Shapes of Molecules
Lewis structures are useful but don’t allow prediction of
the shape of a molecule.
●●
H
O
●●
H
O
H
●●
●●
H
Can use a simple theory based on electron repulsion to
predict structure (for non-transition elements).
Shapes of Molecules
• Molecular shape - the way bonded atoms arrange
themselves in three dimensions.
– Molecular shape affects molecular properties, including reactivity.
• Valence Shell Electron Pair Repulsion (VSEPR) theory molecules assume a shape that allows them to minimize
the repulsions between electron pairs in the valence shell
of the central atom.
– Electron pairs include both lone pair electrons and bonding pair
electrons.
– Electron group geometry – distribution of e- pairs.
– Molecular geometry – distribution of nuclei.
1
2 Electron Pairs
• 2 Pairs: e.g. BeH2
• the molecule is LINEAR to maximize the distance
between bond pairs.
• H – Be – H bond angle is 180°
# Electron
Groups
Electron
Group
Geometry
Lone Pairs
Molecular
Geometry
Bond
Angles
Example
2
linear
0
linear
180°
BeCl2
3 Electron Pairs
• 3 Pairs: e.g. BH3
• the electron pairs arrangement is trigonal planar the
H–B–H bond angle is 120°
# Electron
Groups
Electron
Group
Geometry
3
Trigonal
Planear
Lone Pairs
Molecular
Geometry
Ideal Bond
Angles
Example
0
Trigonal
Planear
120°
BF3
1
Bent
120°
O3
2
4 Electron Pairs
• 4 Pairs: e.g. CH4
• the electron pairs arrangement is tetrahedral the H–C–H
bond angle is 109.5°
# Electron
Groups
4
Electron
Group
Geometry
Tetrahedral
Lone Pairs
Molecular
Geometry
Ideal Bond
Angles
Example
0
Tetrahedral
109.5°
CH4
1
Trigonal
Pyramidal
109.5°
NH3
2
Bent
109.5°
H2O
Structures of
Molecules that
Have Four Electron
Pairs Around the
Central Atom
3
The Molecular Structure of Methane
The Molecular Structure of Ammonia is a Trigonal Pyramid
4
The Structure and Charge Distribution of the Ammonia Molecule
The Tetrahedral Arrangement of Oxygen In a Water Molecule
5
The Charge Distribution in the Water Molecule
Lone Pairs vs. Bond Pairs
• In a tetrahedron, the expected bond angles are
109.5°
– H – C – H bond angle = 109.5°
– H – N – H bond angle = 107°
– H – O – H bond angle = 104.5°
• The slight differences are explained by the lone
pairs occupying more space than bond pairs
around the central atoms
6
Bonding Pair vs Lone Pair Electrons
a)
In a bonding pair of
electrons the electrons are
shared by two nuclei
b) In a lone pair, both
electrons must be close to a
single nucleus
The Bond Angles In the CH4, NH3, and H2O Molecules
7
Multiple Bonds
• for the purposes of VSEPR multiple bonds behave
like single bonds
• Consider CO2
●●
●●
O═C═O
●●
●●
– so the central carbon would have 2 bonding pairs of
electrons around it
– carbon dioxide is linear.
The Carbon Dioxide Molecule
8
5 Electron Pairs
• 5 Pairs: e.g. PCl5
• the electron pairs
arrangement is trigonal
bipyramidal the Cl–P–Cl
bond angles are 90° and120°
# Electron
Groups
5
Electron
Group
Geometry
Trigonal
bipyramidal
Lone Pairs
Molecular
Geometry
Ideal Bond
Angles
Example
0
Trigonal
bipyramidal
90° and 120°
PCl5
1
Seesaw
90° and 120°
SF4
2
T-shaped
90°
IF3
3
Linear
180°
XeF2
Location of Lone Pairs: the I3- Ion
• Three distinct choices for the structure:
• The order of repulsive forces is:
– LP-LP > LP-BP > BP-BP
LP = lone-pair; BP = bond pair
• Therefore the correct structure is c (linear)
9
Structures of
Molecules
with Five
Electron
Pairs Around
the Central
Atom
6 Electron Pairs
• 6 Pairs: e.g. SF6
• the electron pairs
arrangement is trigonal
bipyramidal the F–S–F
bond angles are 90°
# Electron
Groups
6
Electron
Group
Geometry
Octahedral
Lone Pairs
Molecular
Geometry
Ideal Bond
Angles
Example
0
Octahedral
90°
SF6
1
Square
pyramidal
90°
IF5
2
Sqaure
Planar
90°
XeF4
10
Possible Electron Pair Arrangements for XeF4
The order of repulsive forces is:
LP-LP > LP-BP > BP-BP
LP = lone-pair; BP = bond pair
The lone pairs will locate as far apart as possible
Therefore correct structure is b (square planar)
