Section 8.13
Molecular Structures: The VSEPR Model
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Section 8.13
Molecular Structures: The VSEPR Model
Objectives
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Chapter 8
Questions to Consider
• What is meant by the term “chemical bond”?
• Why do atoms bond with each other to form
compounds?
• How do atoms bond with each other to form
compounds?
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Section 8.13
Molecular Structures: The VSEPR Model
Objectives
1. To understand the VSEPR theory model
2. To learn to predict electronic geometries from the
number of regions of high e- density
1. Download VSEPR Charts Here
3. To understand electronic structure and bond angles
OK State summary page
http://intro.chem.okstate.edu/1314F97/Chapter9/VS
EPR.html
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Section 8.13
Molecular Structures: The VSEPR Model
A. The VSEPR Model
• Valence shell electron pair repulsion (VSEPR) model
– Each pair of e-s in a valence shell are significant.
– e- pairs about the central atom repel each other.
– They arrange to minimize repulsion forces.
– Electronic geometry is determined by the number of
e- pairs (RHED) about a central atom.
– Region of high e- density – (RHED)
any e- pair or bond I, II, or III
– Central atom – any atom bonded to > 1 other atom
– Bonded atom – any atom bonded to a central atom
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Section 8.13
Molecular Structures: The VSEPR Model
A. Molecular Structure
• Linear structure – atoms in a line
– Carbon dioxide CO2
– OCTET?
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Section 8.13
Molecular Structures: The VSEPR Model
A. Molecular Structure
• Trigonal planar – atoms in a triangle
– Boron trifluoride BF3
– OCTET?
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Section 8.13
Molecular Structures: The VSEPR Model
A. Molecular Structure
• Tetrahedral structure
– methane CH4
– OCTET?
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Section 8.13
Molecular Structures: The VSEPR Model
A. Molecular Structure
• Trigonal bipyramidal
– PF5
– OCTET?
– 120o equatorial – 90o axial trigonal bipyramidal
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Section 8.13
Molecular Structures: The VSEPR Model
A. Molecular Structure
• Octahedral
– SF6
OK State Website
– OCTET?
– All 90o
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Section 8.13
Molecular Structures: The VSEPR Model
B. The VSEPR Model
• Rules for using the VSEPR model to predict the egeometry:
1. Draw the Lewis-Dot
• Bonded atoms follow octet
• Central atom doesn’t have to (exceptions)
• Any extra e-s get paired around central atom
2. Count the regions of high e-density (RHED)
(e- pair OR bond I, II, III) about the Central Atom
3. Refer to Table to determine e- geometry
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Section 8.13
Molecular Structures: The VSEPR Model
B. The VSEPR Model
BeCl2
• Two Bonds (2 RHED)
– linear
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Section 8.13
Molecular Structures: The VSEPR Model
B. The VSEPR Model
BF3
• Three Bonds (3 RHED)
– trigonal planar
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Section 8.13
Molecular Structures: The VSEPR Model
B. The VSEPR Model
CCl4
• Four Bonds (4 RHEDS)
– tetrahedral
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Section 8.13
Molecular Structures: The VSEPR Model
B. The VSEPR Model
PF5
• Five Bonds (5 RHEDS)
• Trigonal bipyramidal
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Section 8.13
Molecular Structures: The VSEPR Model
B. The VSEPR Model
SF6
• 6 Bonds (6 RHEDS)
– All 90o –octahedral
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Section 8.13
Molecular Structures: The VSEPR Model
B. The 5 types of electronic geometries
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Section 8.13
Molecular Structures: The VSEPR Model
B. The VSEPR Model Practice
• Draw the Lewis Dot Structure and predict the
Electronic Geometry of the following:
• BeCl2
• CH3F
• SF6
• BF3
• PCl5
• Electronic geometries are 1 of 5 options
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Section 8.13
Molecular Structures: The VSEPR Model
B. The VSEPR Model Practice
•
•
•
•
•
•
Electronic Geometry:
BeCl2 Linear
CH3F Tetrahedral
SF6
Octahedral
BF3
Trigonal Planar
PCl5
Trigonal Bipyramidal
• Electronic geometries are 1 of 5 options
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Section 8.13
Molecular Structures: The VSEPR Model
A. Molecular Structure
• Draw the Lewis Dot and Predict the
Electronic Geometry of the following:
•
•
•
•
•
•
•
H2O
NO2SO3-2
XeF2
ClF3
BrF5
IF4-
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Section 8.13
Molecular Structures: The VSEPR Model
A. Molecular Structure
• Electronic Geometry:
•
•
•
•
•
•
•
H2O
NO2SO3-2
XeF2
ClF3
BrF5
IF4-
Another Example
Tetrahedral
Trigonal Planar
Tetrahedral
Trigonal Bipyramidal
Trigonal Bipyramidal
Octahedral
Octahedral
CH4
BF3
CCl4
PCl5
SF6
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Section 8.13
Molecular Structures: The VSEPR Model
Objectives Review
1. To understand the VSEPR theory model
2. To learn to predict electronic geometries from the
number of regions of high e- density
3. To understand electronic structure and bond angles
4. Work Session:
1. How does drawing a Lewis Dot help in determining a
molecule’s shape?
