Chapter 10 Chemical Bonding II
Structure Determines Properties!
• properties of molecular substances depend on
the structure of the molecule
• the structure includes many factors, including:
the skeletal arrangement of the atoms
the kind of bonding between the atoms
ionic, polar covalent, or covalent
the shape of the molecule
• bonding theory should allow you to predict the
shapes of molecules
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Molecular Geometry
• Molecules are 3-dimensional objects
• We often describe the shape of a molecule
with terms that relate to geometric figures
• These geometric figures have characteristic
“corners” that indicate the positions of the
surrounding atoms around a central atom in
the center of the geometric figure
• The geometric figures also have characteristic
angles that we call bond angles
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Using Lewis Theory to Predict
Molecular Shapes
• Lewis theory predicts there are regions of
electrons in an atom based on placing shared
pairs of valence electrons between bonding
nuclei and unshared valence electrons located
on single nuclei
• this idea can then be extended to predict the
shapes of molecules by realizing these regions
are all negatively charged and should repel
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VSEPR Theory
• electron groups around the central atom will be
most stable when they are as far apart as
possible – we call this valence shell electron
pair repulsion theory
since electrons are negatively charged, they should
be most stable when they are separated as much as
possible
• the resulting geometric arrangement will allow
us to predict the shapes and bond angles in the
molecule
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Electron Groups
• the Lewis structure predicts the arrangement of valence
•
•
electrons around the central atom(s)
each lone pair of electrons constitutes one electron group
on a central atom
each bond constitutes one electron group on a central
atom
 regardless of whether it is single, double, or triple
••
•O
•
••
N
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••
O ••
••
there are 3 electron groups on N
1 lone pair
1 single bond
1 double bond
5
Molecular Geometries
• there are 5 basic arrangements of electron groups
around a central atom
 based on a maximum of 6 bonding electron groups
 though there may be more than 6 on very large atoms, it is very rare
• each of these 5 basic arrangements results in 5 different
basic molecular shapes
 in order for the molecular shape and bond angles to be a
“perfect” geometric figure, all the electron groups must be
bonds and all the bonds must be equivalent
• for molecules that exhibit resonance, it doesn’t matter
which resonance form you use – the molecular
geometry will be the same
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Linear Geometry
• when there are 2 electron groups around the central
•
•
atom, they will occupy positions opposite each other
around the central atom
this results in the molecule taking a linear geometry
the bond angle is 180°



Cl

Be

Tro, Chemistry: A Molecular Approach
Cl





O

C
O



7
Trigonal Geometry
• when there are 3 electron groups around the central
•
•
atom, they will occupy positions in the shape of a
triangle around the central atom
this results in the molecule taking a trigonal planar
geometry
the bond angle is 120°




F
B
F






F



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Not Quite Perfect Geometry
Because the bonds are
not identical, the
observed angles are
slightly different from
ideal.
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Tetrahedral Geometry
• when there are 4 electron groups around the central
•
•
atom, they will occupy positions in the shape of a
tetrahedron around the central atom
this results in the molecule taking a tetrahedral
geometry
the bond angle is 109.5°