Predicting a VSEPR Structure
1. Draw Lewis structure.
2. Put electron pairs as far apart as possible.
• Double and triple bonds count as one bonding pair.
3. Determine positions of atoms from the way
electron pairs are shared.
4. Determine the name of molecular structure from
positions of the atoms.
11
Molecular Shape Example
Determine the shape for each of the following
molecules, and include bond angles:
• HCN
• PH3
• SF4
• O3
• KrF4
Shapes of Molecules
• Molecular geometry for larger molecules is
possible.
– Geometry is assigned for each central atom.
– Hybridization on each central atom assists in
determination of overall geometry.
12
The Molecular Structure of Methanol
a)
The arrangement of electron
pairs around the carbon atom
b) The arrangement of bonding
and lone pairs around the
oxygen atom
c)
The molecular structure
Molecular Orbital Theory
• Atomic orbitals are isolated on atoms.
• Molecular orbitals span two or more atoms.
• LCAO
– Linear combination of atomic orbitals.
Ψ1 = c111 + c122
Ψ2 = c211 + c222
 1   c11 c12   1 
   
  
 2   c21 c22    2 
13
Combining Atomic Orbitals
Electron Waves in Two Different Atoms Interact as the Atoms Come Together
14
Overlap as a Function of Atomic Distance
• (Left column) Two
hydrogen atoms
approaching with 1s orbitals
in phase result in an
enhanced amplitude in the
internuclear space
• (Right column) Out-ofphase orbitals in the two
approaching hydrogen
atoms cancel each out in the
internuclear space, resulting
in diminished amplitude
(a node) between the atoms
Molecular Orbitals of Hydrogen
15
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 =
No. e- in bonding MOs - No. e- in antibonding MOs
2
16
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).
The Relative Sizes of the Lithium 1s and 2s Atomic Orbitals
17
The Molecular Orbital Energy-Level Diagram of the Li2 Molecule
Molecular Orbitals of the Second Period
18
Expected MO Diagram of C2
The s Orbital and the pz Orbital
The s bonding orbital and the pz orbital are located in a similar region
of space and have the same symmetry with respect to the molecule
19
Modified MO Diagram of C2
The Responses of Different Materials to a Magnetic Field Reveal
Pairing of Electrons in the Bonds
20
Apparatus
Used to
Measure the
Paramagnetism
of a Sample
Molecular Orbital Summary of Second Row Diatomics
21
Orbital Energy-Level Diagram for the HF Molecule
The Electron Probability Distribution in the Bonding
Molecular Orbital of the HF Molecule
22
MO Diagrams of Heteronuclear Diatomics
The molecular
orbital energy-level
diagram for NO
Illustrates the effect
of differing
electronegativities
on bonding
The Molecular Orbitals of NO
The portion of the orbital in the vicinity of the oxygen atom is more
compact and more concentrated than that near to the nitrogen, reflecting
the greater electronegativity of oxygen
23
Delocalized Electrons
The Sigma System for Benzene
24
The Pi System for Benzene
Benzene
25
Ozone
The Molecular Orbital System of the NO3- Ion
(a) The p Orbitals used
to form the 
bonding system in
the NO3-.
(b) A representation of
the delocalization of
the electrons in the
 molecular orbital
system of the NO3ion
26