2. If double bonds contain 4 e-s instead of 2, why
should they be treated the same as single bonds
when determining the molecular structure?
3. Predict the e- geometry of: SO3-2, HCN, Cl2O, PCl3
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Section 8.13
Molecular Structures: The VSEPR Model
Objectives
•
•
•
To learn to predict MOLECULAR geometries from the
number of paired and unpaired e-s
To predict the polarity of a molecule
To understand the effect of an unshared pair on bond
angles
OK State summary page
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Section 8.13
Molecular Structures: The VSEPR Model
A. Molecular Structure
• The molecular geometry is a three dimensional
arrangement of the atoms in a molecule. What do they
actually look like?
– Water - bent
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Section 8.13
Molecular Structures: The VSEPR Model
A. Molecular Structure
• How do we predict the Molecular
Geometry?
• Same as Electronic Geometry, only more
options…
• 1. Draw the Lewis Dot
• 2. Count the RHEDS
• 3. Determine the General Formula (ABxUy)
• 4. Refer to Table
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Section 8.13
Molecular Structures: The VSEPR Model
A. Molecular Structure
• AB2U
• Trig Planar
• Bent
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Section 8.13
Molecular Structures: The VSEPR Model
A. Molecular Structure
• AB3U
• Tetrahedral
• Trigonal Pyramidal
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Section 8.13
Molecular Structures: The VSEPR Model
A. Molecular Structure
• AB2U2
• Tetrahedral
• Bent
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Section 8.13
Molecular Structures: The VSEPR Model
A. Molecular Structure
• AB4U
• Trigonal Bipyramidal
• Seesaw
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Section 8.13
Molecular Structures: The VSEPR Model
A. Molecular Structure
• AB3U2
• Trigonal Bipyramidal
• T - shaped
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Section 8.13
Molecular Structures: The VSEPR Model
A. Molecular Structure
• AB2U3
• Trigonal Bipyramidal
• Linear
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Section 8.13
Molecular Structures: The VSEPR Model
A. Molecular Structure
• AB5U
• Octahedral
• Square Pyramidal
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Section 8.13
Molecular Structures: The VSEPR Model
A. Molecular Structure
• AB4U2
• Octahedral
• Square Planar
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Section 8.13
Molecular Structures: The VSEPR Model
A. Molecular Geometries
Seesaw
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Section 8.13
Molecular Structures: The VSEPR Model
A. Molecular Structure
• Draw the Lewis Dot and Predict the ELECTRONIC and
MOLECULAR Geometry of the following:
•
•
•
•
•
•
•
•
H2O Find the General Formula ABxUy
I 3O3
ICl3
NH3
IF5
XeF4
SF4
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Section 8.13
Molecular Structures: The VSEPR Model
A. Molecular Structure
• ELECTRONIC and MOLECULAR Geometry
•
•
•
•
•
•
•
H2O
I 3O3
ICl3
NH3
IF5
XeF4
SF4
Tetrahedral
Trig Bipyramidal
Trig Planar
Trig Bipyramidal
Tetrahedral
Octa
Octa
Trig Bipyramidal
Angular
Linear
Angular
T-shaped
Trig Pyramid
Squ Pyramid
Squ Planar
See-Saw
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Section 8.13
Molecular Structures: The VSEPR Model
A. Molecular Structures
If y=0 in ABxUy, then molecular geom = e- geom
Trigonal Planar
Bent
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Section 8.13
Molecular Structures: The VSEPR Model
A. Molecular Structure
• Draw the Lewis Dot and predict both the
ELECTRONIC and MOLECULAR Geometry of
the following: Show the dipole moment…
•
•
•
•
•
•
•
BeF2
SO2
BBr3
NH3
CHCl3
BrF5
XeF2
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Section 8.13
Molecular Structures: The VSEPR Model
A. Molecular Structure
• Draw the Lewis Dot and predict both the
ELECTRONIC and MOLECULAR Geometry of
the following:
•
•
•
•
•
•
•
SF4
ClF3
XeF4
CH2ClBr
SbCl5
SeF6
BI3
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Section 8.13
Molecular Structures: The VSEPR Model
B. Molecular Structure - Polarity
• Consider HF and electronegativities:
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Section 8.13
Molecular Structures: The VSEPR Model
The Electron Probability Distribution in the Bonding Molecular
Orbital of the HF Molecule
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Section 8.13
Molecular Structures: The VSEPR Model
B. Molecular Structure - Polarity
• We can use the same analysis on a molecular
level to determine if it is polar:
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Section 8.13
Molecular Structures: The VSEPR Model
