Valence Shell Electron-Pair Repulsion (VSEPR) model helps to determine the molecular structure and geometry in 3-D in the space. Even though it is not as precise as other models, the p , extremelyy useful model. VSERP model is a simple, The VSEPR model deals with molecules of the general type:
The VSEPR model is based on the assumption that: Bonding and nonbonding electron pairs in the valence shell are positioned around the central atom such that electron-pair repulsions are minimized.
A central atom can be surrounded by: Bonding pairs (B) • can be single, double or triple bonds
ABx
where the central atom A is surrounded by x of B atoms, and x can take values from 2 to 6. Dr. A. Al-Saadi
The VSEPR model • Molecular geometry (going from two-dimensional “Lewis structure” to three-dimensional “VSEPR model” shapes) • Polarity Valence bond theory Hybridization of atomic orbitals Molecular orbital (MO) theory • MO energy diagram • Delocalization of orbitals
Lone pairs (E)
Electron domains
ABx Ey 3
Dr. A. Al-Saadi
where the number of electron domains = x + y
4
1
27-Nov-11
Chapter 9
Section 1
Chapter 9
Counting Electron Domains
A central atom can be surrounded by: Bonding pairs (B) • can be single, double or triple bonds
Lone pairs (E)
Section 1
Counting Electron Domains
Electron domains
A central atom can be surrounded by: Bonding pairs (B) • Can be single, double or triple bonds
Lone pairs (E)
F Be
F O
Cl F
C
F
C
H
O
F
2
4
2 5
Dr. A. Al-Saadi
Chapter 9
Dr. A. Al-Saadi
F
P
F
F
4
5 6
Dr. A. Al-Saadi
Section 1
Chapter 9
Section 1
Arrangement of Electron Domains around the Central Atom
Electron-Domain Geometry
Number of Electron Domains
Electron-Domain Geometry
Linear
3
Trigonal planar
4
Tetrahedral
The repulsion of electro on pairs is minimized
The repulsion of electro on pairs is minimized
2
F
H
H
Arrangement of Electron Domains around the Central Atom Number of Electron Domains
N
Total # of electron domains
Total # of electron domains
Cl
Electron domains
7
Dr. A. Al-Saadi
5
Trigonal bipyramidal
6
Octahedral
8
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27-Nov-11
Chapter 9
Section 1
Chapter 9
Molecular Geometry
Steps of Determining Molecular Geometry 1.
Draw the molecule’s Lewis structure.
2.
Count the number of electron domains on the central atom.
3.
Determine the electron-domain geometry.
4.
Determine the molecular geometry.
Lewis structure
Section 1
Electrondomain geometry
Molecular geometry
180º
AB2
180º
Cl
Be
Cl
Linear 120º
120º
F Electron-domain geometry
Lewis structure
AB3
Molecular geometry
B F
F
9
Dr. A. Al-Saadi
Chapter 9
Lewis structure
Section 1
Molecular Geometry
AB4
F
Square q planar??
F
90º
C
F
Electrondomain geometry
Molecular geometry
Chapter 9
Section 1
Molecular Geometry
Is there another possible orientation that can further minimize the repulsion between the four electron domains around the C atom??
F
Trigonal planar
Dr. A. Al-Saadi
Lewis structure
Electrondomain geometry
10
Molecular geometry
PCl5 AB5
109.5º
F
SF6
109.5º
AB6
C F
Dr. A. Al-Saadi
F F
Tetrahedral 11
Dr. A. Al-Saadi
12
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27-Nov-11
Chapter 9
Section 1
Chapter 9
Molecular Geometry for ABx Molecules
Section 1
Molecular Geometry for ABx Molecules # of atoms # lone bonded to central pairs on Class atom central atom
AB2
Trigonal bipyramidal
13
0
AB4
4
0
tetrahedral tetrahedral
AB5
5
0
trigonal bipyramidal
trigonal bipyramidal
AB6
6
0
Octahedral
Octahedral
14
Dr. A. Al-Saadi
Chapter 9
Other subclasses
# lone pairs on central atom
# bonding pairs on central atom
AB2
None
-
-
-
AB2E
1
2
3
AB4
AB3E AB2E2
1
3
4
2
2
4
AB5
AB4E AB3E2
1
4
2 3
2
5 5 5
Dr. A. Al-Saadi
AB5E AB4E2
1 2
3 5 4
6 6
Lewis structure
Section 1
AB2E Class Molecules
Total # of electron domains
AB3
AB6
linear trigonal planar l
3
Section 1
AB2E3
linear trigonal planar l
AB3
Other Subclasses of Molecules Main Class
0
Octahedral
Dr. A. Al-Saadi
Chapter 9
2
Molecular geometry
Electron-domain geometry
Electrondomain geometry
Molecular geometry
SO2 Molecular geometry is V-shaped or bent
• Arrangement of the electron domains is trigonal planar.