F






F
C
F






F



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Methane
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Trigonal Bipyramidal Geometry
• when there are 5 electron groups around the central atom, they
•
•
•
•
•
will occupy positions in the shape of a two tetrahedra that are
base-to-base with the central atom in the center of the shared
bases
this results in the molecule taking a trigonal bipyramidal
geometry
the positions above and below the central atom are called the
axial positions
the positions in the same base plane as the central atom are
called the equatorial positions
the bond angle between equatorial positions is 120°
the bond angle between axial and equatorial positions is 90°
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Trigonal Bipyramidal Geometry
••
• Cl
• ••
••
• Cl
• ••
••
• Cl•
• • •• •
Cl •
P ••
••
Cl ••
••
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Octahedral Geometry
• when there are 6 electron groups around the central
•
atom, they will occupy positions in the shape of two
square-base pyramids that are base-to-base with the
central atom in the center of the shared bases
this results in the molecule taking an octahedral
geometry
 it is called octahedral because the geometric figure has 8
sides
• all positions are equivalent
• the bond angle is 90°
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Octahedral Geometry
••
•F
• ••
• ••
•F
••
••
•F •
• •
S
• F•
• •• •
••
F ••
••
•• •
F•
••
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The Effect of Lone Pairs
• lone pair groups “occupy more space” on the central
atom
because their electron density is exclusively on the
central atom rather than shared like bonding electron
groups
• relative sizes of repulsive force interactions is:
Lone Pair – Lone Pair > Lone Pair – Bonding Pair > Bonding Pair – Bonding Pair
• this effects the bond angles, making them smaller
than expected
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Molecules with lone pairs or different kinds of surrounding atoms
will have distorted bond angles and different bond lengths, but the
shape will be a derivative of one of the basic shapes
Derivative of Trigonal Geometry
• when there are 3 electron groups around the central
•


atom, and 1 of them is a lone pair, the resulting
shape of the molecule is called a trigonal planar bent shape
the bond angle is < 120°



O
S
O


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




O
S
O







O
S
O


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Derivatives of Tetrahedral Geometry
• when there are 4 electron groups around the central
atom, and 1 is a lone pair, the result is called a
pyramidal shape
 because it is a triangular-base pyramid with the central
atom at the apex
• when there are 4 electron groups around the central
atom, and 2 are lone pairs, the result is called a
tetrahedral-bent shape
 it is planar
 it looks similar to the trigonal planar-bent shape, except the
angles are smaller
• for both shapes, the bond angle is < 109.5°
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Bond Angle Distortion from Lone Pairs
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Tetrahedral-Bent Shape
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Derivatives of the
Trigonal Bipyramidal Geometry
• when there are 5 electron groups around the central atom, and
•
some are lone pairs, they will occupy the equatorial positions
because there is more room
when there are 5 electron groups around the central atom, and
1 is a lone pair, the result is called see-saw shape
 aka distorted tetrahedron
• when there are 5 electron groups around the central atom, and
•
•
•
2 are lone pairs, the result is called T-shaped
when there are 5 electron groups around the central atom, and
3 are lone pairs, the result is called a linear shape
the bond angles between equatorial positions is < 120°
the bond angles between axial and equatorial positions is < 90°
 linear = 180° axial-to-axial
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See-Saw Shape
••
•F•
• •
••
•• • •
•
•F
S
F
•
•
••
••
•F•
• •• •
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T-Shape
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Linear Shape
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Derivatives of the
Octahedral Geometry
• when there are 6 electron groups around the central
•
atom, and some are lone pairs, each even number lone
pair will take a position opposite the previous lone pair
when there are 6 electron groups around the central
atom, and 1 is a lone pair, the result is called a square
pyramid shape
 the bond angles between axial and equatorial positions is < 90°
• when there are 6 electron groups around the central
atom, and 2 are lone pairs, the result is called a square
planar shape
 the bond angles between equatorial positions is 90°
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Square Pyramidal Shape
••
•• •• F •• ••
•F
F•
• ••
•
Br ••
•• •• ••
F•
•F
• ••
•••
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Practice – Predict the Molecular Geometry and
Bond Angles in SiF5─
-1
••
Si Least Electronegative
•• •• F •• ••
5 Electron Groups on Si
•
•
F
F•
Si Is Central Atom
• ••
••
Si
5 Bonding Groups
Si = 4e─
••
••
•
0 Lone Pairs
•F
F
F5 = 5(7e─) = 35e─
•
• ••
─
••
(─) = 1e
total = 40e─
Shape = Trigonal Bipyramid
Bond Angles
Feq-Si-Feq = 120°
Feq-Si-Fax = 90°
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Practice – Predict the Molecular Geometry and
Bond Angles in ClO2F
Cl Least Electronegative
Cl Is Central Atom
Cl = 7e─
O2 = 2(6e─) = 12e─
F = 7e─
Total = 26e─
••
•O
•
••
••
•O•
• •
Cl
••
••
F ••
••
4 Electron Groups on Cl
3 Bonding Groups
1 Lone Pair
Shape = Trigonal Pyramidal
Bond Angles
O-Cl-O < 109.5°
O-Cl-F < 109.5°
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Representing 3-Dimensional Shapes on
a 2-Dimensional Surface
• one of the problems with drawing molecules is trying
•
•
•
•
to show their dimensionality
by convention, the central atom is put in the plane of
the paper
put as many other atoms as possible in the same plane
and indicate with a straight line
for atoms in front of the plane, use a solid wedge
for atoms behind the plane, use a hashed wedge
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••
•F
• ••
• ••
•F
••
••
•F •
• •
S
• F•
• •• •
••
F ••
••
•• •
F•
••
Tro, Chemistry: A Molecular Approach
SF6
F
F
F
S
F
F
F
32
Multiple Central Atoms
• many molecules have larger structures with many
•
•
interior atoms
we can think of them as having multiple central atoms
when this occurs, we describe the shape around each
central atom in sequence

