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New Jersey Center for Teaching and Learning
AP Chemistry
Progressive Science Initiative
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Unit 4: Presentation C
Molecular Shapes and
Dipole Moments
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Molecular Shape and VSEPR Theory
Chemical Bonding
The molecular shape is determined by the number of bonded
and unbonded pairs of electrons around an atom.
Water's bent shape gives rise to
its properties such as having an
unusually high boiling point for
a molecule so small.
A molecule will arrange its atoms and bonds to minimize
the repulsions between electron pairs.
104.5
In methane (CH4), the
molecule adopts a 3D
tetrahedral arrangement to
minimize repulsions.
In water (H2O), the un-bonded
electrons repel more strongly than
the bonded electrons resulting in a
bent configuration with a reduced
bond angle compared to methane.
***Note: Un-bonded electrons repel more strongly as they have no nuclei to reduce
or shield their charge.
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Molecular Shape and VSEPR Theory
The "AB" system can be used to determine the molecular shape by
tracking how many atoms and unbonded pairs are attached to an atom.
Molecular Shape and VSEPR Theory
Basic Molecular Shapes
# of unbonded
"AB"
pairs of
Designation electrons on "A"
atom
AB2
O
AB2
AB4
O
S
?
AB3
AB4
O
Shape
Bond
Angles
0
Linear
180
0
trigonal
planar
120
0
tetrahedral
109.5
AB5
0
trigonal 90, 120,
bypyramidal 180
AB6
0
octahedral 90, 180
Example
**Note: Pi bonds act with the sigma bonds to contribute to the repulsions that result in the
molecular shape, however they do not act as a separate constituent around the "A" atom.
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Molecular Shape and VSEPR Theory
Un-bonded pairs contribute to the shape of the molecule as well,
most often shrinking the bond angles as they repel more strongly
than bonded pairs.
AB5 molecules
PF5
SF4
trigonal bipyramidal
see saw
IF3
I3-
T-shape
linear
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Molecular Shape and VSEPR Theory
Unbonded pairs contribute to the shape of the molecule as well,
most often shrinking the bond angles as they repel more strongly
than bonded pairs.
AB6 molecules
1 Which of the following would have a see-saw shape?
A I only
B II only
C III only
I. XeO2F2
II. IBr3
III. SeH2
D I and II only
SF6
XeCl4
ClF5
octahedral
square planar
square pyramidal
E II and III only
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1 Which of the following would have a see-saw shape?
A I only
I. XeO2F2
D I and II only
B II only
III. SeH2
Answer
C III only
2 Which of the following is ranked properly from
largest to smallest bond angles within the molecule?
A I only
II. IBr3
B II only
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A
C III only
D II and III only
E II and III only
E I, II, and III
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I. CH4, PCl3, SF5
II. XeF2, H2O, XeF4
III. NO3-, NO2-, CH4
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2 Which of the following is ranked properly from
largest to smallest bond angles within the molecule?
II. XeF2, H2O, XeF4
B II only
III. NO , NO , CH4
C III only
D II and III only
Answer
3
3 Which of the following does NOT have a bent shape?
A BeCl2
I. CH4, PCl3, SF5
A I only
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2
E
B SeH2
C SCl2
D OH2
E All have a bent shape
E I, II, and III
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3 Which of the following does NOT have a bent shape?
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4 Which of the following has a planar shape?
A C2H4
B SeH2
B PH3
C SCl2
D OH2
Answer
A BeCl2
C SiH4
A
E All have a bent shape
D PF5
E SeCl6
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4 Which of the following has a planar shape?
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5 Which of the following contribute to the shape of
the molecule?
A Only the number of bonded e- pairs around atom
B PH3
B Only the number of un-bonded and bonded e- pairs
around the atom
C SiH4
D PF5
Answer
A C2H4
A
C Only the atomic radii of atoms
D Only the atomic radii and bonded e- pairs around
the atom
E SeCl6
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E None of these are correct
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5 Which of the following contribute to the shape of
the molecule?
A Only the number of bonded e- pairs around atom
Answer
B Only the number of un-bonded and bonded e- pairs
around the atom
C Only the atomic radii of atoms
B
D Only the atomic radii and bonded e- pairs around
the atom
E None of these are correct
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6 Which of the following is TRUE regarding the effect
of substitution of un-bonded pairs of electrons in
place of bonded pairs of electrons on the molecular
shape?
A The bond angle increases due to the decreased
repulsions
B The bond angle decreases due to the decreased
repulsions
C The bond angle decreases due to the increased
repulsions
D The bond angle increased due to the increased
repulsions
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6 Which of the following is TRUE regarding the effect
of substitution of un-bonded pairs of electrons in
place of bonded pairs of electrons on the molecular
shape?
A The bond angle increases due to the decreased
repulsions
Answer
B The bond angle decreases due to the decreased
C
repulsions
C The bond angle decreases due to the increased
repulsions
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7 Which of the following would be the correct shape of
the BF3 molecule?
A bent
B trigonal planar
C trigonal pyramidal
D see-saw
E trigonal bipyramidal
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D The bond angle increased due to the increased
repulsions
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7 Which of the following would be the correct shape of
the BF3 molecule?