Electron bonding pairs O
AB2E
S
O 15
Dr. A. Al-Saadi
Electron lone pair
16
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27-Nov-11
Chapter 9
Section 1
Chapter 9
The Geometry of AB3 Class Molecules # of atoms # lone bonded to central pairs on Class atom central atom
Electron-domain geometry
Molecular geometry
AB3E Class Molecules
3
0
Trigonal planar
Trigonal planar
AB2E
2
1
Trigonal planar
V-shaped / bent
H2 O
Section 1
H
O
• Arrangement of the electron domains is again tetrahedral.
Dr. A. Al-Saadi
N
H
Molecular geometry is Trigonal Pyramid
18
Dr. A. Al-Saadi
Chapter 9
AB2E2 Class Molecules
H
• Arrangement of the electron domains is tetrahedral.
17
Chapter 9
NH3
H
AB3
Dr. A. Al-Saadi
Section 1
Section 1
Geometry AB3 and AB4 Class Molecules # of atoms # lone bonded to central pairs on Class atom central atom
H
Molecular geometry is V-shaped or bent
19
Electron-domain geometry
Molecular geometry
AB3
3
0
Trigonal planar
Trigonal planar
AB2E
2
1
Trigonall planar
V-shaped h d/ bent
AB4
4
0
tetrahedral tetrahedral
AB3E
3
1
tetrahedral
trigonal pyramidal
AB2E2
2
2
tetrahedral
V-shaped / bent
Dr. A. Al-Saadi
20
5
27-Nov-11
Chapter 9
Section 1
Chapter 9
Geometry of Subclasses of AB5 Molecules
Section 1
Geometry of Subclasses of AB5 Molecules
Class
# of atoms # lone bonded to central pairs on Class atom central atom
AB5
PCl5 AB4E
Electron-domain geometry
Molecular geometry
AB5
5
0
Trigonal bipyramidal
Trigonal bipyramidal
AB4E
4
1
Trigonal bipyramidal
See-saw
AB3E2
AB2E3 21
Dr. A. Al-Saadi
Chapter 9
22
Dr. A. Al-Saadi
Section 1
Chapter 9
Geometry of Subclasses of AB5 Molecules Class
# of atoms # lone bonded to central pairs on atom central atom
Section 1
Geometry of Subclasses of AB5 Molecules # of atoms # lone bonded to central pairs on atom central atom
Electron-domain geometry
Molecular geometry
Class
AB5
5
Electron-domain geometry
Molecular geometry
0
Trigonal bipyramidal
Trigonal bipyramidal
AB5
5
0
Trigonal bipyramidal
Trigonal bipyramidal
AB4E
4
1
Trigonal bipyramidal
See-saw
AB4E
4
1
Trigonal bipyramidal
See-saw
AB3E2
3
2
Trigonal bipyramidal
T-shaped
AB3E2
3
2
Trigonal bipyramidal
T-shaped
AB2E3
2
3
F F Dr. A. Al-Saadi
Cl F
23
Dr. A. Al-Saadi
Trigonal bipyramidal
Linear 24
6
27-Nov-11
Chapter 9
Section 1
Chapter 9
Geometry of Subclasses of AB6 Molecules # of atoms # lone bonded to central pairs on Class atom central atom
AB6
6
0
Electron-domain geometry
Geometry of Subclasses of AB6 Molecules # of atoms # lone bonded to central pairs on Class central atom atom
Molecular geometry
Octahedral Octahedral
25
Dr. A. Al-Saadi
Chapter 9
0
Octahedral Octahedral
AB5E
5
1
O t h d l Octahedral
Square pyramidal
26
Dr. A. Al-Saadi
Chapter 9
Electron-domain geometry
Molecular geometry
6
Section 1
# of atoms # lone bonded to central pairs on central atom atom
Electron-domain geometry
AB6
Section 1
Geometry of Molecules with Lone Pairs on the Central Atom: a Summary
Geometry of Subclasses of AB6 Molecules Class
Section 1
Molecular geometry
AB6
6
0
Octahedral Octahedral
AB5E
5
1
O t h d l Octahedral
Square pyramidal
AB4E2
4
2
Octahedral
Square planar
E
E
E2
Dr. A. Al-Saadi
27
Dr. A. Al-Saadi
28
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27-Nov-11
Chapter 9
Section 1
Chapter 9
Geometry of Molecules with Lone Pairs on the Central Atom: a Summary
Section 1
Deviation from Ideal Bond Angles
E
Bond angles in methane, ammonia, and water molecules.
Lone pairs require more space than bonding pairs and tend to slightly squeeze the angles between the bonding pairs.
E2
E
E3
E
E
E2
E2 29
Dr. A. Al-Saadi
Chapter 9
30
Dr. A. Al-Saadi
Section 1
Chapter 9
Deviation from Ideal Bond Angles
Section 1
Deviation from Ideal Bond Angles
(a) For a bonding pair in a molecules, the electrons are attracted towards two nuclei. (b) For a lone pair, it is localized on only one nucleus and occupies more space around the nucleus.