B. Molecular Structure - Polarity
• Draw the Lewis Dot, analyze each bond, and
predict the overall polarity of the molecule:
•
•
•
•
•
•
•
BeBr2
XeBr2
BeFI
CH3Cl
SbCl5
SeF5Br
BFI2
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Section 8.13
Molecular Structures: The VSEPR Model
B. Molecular Structure - Polarity
• Draw or look over the Lewis Dot and predict the
overall polarity of the molecule:
•
•
•
•
•
•
•
SF4
ClF3
XeF4
CH3Br
SbCl5
NH3
BrI5
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Section 8.13
Molecular Structures: The VSEPR Model
B. Molecular Structure -
• The effect of lone e- pairs on bond angles:
• Lone pairs of e- repel each other MORE than
bonding pairs of e-s thereby REDUCING the
bond angles:
•
•
•
•
LP/LP >> LP/BP > BP/BP
CH4
NH3
H2O
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Section 8.13
Molecular Structures: The VSEPR Model
Objectives Review
•
•
•
•
To learn to predict MOLECULAR geometries from the
number of paired and unpaired e-s
To predict the polarity of a molecule
To understand the effect of an unshared pair on bond
angles
Work Session:
– When are the molecular geometry and the egeometry equal?
– What causes the name for the molecular geometry
to be different from the e- geometry?
– Predict the molecular geometry AND polarity of:
SO3, HCN, Cl2O, PCl3
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Section 8.13
Molecular Structures: The VSEPR Model
Objectives
•
•
•
To understand the Valence Bond (VB) Theory
To describe HOW the exceptions to the octet rule occur
To use the VB theory to predict the orbital hybridization
about a central atom
OK State summary page
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Section 8.13
Molecular Structures: The VSEPR Model
B. The VB Theory
• Let’s consider the orbital diagram for Be
•
•
•
•
•
•
Be ______ ______ ______ ______ ______
1s
2s
2p
Now, let’s consider BeCl2:
Cl—Be—Cl
How can it bond if the valence 2s orbital is full?
Hybridization of the 2s and one of the 2p suborbitals to
form two sp orbitals
•
•
Be ______
1s
______
______
sp
sp
______
Be ______
sp 1s sp
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Section 8.13
Molecular Structures: The VSEPR Model
B. The VB Theory
•
•
•
Let’s visualize the hybrid orbitals:
s
p
sp
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Section 8.13
Molecular Structures: The VSEPR Model
B. The VB Theory
•
Let’s visualize the hybrid orbitals:
•
•
Be
1s2s
sp
sp
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Section 8.13
Molecular Structures: The VSEPR Model
B. The VB Theory
•
The sp hybridization of Be
•
Be
•
3p of Cl
•
•
______ ______ ______
______
3p of Cl
sp
sp
sp
Be ______
sp
3p of Cl
______ ______ ______
3p of Cl
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Section 8.13
Molecular Structures: The VSEPR Model
B. The VB Theory
•
•
Hybridization of other orbitals occurs in a similar fashion:
www.mikeblaber.org
sp2 (3 RHEDS)
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Section 8.13
Molecular Structures: The VSEPR Model
B. The VB Theory
•
www.mikeblaber.org
sp3 (4 RHEDS)
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Section 8.13
Molecular Structures: The VSEPR Model
B. The VB Theory
•
www.mikeblaber.org
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Section 8.13
Molecular Structures: The VSEPR Model
B. The VB Theory
•
www.mikeblaber.org
sp3d (5 RHEDS)
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Section 8.13
Molecular Structures: The VSEPR Model
B. The VB Theory Hybridization Summary
www.mikeblaber.org
Valence Electron
Pair Geometry
Number of
Orbitals
Hybrid Orbitals
Linear
Trigonal Planar
Tetrahedral
2
3
4
sp
sp2
sp3
Trigonal
Bipyramidal
5
sp3d
Octahedral
6
sp3d2
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Section 8.13
Molecular Structures: The VSEPR Model
C. The VB Theory Hybridization
• Draw or look over the Lewis Dot and Predict the
VB Hybridization of the following:
•
•
•
•
•
•
•
•
H2O
I 3O3
ICl3
NH3
IF5
XeF4
SF4
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Section 8.13
Molecular Structures: The VSEPR Model
C. The VB Theory Hybridization
• Draw or look over the Lewis Dot and Predict the
VB Hybridization of the following:
•
•
•
•
•
•
•
BeF2
SO2
BBr3
NH3
CHCl3
BrF5
XeF2
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Section 8.13
Molecular Structures: The VSEPR Model
The Orbitals Used to Form the Bonds in PCl5
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Section 8.13
Molecular Structures: The VSEPR Model
The Orbitals for CO2
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Section 8.13
Molecular Structures: The VSEPR Model
C. The VB Theory Hybridization
• How about Double and triple bonds?
