General Chemistry
M. R. Naimi-Jamal
Faculty of Chemistry
Iran University of Science & Technology
‫فصل نهم‪:‬‬
‫شکل هندسی مولکول‬
‫اوربیتال مولکولی‬
The Shapes of Molecules
H
O
H
Terminology
VSEPR Theory
Valence Shell Electron Pair Repulsion theory
Electron pairs repel each other whether they are
in chemical bonds (bond pairs) or unshared
(lone pairs). Electron pairs assume orientations
about an atom to minimize repulsions.
Molecular Geometry
• Electron group geometry – distribution of e- pairs.
• Molecular geometry – distribution of nuclei.
• Most important factor in determining geometry is
relative repulsion between electron pairs.
Balloon Analogy
No. of e- Pairs
Around Central
Atom
2
Example
F—Be—F
Geometry
linear
180
F
3
F
planar
trigonal
B
F
120
H
4
C
H
109
tetrahedral
H
H
Structure Determination by VSEPR
••
H
Ammonia, NH3
N
H
H
1. Draw electron dot structure
2. Count BP’s and LP’s of the central atom: 4 pairs
3. The 4 electron pairs are at the corners of a
tetrahedron.
lone pair of electrons
in tetrahedral position
N
H
H
H
Structure Determination by VSEPR
Ammonia, NH3
The electron pair geometry is tetrahedral.
lone pair of electrons
in tetrahedral position
N
H
H
H
The MOLECULAR GEOMETRY — the positions
of the atoms — is PYRAMIDAL.
Structure Determination by VSEPR
Water, H2O
••
1. Draw electron dot structure
H O
••
2. Count BP’s and LP’s = 4
3. The 4 electron pairs are at the corners of a
tetrahedron.
O
H
H
The electron pair
geometry is
TETRAHEDRAL.
H
Structure Determination by VSEPR
Water, H2O
••
H O H
••
O
H
H
The molecular
geometry is
bent.
The electron pair
geometry is
TETRAHEDRAL.
Structure Determination by VSEPR
Formaldehyde, CH2O
•
•
O
•
•
1. Draw electron dot structure
H C H
2. Count BP’s and LP’s = 3
3. There are 3 electron pairs are at the corners
of a planar triangle.
•
•
O
•
•
The electron pair geometry
is PLANAR TRIGONAL
with 120o bond angles.
C
H
H
Structure Determination by VSEPR
Formaldehyde, CH2O
•
•
O
•
•
The electron pair
geometry is PLANAR
TRIGONAL
C
H
H
The molecular
geometry is also planar
trigonal.
Structure Determination by VSEPR
Methanol, CH3OH
1. Draw electron dot structure
H
••
H—C—O—H
••
1
H
2
2. Define bond angles 1 and 2
Structure Determination by VSEPR
Methanol, CH3OH
Define bond angles 1 and 2
Angle 1 = 109o
Angle 2 = 109o
H
••
H—C—O—H
••
109
H 109
In both cases the atom is surrounded by 4 electron pairs.
Structure Determination by VSEPR
Acetonitrile, CH3CN
Draw the electron dot structure
Structure Determination by VSEPR
Acetonitrile, CH3CN
Define bond angles 1 and 2
H
109o
109
H 180
N
••
H—C—C
Angle 1 =
Angle 2 = 180o
One C is surrounded by 4 electron “lumps” and the
other by 2 “lumps”
Phenylalanine, an amino acid
1
H
C
H
H
C
C
C
C
H
C
H
H 2 H
O
3
C
C
C
O
H
N
H
H
5
4
H
Phenylalanine
STRUCTURES WITH
CENTRAL ATOMS THAT
DO NOT OBEY THE
OCTET RULE
Compounds with 5 or More Pairs Around the Central Atom
90
F
F
P
Trigonal bipyramid
F
120 5 electron pairs
F
F
90
6 electron pairs
F
F
S
F
F
Octahedron
F
F
90
Sulfur Tetrafluoride, SF4
Number of valence electrons = 34
••
Central atom = S
••
•• F
••
Dot structure
••
••
•• F
••
S
•• F ••
••
Electron pair geometry = ?
F ••
••
Sulfur Tetrafluoride, SF4
Number of valence electrons = 34
Central atom = S
Dot structure
Electron pair geometry
= trigonal bipyramid (because
90
there are 5 pairs around the S)
••
••
•• F
••
••
•• F
••
••
S
••
F ••
••
•• F ••
••
F
S
F
F
F
120
Sulfur Tetrafluoride, SF4
Lone pair is in the equator
because it requires more
room.
90
F
••
S
F
F
F
120
••
•• F
••
••
•• F
••
••
S
•• F ••
••
••
F ••
••
Hybridization of Atomic Orbitals
sp3 Hybridization
Bonding in Methane
sp3 Hybridization in Nitrogen
Bonding in Nitrogen
sp2 Hybridization in Boron
Orbitals in Boron
sp Hybridization in Be
Orbitals in Beryllium
sp3d and sp3d2 Hybridization
Hybrid Orbitals and VSEPR
• Write a plausible Lewis structure.
• Use VSEPR to predict electron geometry.
• Select the appropriate hybridization.
Multiple Covalent Bonds
• Ethylene (H2C=CH2) has a double bond in
its Lewis structure.
• VSEPR says trigonal planar at carbon.
Ethylene
Acetylene
• Acetylene, C2H2, has a triple bond.
• VSEPR says linear at carbon.
Applying VSEPR Theory
• Draw a plausible Lewis structure.
• Determine the number of e- groups and identify
them as bond or lone pairs.
• Establish the e- group geometry.
• Determine the molecular geometry.
• Multiple bonds count as one group of electrons.
• More than one central atom can be handled
individually.
Molecular Orbital Theory
• Atomic orbitals are isolated on atoms.
• Molecular orbitals span two or more atoms.
• LCAO:
– Linear Combination of Atomic Orbitals.
Ψ1 = φ1 + φ2
Ψ2 = φ1 - φ2
Combining Atomic Orbitals
Molecular Orbitals of Hydrogen
Basic Ideas Concerning MOs
• Number of MOs = Number of AOs.
• Bonding and antibonding MOs formed from AOs.
• e- fill the lowest energy MO first.
• Pauli exclusion principle is followed.
• Hund’s rule is followed
Bond Order
• Stable species have more electrons in bonding
orbitals than antibonding.
Bond Order =
# e- in bonding MOs - # e- in antibonding MOs
2
Diatomic Molecules of the First-Period
BO = (e-bond - e-antibond )/2
BOH += (1-0)/2 = ½
2
BOH = (2-0)/2 = 1
2
BOHe + = (2-1)/2 = ½
2
BOHe = (2-2)/2 = 0
2
Molecular Orbitals of the Second Period
• First period use only 1s orbitals.
• Second period have 2s and 2p orbitals
available.
• p orbital overlap:
– End-on overlap is best – sigma bond (σ).
– Side-on overlap is good – pi bond (π).
Molecular Orbitals of the Second Period
Combining p orbitals
Expected MO Diagram of A2
MO Diagram of A2 , Z ≥ 8
Modified MO Diagram of A2 , Z < 8
MO Diagrams of 2nd Period Diatomics
Benzene
Benzene
Ozone
Chapter 9 Questions
7, 16, 22, 25, 33
34, 35, 43