B trigonal planar
Answer
A bent
C trigonal pyramidal
B
D see-saw
E trigonal bipyramidal
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Molecular Polarity
Molecules in which the electrons are not evenly distributed
experience a dipole moment when in an electric field and are
said to be polar.
Two factors contribute to the polarity of a molecule:
Polarity of bonds
Polar bonds are necessary for a molecule to be polar
but do not guarantee polarity.
Symmetry
To be polar a molecule must be asymmetrical to ensure
an uneven distribution of electrons.
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Molecular Polarity
Molecular Polarity
Certain shapes are asymmetrical in nature due to unbonded
electrons and can, therefore, result in polar molecules.
Asymmetrical Shapes
If the molecule is asymmetrical, the more polar the bonds, the
more polar the molecule
H2O
Bent, Trigonal pyramidal
H2S
S
T-Shape, See-saw, Square pyramidal
Asymmetry and polarity can also
result from a heterogenous group of
atoms attached to the central atom
thereby creating asymmetrical
differences in electronegativity.
CHCl3 is a classic example.
H
+
Dipole moment = 1.85 D
-
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H
Dipole moment = 0.97 D
Both molecules are asymmetrical and exhibit a dipole
moment but water's dipole moment is significantly
higher than hydrogen sulfide due to the greater
electronegativity difference between O and H.
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Molecular Polarity
Molecular Polarity
Polarity influences solubility. Polar solutes are more soluble/
miscible in polar solvents and non-polar solutes are more
soluble in non-polar solvents.
Polarity influences solubility. Polar solutes are more soluble/
miscible in polar solvents and non-polar solutes are more
soluble in non-polar solvents.
Since water is a polar molecule, it is an excellent solvent for polar
solutes such as ethanol (C2H5OH), soluble ionic salts (NaCl,
K3PO4, etc), or polar gases like ammonia (NH3).
Many pigments used to create color in paints are non-polar and
require a non-polar solvent such as hexane (C6H14) to dissolve.
Many molecules have non-polar and polar regions and require
an "amphipathic" solvent that also shares those qualities.
Acetone (CH3COCH3) is an excellent choice for these
applications.
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9 Which of the following molecules would have the
highest dipole moment?
A NH3
A NH3
B PH3
B PH3
C BF3
C BF3
D CH4
D CH4
E OF2
E OF2
Answer
9 Which of the following molecules would have the
highest dipole moment?
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A
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10 Which of the following is TRUE regarding molecular
polarity?
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10 Which of the following is TRUE regarding molecular
polarity?
A Polar molecules must be asymmetrical and have
polar bonds
B Polar molecules must have polar bonds and can be
symmetrical or asymmetrical
B Polar molecules must have polar bonds and can be
symmetrical or asymmetrical
C Polar molecules cannot have polar bonds
C Polar molecules cannot have polar bonds
D Polar molecules do not require polar bonds but do
require asymmetry
D Polar molecules do not require polar bonds but do
require asymmetry
E Polar molecules do not require polar bonds or
asymmetry
E Polar molecules do not require polar bonds or
asymmetry
Answer
A Polar molecules must be asymmetrical and have
polar bonds
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11 Which of the following would be LEAST soluble in
benzene (C6H6)?
A C2H2
A C2H2
B SiH4
B SiH4
C I2
C I2
D CH2F2
D CH2F2
E CO2
E CO2
Answer
11 Which of the following would be LEAST soluble in
benzene (C6H6)?
A
D
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12 Which of the following explains the low dipole
moment of CCl4 (0.0 D)?
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12 Which of the following explains the low dipole
moment of CCl4 (0.0 D)?
A There is a small electronegativity difference
between C and Cl
B The bonds are non-polar
B The bonds are non-polar
C The molecule is symmetrical
C The molecule is symmetrical
D The atoms in the molecule are small
D The atoms in the molecule are small
E The IR spectra for the molecule has few peaks
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a pull tab]
E The IR spectra for the molecule
hasis few
peaks
Answer
A There is a small electronegativity difference
between C and Cl
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13 A symmetrical molecule can be polar so long as it
has polar bonds.
C
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13 A symmetrical molecule can be polar so long as it
has polar bonds.
True
False
False
Answer
True
False
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14 Which of the following molecules would be expected
to have the highest dipole moment?
A C8H18
A C8H18
B BeCl2
B BeCl2
C CO2
C CO2
D HCN
D HCN
E H2S
E H2S
Answer
14 Which of the following molecules would be expected
to have the highest dipole moment?
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D
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Application
Fat soluble vitamins dissolve best in non-polar solvents such as
bile salts released from the liver. The structure for vitamin K is
below. Do you think it is a fat soluble or water soluble vitamin?
Fat soluble vitamins dissolve best in non-polar solvents such as
bile salts released from the liver. The structure for vitamin K is
below. Do you think it is a fat soluble or water soluble vitamin?
Answer
Application
Vitamin K is mostly nonpolar,
so it is fat soluble
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In the next section we will examine the forces
that exist between these molecules and how the
strength and types of these forces influence the
properties of these molecules....see you there!