Chapter 9: Molecular Geometry &
Bonding Theories
Molecular geometry
Lewis structures account for formulas of covalent
compounds, but do not indicate their shapes
show the number and types of bonds between
atoms
e.g., H2O, NH3, CCl4
actual molecular shapes are bent, trigonal
pyramidal, and tetrahedral, respectively
How can we predict these molecular geometries?
How to characterize the shape of a molecule?
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The overall shape of a molecule is determined by its bond
angles
e.g., consider CCl4
note that the C-Cl bond lengths
are all equal (1.78 Å)
all 6 Cl-C-Cl bond angles are
109.50
these angles are characteristic of a tetrahedron
If the overall shape of a molecule is determined by its bond
angles, how do we determine the bond angles?
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Predicting geometries: The VSEPR model
We will first consider the geometries of molecules of the
general type ABn
A is the central atom and is bonded to n atoms of type B,
e.g., CO2, NH3, SF4.
Molecules of the type ABn exhibit a limited number of
shapes
these geometries can be predicted, when A is a nonmetal,
by the Valence Shell Electron Pair Repulsion model
The VSEPR Model
Atoms are bonded to one another in molecules by the
sharing of pairs of valence shell electrons
Electron pairs or domains repel one another - they seek to
stay out of each other's way
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The "best" arrangement of a given number of e- domains
around a central atom in a molecule of type ABn is the one
which minimizes e-- e- repulsion
How do e- domains orient themselves to minimize
repulsion?
5 shapes to learn: Table 9.1
Lewis structures show two types of valence e- domains
e.g., NH3
e- domain repulsion around N is minimized when the 4 edomains about N are arranged tetrahedrally -- this is the
electron domain geometry
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e.g., how are the e- domains arranged spatially about
oxygen in H2O? Around S in SO32-?
How are the H atoms arranged about the N atom in NH3?
arrangement of the atoms in space: molecular geometry
Can we predict the molecular geometry from the e- domain
geometry? Are the two different?
e.g., NH3:
3 bonding pairs point towards vertices of a tetrahedron;
molecular geometry is trigonal pyramidal (looks like a
tetrahedron with one atom missing)
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Steps in predicting geometries using VSEPR model:
-draw
Lewis structure
-count
e- domains about central atom; arrange as to
minimize e- domain repulsion (TABLE 9.1)
-describe
molecular geometry in terms of bonding edomains (TABLE 9.2)
REMEMBER: e- domain geometry causes the
molecular geometry!
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VSEPR examples
molecules / ions which obey the octet rule (i.e., 4 or
less valence shell e- domains)
e.g., predict the e- domain and molecular geometries of
Cl2O
SO32-
CS2
H3O+
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How do multiple bonds effect VSEPR geometries?
e.g., predict the electron-domain and molecular geometries
of NO3-
Which has a greater influence on molecular geometry, lone
pairs or bonding pairs? why?
e.g., consider CH4, NH3, and H2O: all have tetrahedral edomain geometries, yet their bond angles differ slightly :
What is the effect of multiple bonding on observed
molecular geometries?
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e.g., H2CO: why aren't the observed H-C-H and H-C-O bond
angles 120o in this compound?
Geometries of molecules with expanded valence shells
central atom from period 3 & beyond: may be surrounded
by > 4 e- pairs (common cases: 5 or 6 e- pairs); why is this
so?
geometries of molecules with 5 or 6 e- domains around
central atom are found in table 9.3
Central atom with 5 valence e- domains: preferred
electron-pair geometry is trigonal bipyramid
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The axial & equatorial positions in a trigonal bipyramid are
nonequivalent
If a molecule has 5 e- domains about its central atom, one
of which is nonbonding, is the lone pair axial or
equatorial?
In order to best minimize e--e- repulsion, lone pairs are
placed in equatorial positions; why?
This gives seesaw, T-shaped, or linear molecular
geometries for < 5 bonding pairs
e.g., find the electron pair and molecular geometries of SF4
and XeF2.
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Six e- pairs about the central atom: preferred geometry is
octahedral, i.e.,
six e- pairs: all positions equivalent
e.g., predict the molecular geometries of ClF3 and ICl4-
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How to predict the geometry of molecules with no single
central atom?
e.g., CH3COOH (acetic acid):
H
H
O
C
C
O
H
H
For a given atom:
count e- domains as before
find e- domain geometry
find molecular geometry
e.g., find the H-C-H, C-C-O, and C-O-H bond angles in
acetic acid.
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Problems du Jour
Predict the molecular geometry of
AsF3
I3-
OCN-
Give approximate values for the indicated bond angles in:
H
O
Cl
O
O
H
H
C
C
N
H
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