• C2H4
1 σ & 1 π bond
• C2H2
1 σ & 2 π bonds
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Section 8.13
Molecular Structures: The VSEPR Model
C. The VB Theory Hybridization
• C2H2
1 σ & 2 π bonds
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Section 8.13
Molecular Structures: The VSEPR Model
Objectives Review
•
•
•
•
To understand the Valence Bond (VB) Theory
To describe HOW the exceptions to the octet rule occur
To use the VB theory to predict the orbital hybridization
about a central atom
Work Session: Draw the Lewis Dot and predict the
hybridization about the central atom:
AlCl3, NCl3, SiH4, SF6, MgCl2, IF4-, AlH4-, NH4+, PCI3,
CIO3-
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Section 8.13
Molecular Structures: The VSEPR Model
B. The VSEPR Model
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Section 8.13
Molecular Structures: The VSEPR Model
VSEPR: Two Electron Pairs
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Section 8.13
Molecular Structures: The VSEPR Model
VSEPR: Three Electron Pairs
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Section 8.13
Molecular Structures: The VSEPR Model
VSEPR: Four Electron Pairs
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67
Section 8.13
Molecular Structures: The VSEPR Model
VSEPR: Iodine Pentafluoride
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Section 8.13
Molecular Structures: The VSEPR Model
Concept Check
Determine the shape for each of the following
molecules, and include bond angles:
HCN
PH3
SF4
HCN – linear, 180o
PH3 – trigonal pyramid, 109.5o (107o)
SF4 – see saw, 90o, 120o
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69
Section 8.13
Molecular Structures: The VSEPR Model
Concept Check
Determine the shape for each of the following
molecules, and include bond angles:
O3
KrF4
O3 – bent, 120o
KrF4 – square planar, 90o, 180o
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70
Section 8.13
Molecular Structures: The VSEPR Model
Concept Check
True or false:
A molecule that has polar bonds will always
be polar.
-If true, explain why.
-If false, provide a counter-example.
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Section 8.13
Molecular Structures: The VSEPR Model
Let’s Think About It
• Draw the Lewis structure for CO2.
• Does CO2 contain polar bonds?
• Is the molecule polar or nonpolar overall? Why?
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Section 8.13
Molecular Structures: The VSEPR Model
Concept Check
True or false:
Lone pairs make a molecule polar.
-If true, explain why.
-If false, provide a counter-example.
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Section 8.13
Molecular Structures: The VSEPR Model
Let’s Think About It
• Draw the Lewis structure for XeF4.
• Does XeF4 contain lone pairs?
• Is the molecule polar or nonpolar overall? Why?
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74
Section 8.13
Molecular Structures: The VSEPR Model
Arrangements of Electron Pairs Around an Atom Yielding
Minimum Repulsion
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75
Section 8.13
Molecular Structures: The VSEPR Model
Arrangements of Electron Pairs Around an Atom Yielding
Minimum Repulsion
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76
Section 8.13
Molecular Structures: The VSEPR Model
Structures of Molecules That
Have Four Electron Pairs
Around the Central Atom
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Section 8.13
Molecular Structures: The VSEPR Model
Structures of Molecules
with Five Electron Pairs
Around the Central Atom
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