shape around left C is tetrahedral
shape around center C is trigonal planar
shape around right O is tetrahedral-bent
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H O 
|
||  
HCCOH
|

H
33
Describing the Geometry of Methanol
Describing the Geometry of Glycine
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Practice – Predict the Molecular Geometries in
H3BO3
oxyacid, so H attached to O
B Least Electronegative
B Is Central Atom
B = 3e─
O3 = 3(6e─) = 18e─
H3 = 3(1e─) = 3e─
Total = 24e─
H
••
O
••
••
•O
•
B
H
••
O
••
34 Electron
Electron Groups
Groups on
on B
O
H
O has
B
23 Bonding Groups
20 Lone Pairs
Shape on B = Trigonal Planar
Shape on O = Tetrahedral Bent
35
Polarity of Molecules
• in order for a molecule to be polar it must
1) have polar bonds
 electronegativity difference - theory
 bond dipole moments - measured
2) have an unsymmetrical shape
 vector addition
• polarity affects the intermolecular forces of attraction
 therefore boiling points and solubilities
 like dissolves like
• nonbonding pairs affect molecular polarity, strong
pull in its direction
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Molecule Polarity
The H-Cl bond is polar. The bonding
electrons are pulled toward the Cl end of the
molecule. The net result is a polar molecule.
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Molecule Polarity
The O-C bond is polar. The bonding
electrons are pulled equally toward both O
ends of the molecule. The net result is a
nonpolar molecule.
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Molecule Polarity
The H-O bond is polar. The both sets of
bonding electrons are pulled toward the O
end of the molecule. The net result is a
polar molecule.
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Molecule Polarity
The H-N bond is polar. All the sets of
bonding electrons are pulled toward the N
end of the molecule. The net result is a
polar molecule.
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Molecular Polarity Affects
Solubility in Water
• polar molecules are attracted to
other polar molecules
• since water is a polar molecule,
other polar molecules dissolve
well in water
and ionic compounds as well
• some molecules have both polar
and nonpolar parts
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A Soap Molecule
Sodium Stearate
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Practice - Decide Whether the Following Are Polar
••
N
••
•O
•
••
Cl ••
••
Trigonal
Bent
3.0
3.0
Cl
N
••
•O
•
••
3.5
polar
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S
S
••
O ••
Trigonal
Planar
3.5 O
O
1) polar bonds, N-O
2) asymmetrical shape
••
•O•
• •
2.5
O3.5
1) polar bonds, all S-O
2) symmetrical shape
nonpolar
3.5 O
44
Problems with Lewis Theory
• Lewis theory gives good first approximations of
the bond angles in molecules, but usually cannot
be used to get the actual angle
• Lewis theory cannot write one correct structure
for many molecules where resonance is
important
• Lewis theory often does not predict the correct
magnetic behavior of molecules
e.g., O2 is paramagnetic, though the Lewis structure
predicts it is diamagnetic
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