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MOLECULAR STRUCTURE
AND COVALENT BONDING
GENERAL CHEMISTRY
LECTURE
10
1. Valence Shell Electron Pair Repulsion (VSEPR) Theory
– Determine the shape (electronic and molecular geometry) of the molecule
2. Polar Molecules: The Influence of Molecular Geometry
– Determine the polarity of the molecule
3. Valence Bond (VB) Theory
– Determine the hybrid orbitals used for bonding
SCOPE
• Electron group domains around the central atom are arranged as far apart as possible to minimize repulsions.
• There are five basic molecular shapes based on the number of regions of high electron density around the central atom.
VSPER Theory
1. Two electron group domains around the central atom.
2. Three electron group domains around the central atom.
VSPER Theory
3. Four electron group domains around the central atom.
4. Five electron group domains around the central atom.
VSPER Theory
5. Six electron group domain around the central atom.
VSPER Theory
MOLECULAR STRUCTURE
AND COVALENT BONDING
GENERAL CHEMISTRY
LECTURE
10
Electron-group repulsions and the five
basic molecular shapes.
• Electronic geometry – based on the number of electron domains around
the central atom
• Molecular geometry – accounts the number of nonbonding
electrons/lone pairs
• The same electron-group arrangement can give rise to different molecular shapes.
VSPER Theory
• Lone pairs of electrons require more volume than shared electrons.
• Criteria for the ordering of the repulsions: a) Lone pair to lone pair is the strongest repulsion.
b) Lone pair to bonding pair is intermediate repulsion.
c) Bonding pair to bonding pair is weakest repulsion.
lp/lp > lp/bp > bp/bp
VSPER Theory
The Different Types of Shapes
according to Electron Domains
Examples CO2 BF3
NO2
-
Total Electron Domains: 4 pairs
Examples
CCl4
NH3
H2O
Total Electron Domains: 5 pairs
Examples
PCl5
SF4
ClF3
XeF2
Total Electron Domains: 6 pairs
Examples
SF6
BrF5
XeF5
The Steps in Determining
a Molecular Shape.
Example: Methane, CH4
Electronic and molecular geometries are the same: tetrahedral.
VSPER Theory
Exercise No. 1 Predicting Molecular Shapes
Give the molecular shape and predict the bond angles (relative to the ideal angles) of (a) PF3 (b) COCl2 and (c) BrF5.
SOLUTION: (a) For PF3 N = 8 x 4 = 32 A = 5 + 7 x 3 = 26 S = 6 -> 3 bonds A-S = 20
PF F
F
4 electron group domain The electronic geometry is tetrahedral arrangement.
The F-P-F bond is <109.5o
The final shape (molecular geometry) is trigonal pyramidal.
PF F
F
(b)For COCl2, C has the lowest EN and will be the center atom. N = 8 x 4 = 32 A = 4 + 6 + (7 x 2) = 24 S = 8 -> 4 bonds A-S = 16
C
Cl
O
Cl
The Cl-C-Cl bond angle is < 120o due to the electron density of the C=O.
C
Cl
O
Cl
Predicting Molecular Shapes
SOLUTION:
3 electron group domain The electronic geometry is trigonal planar The final shape (molecular geometry) is trigonal planar
Exercise No. 1
111o
124.5o
Predicting Molecular Shapes
(b)For BrF5, N = 8 x 6 = 48 A = 7 x 6 = 42 S = 6 e -> 10 e -> 5 bonds A-S = 42-10 = 32
SOLUTION:
Br
F
F F
F
F
6 electron group domain The electronic geometry is octahedral arrangement.
The final shape (molecular geometry) is square pyramidal.
Exercise No. 1
Predicting Molecular Shapes with More Than One Central Atom
SOLUTION:
Determine the shape around each of the central atoms in acetone, (CH3)2C=O.
C C C
OH
H
H
HH
H
tetrahedral tetrahedral
trigonal planar
C
O
HC
HHH
C
HH
Exercise No. 2
3. What is the shape of each of the indicated atoms in the molecule below?
1. Trigonal planar
2. Tetrahedral
3. Trigonal planar
4. Bent or angular
Predicting Molecular Shapes with More Than One Central Atom
Exercise No. 3
• Molecular geometry affects molecular polarity.
– The bond dipoles either cancel or reinforce each other.
linear molecule nonpolar
A B
A B A
angular molecule
A
polar
Polarity and Molecular Geometry
• Polar molecules must meet the following requirements:
– One polar bond or one lone pair of electrons on central atom.
– Bonds and lone pairs are not symmetrically arranged.
– Their polarities do NOT cancel.
– Net dipole moment is NOT equal to zero.
Polarity and Molecular Geometry
Bond Polarity, Bond Angle and
Bond Dipole Moment
1. Covalent bonds are formed by the overlap of atomic orbitals.
Valence Bond (VB) Theory
2. Atomic orbitals on the central atom can mix and exchange their character with other atoms in a molecule.
- Hybridization of atomic orbitals
3. Hybrid orbitals have the same shapes as predicted by VSEPR.
Valence Bond (VB) Theory
The number of hybrid orbitals obtained is equal to the number of atomic orbitals mixed.
The type of hybrid orbitals obtained varies with the types of atomic orbitals mixed.
sp sp2 sp3 sp3d sp3d2
Types of Hybrid Orbitals
Hybrid Orbitals
The sp Hybrid Orbitals in Gaseous BeCl2
The sp Hybrid Orbitals in Gaseous BeCl2
Shape: Linear
sp-p
The sp2 Hybrid Orbitals in BF3
The sp2 Hybridized Orbitals
Shape: Trigonal planar
sp2-p
The sp3 Hybrid Orbitals in CH4
The sp3 Hybridized Orbitals
The sp3 Hybridized Orbitals
Shape: Tetrahedral
sp3-p sp3-s
The sp3 Hybrid Orbitals in NH3 and H2O
The sp3d Hybrid Orbitals in PCl5
The sp3d Hybridized Orbitals
Shape: Trigonal bipyramidal
sp3d-p
The sp3d2 Hybrid Orbitals in SF6
The sp3d2 Hybridized Orbitals
Shape: Octahedral
sp3d2-p
Summary: Hybridization
Regions of High Electron Density
Electronic Geometry
Hybridization
2 Linear sp
3 Trigonal planar sp2
4 Tetrahedral sp3
5 Trigonal bipyramidal
sp3d
6 Octahedral sp3d2
Summary: Hybridization
Molecule Lewis Structure
Electron group
domain
Electronic geometry
Hybrid Orbitals
CO2
PO43-
XeF4
SbF5
2 Linear sp
4 Tetrahedral
Octahedral 6
sp3
sp3d2
5 Trigonal bipyramidal
sp3d
4. What is the hybridization of each of the indicated atoms in the molecule below?
1. sp2
2. sp3
3. sp2
4. sp3
Exercise No. 4
Take home quiz
Consider the following ACln species:
SCl2, OCl2, PCl3, SiCl4, SiCl62-
1) Determine the EGG and MGG of each compound
2) Arrange the compounds in decreasing Cl-A-Cl bond angles
3) Are all the molecules polar? Give the list of all the polar compounds from the given
4) Identify the orbitals involved in the bond A-Cl
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