Molecular Polarity
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C ONCEPT
Concept 1. Molecular Polarity
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Molecular Polarity
Lesson Objectives
• The student will determine whether bonds are polar or non-polar.
• The student will determine whether simple molecules are polar or non-polar.
Introduction
Two atoms of the same electronegativity will share electrons equally in a pure covalent bond. Any molecule
that contains atoms of only one element, like H2 or F2 , has pure covalent bonding. Two atoms with different
electronegativities will have either the distorted electron distribution of a polar bond or the complete electron
transfer of an ionic bond. Table 1.1 interprets the bonding between two elements in terms of the difference in
their electronegativities.
TABLE 1.1: Electronegativity Difference and Type of Bond
Electronegativity Difference
0.0
0.5
1.0
1.5
2.0
2.5
3.0
Ionic Character (%)
Covalent Character (%)
Bond Type
0
5
20
40
60
75
90
100
95
80
60
40
25
10
Covalent
Covalent
Covalent
Polar
Polar
Ionic
Ionic
The Cause of Polar Bonds
When covalent bonds form between identical atoms such as in H2 , N2 , O2 , and so on, the electrons shared in the
bonds are shared exactly equally. The two atoms have the same electronegativity and therefore, the same pull on the
shared electrons. The center of negative charge for the entire molecule will be in the exact center of the molecule.
This will coincide with the center of positive charge for the molecule. When the center of negative charge and the
center of positive charge coincide, there is no charge separation and no dipole.
The center of positive and negative charge in a molecule. (Source: Richard Parsons. CC-BY-SA)
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If the two atoms sharing the bonding pair of electrons are not of the same element, the atom with the greater
electronegativity will pull the shared electrons closer to it. Because of the resulting uneven distribution of electrons,
the center of negative charge will not coincide with the center of positive charge and a dipole is created on the
molecule. When the centers of positive and negative charge do not coincide, a charge separation exists and a dipole
is present.
Sketch of a polar molecule showing the separation of the centers of positive and negative charge. (Source: Richard
Parsons. CC-BY-SA)
The end of the molecule with the more electronegative atom will have a partial negative charge and the end of the
molecule with the more electropositive atom will have a slight positive charge. The symbols δ+ and δ− are used
because these are not full 1+ and 1− charges. This polarity is much less of a charge separation than in an ionic bond.
These charges are only fractions of full 1+ and 1− charges. How much polarity a bond will experience depends on
the difference in the electronegativities of the atoms.
No Dipole is Present on Symmetrical Molecules
In the case of a symmetrical molecule with polar bonds, the symmetry of the electron displacements of two or more
electron pairs will keep the center of negative charge in the center of the molecule coincident with the center of
positive charge. When the centers of positive and negative charge are coincident, no dipole will occur.
The Lewis structure for carbon dioxide. (Source: Richard Parsons. CC-BY-SA)
For example, in the CO2 molecule, both carbon-oxygen bonds are polar but the shift of bonding electrons toward the
oxygen is the same on both sides of the carbon atom and the center of negative charge remains in the center.
All bonds between non-metallic atoms that are not the same atom will be polar bonds. However, having polar bonds
is not enough to guarantee than the molecule will be polar. If the molecule has those polar bonds in a symmetrical
shape, the center of negative charge does not move away from the center of positive charge, and the molecule will not
be polar. For a particular molecule to be symmetrical, all the attached atoms must be the same atom. For example,
CH4 is a symmetrical molecule but CH3Cl is not symmetrical.
TABLE 1.2: Symmetrical Molecular Shapes
Molecular Geometry
Linear
Linear
Trigonal Planar
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Example Molecule
F2
BeH2
BH3
Symmetry
Symmetrical
Symmetrical
Symmetrical
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Concept 1. Molecular Polarity
TABLE 1.2: (continued)
Molecular Geometry
Angular
Pyramidal
Tetrahedral
Trigonal Bipyramidal
Distorted Tetrahedron
T-shaped
Octahedral
Square Pyramidal
Square Planar
Example Molecule
H2 O
NH3
CH4
PCl5
SF4
ClF3
SF6
ClF5
XeF4
Symmetry
Not Symmetrical
Not Symmetrical
Symmetrical
Symmetrical
Not Symmetrical
Not Symmetrical
Symmetrical
Not Symmetrical
Symmetrical
All molecules that have polar bonds and are not symmetrical will have a dipole on the molecule.
Lesson Summary
• Covalent bonds between atoms that are not identical will produce polar bonds.
• Molecules with polar bonds and non-symmetrical shapes will have a dipole.
Review Questions
1.
2.
3.
4.
5.
What two requirements should be satisfied for a molecule to be polar?
BF3 contains polar bonds but the molecule is not polar. Why not?
Which of the following molecules will be polar: ICl5 , XeCl4 , SeCl6 ?
Which of the following molecules will be polar: PCl3 , SCl2 , SiF4 ?
Which of the following molecules will have the largest dipole moment?
a.
b.
c.
d.
e.
C2 H2
CH2Cl2
BF3
CH3CH2 OH
HF
6. Which of the following molecules will have a triple bond?
a.
b.
c.
d.
e.
C2 H2
CH2Cl2
BF3
CH3CH2 OH
HF
7. Which of the following molecules has a central atom with sp2 hydridized orbitals?
a.
b.
c.
d.
e.
C2 H2
CH2Cl2
BF3
CH3CH2 OH
HF
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Further Reading / Supplemental Links
• http://learner.org/resources/series61.html
The learner.org website allows users to view streaming videos of the Annenberg series of chemistry videos. You are
required to register before you can watch the videos but there is no charge. The website has one video that relates to
this lesson called Molecular Architecture.
• http://www.sparknotes.com/testprep/books/sat2/chemistry/section4section8.rhtml
• http://www.up.ac.za/academic/chem/mol_geom/mol_geometry.htm
• http://chemed.chem.purdue.edu/genchem/topicreview/bp/ch8/lewis.html#step
• http://en.wikipedia.org/wiki/Molecular_geometry
Vocabulary
polar bond A covalent bond in which the shared pair of electrons are not shared equally owing to a difference in
the electronegativity of the two atoms.
molecular symmetry The property of a molecule that enables it to undergo inversion through a line, a point, or a
plane, and its new state is indistinguishable from its original state.
dipole A pair of equal and opposite charges separated by a small distance; a molecular dipole is produced when
the centers of positive and negative charge do not coincide.
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