vectors) do not cancel. Instead, they add together to produce a non-zero molecular dipole (shown in red). The water molecule has an overall
polarity and that it is why it is a polar molecule. Note that the water molecule has a
partially negative end near the oxygen atom and a partially positive end at the two
hydrogen atoms. Although the “ends” of the water molecule are not initially obvious,
the V shape produces two oppositely charged regions on the outside of the molecule. This
explains why a stream of water is attracted to a positively charged strip or rod.
From the two examples, carbon dioxide and water, you can see that the shape of the
molecule is as important as the bond polarity.
Both the shape of the molecule and the polarity of the
bonds are necessary to determine if a molecule is polar or
nonpolar.
δ+
Figure 6
Notice how all of the bond dipoles
point into the central carbon atom.
There are no positive and negative
ends on the outer part of the
methane molecule.
Methane is a nonpolar substance and its CH bonds are polar. Does the same explanation we used for carbon dioxide apply to methane? The shape diagram with bond
dipoles (Figure 6) shows that the outer part of the molecule is uniformly positive and
therefore the molecule has no ends that are charged differently. A nearby molecule would
“see” the same charge from all sides of the methane molecule. This is true because the
CH4 molecule is symmetrical. In fact, all symmetrical molecules, such as CCl4 and BF3,
are nonpolar for the same reason.
In all symmetrical molecules, the sum of the bond dipoles is
zero and the molecule is nonpolar.
The theory created by combining the concepts of covalent bonds, electronegativity, bond
polarity, and VSEPR logically and consistently explains the polar or nonpolar nature of
molecules. We are now ready to put this combination of concepts to a further test—to
predict the polarity of a molecule.
254 Chapter 4
NEL
Section 4.4
Predicting the Polarity of a Molecule
SAMPLE problem
Predict the polarity of the ammonia, NH3 , molecule, including your reasoning.
First, draw the Lewis structure.
H N H
H
LEARNING
Based on the Lewis structure, draw the shape diagram.
N
H
H
H
Add the electronegativities of the atoms, from the periodic table, and assign d and d
to the bonds.
N
δ+
δ–
3.0
H
2.1
H
H
δ+
2.1
δ+
2.1
Draw in the bond dipoles.
N
δ+
H
2.1
H
δ+
H
Vectors and Vector Addition
A vector quantity is a quantity that
has a size and a direction and is
often represented by a vector,
which is an arrow pointing in a
particular direction. When we want
to add vector quantities, we usually add the vectors (arrows) “head
to tail” to obtain the final
(resultant) vector. Bond dipoles
are vector quantities because they
have a size (difference in electronegativities) and a direction,
defined as pointing toward the
negative end of the bond.
head-to-tail addition
(resultant vectors in red)
δ–
3.0
TIP
δ+
2.1
2.1
The ammonia molecule is polar because it has polar bonds that do not cancel to zero.
The electron pairs are in a tetrahedral arrangement, but one of these pairs is a lone pair
and three are bonding pairs. Therefore, the bond dipoles do not cancel.
resultant
resultant: none
SUMMARY
Theoretical Prediction of
Molecular Polarity
To use molecular shape and bond polarity to determine the polarity of a
molecule, complete these steps.
resultant
Step 1 Draw a Lewis structure for the molecule.
Step 2 Use the number of electron pairs and VSEPR rules to determine the
shape around each central atom.
Step 3 Use electronegativities to determine the polarity of each bond.
Step 4 Add the bond dipole vectors to determine if the final result is zero
(nonpolar molecule) or nonzero (polar molecule).
Practice
Understanding Concepts
6. Predict the shape of the following molecules. Provide Lewis and shape structures.
(a) silicon tetrabromide, SiBr4(l)
(b) nitrogen trichloride, NCl3(l)
(c) beryllium fluoride, BeF2(s)
(d) sulfur dichloride, SCl2(l)
LEARNING
TIP
3-D Vector Addition
Adding vectors in three dimensions follows the same rules but it
is mathematically more difficult.
For symmetrical molecules, like
CH4, it is much easier to use the
symmetry of the molecule to reach
the conclusion that the molecule is
nonpolar.
7. Predict the bond polarity for the following bonds. Use a diagram that includes the
partial negative and positive charges and direction of the bond dipole.
(a) CN in hydrogen cyanide
(c) PS in P(SCN)3(s)
(b) NO in nitrogen dioxide
(d) CC in C8H18(l)
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Chemical Bonding
255
8. Predict the polarity of the following molecules. Include a shape diagram, bond
dipoles, and the final resultant dipole (if nonzero) of the molecule.
(a) boron trifluoride, BF3(g)
(c) carbon tetraiodide, CI4(s)
(b) oxygen difluoride, OF2(g)
(d) phosphorus trichloride, PCl3(l)
9. Use the empirical rules from Table 1 to predict the polarity of an octane, C8H18(l),
molecule. Explain your answer without drawing the molecule.
10. Why is N2H4(l) nonpolar?
Applying Inquiry Skills
11. Predict the polarity of hydrogen sulfide, H2S(g), a toxic gas with a rotten-egg odour.
Design an experiment to test your prediction.
Section 4.4 Questions
Understanding Concepts
1. Scientific concepts are tested by their ability to explain cur-
rent observations and predict future observations. To this
end, explain why the following molecules are polar or nonpolar, as indicated by the results of the diagnostic tests.
(a) beryllium bromide, BeBr2(s); nonpolar
(b) nitrogen trifluoride, NF3(g); polar
(c) methanol, CH3OH(l); polar
(d) hydrogen peroxide, H2O2(l); nonpolar
(e) ethylene glycol, C2H4(OH)2(l); nonpolar
2. Predict the polarity of the following molecules. Include
shape diagrams and bond dipoles in your reasoning for
your prediction.
(a) dichlorofluoroethane, CHFCl2(g); a refrigerant (a CFC)
(b) ethene, C2H4(g); monomer of polyethylene
(c) chloroethane, C2H5Cl(g)
(d) methylamine, CH3NH2(g)
(e) ethanol, C2H5OH(l); beverage alcohol
(f) diboron tetrafluoride, B2F4(g)
3. Polar substances are used in a capacitor—a device for
storing electrical energy. For example, a capacitor may store
enough electrical energy to allow you to change the battery
in your calculator without losing what you have stored in
the memory.
256 Chapter 4
(a) Based upon polarity alone, which of water or pentane,
C5H12(l), is a good candidate for use in a capacitor?
Provide your reasoning.
(b) What are some other considerations for choosing the
liquid inside a capacitor?
Applying Inquiry Skills
4. Geometric isomers are substances with the same molecular
formula but a different molecular geometry. One type that
you have seen is the cis (same side) and trans (diagonally
opposite) forms of a substituted alkene; for example, cis1,2-dichloroethene and trans-1,2-dichloroethene, with the
same formula, C2H2Cl2(l).
Predict the polarity of each molecule, including your
reasoning. Design an experiment to distinguish these two
isomers.
Making Connections
5. Various consumer products and books exist to help people
remove stains from clothing, carpets, etc. Discuss how a
knowledge of polar and nonpolar substances is related to
the removal of stains.
NEL