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20/10/2010 1 BeCl 2 SO 2 CH 2 FCl Chem 101 1 H 2 S ICl 4 + Chem 101 2 I 3 -
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BeCl2
SO2
CH2FCl
Chem 101 1
H2S
ICl4+
Chem 101 2
I3-
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SCl6
Chem 101 3
Reading: Chapter 9, sections 9.4-9.7
How can a molecule with polar bonds be nonpolar?
As you read these sections ask yourself:
Why do we need theories of bonding that differ from VSEPR?How does Valence Bond theory differ from the Lewis concept of chemical bonding?How does molecular orbital theory differ from valence bond theory?H d h b id bit l diff f t i
Chem 101 4
How does a hybrid orbital differ from a pure atomic orbital? How are hybrid orbitals related to the VSEPR shapes you learned earlier?How do sigma and pi bonds differ from each other?
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Molecular shape and polarity
Can now take into consideration the effect of bond polarity on the overall molecule
Bond polarity is a measure of
depends on differences in
dipole moment defined as a measure of the
Chem 101 5
In diatomic molecules (HCl) the bond dipole and the molecular dipole moment are the same
The overall molecular dipole moment depends on
polarity of the individual bonds
overall geometry of the molecule
bond dipoles
molecular dipole moment
O C O
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CCl4CHCl3
Chem 101 7
To determine if a molecule is polar…
draw Lewis structure and determine geometry
determine if the polar bonds add together (based on geometry)
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Bonding theories
VSEPR models and Lewis structures
do not explain how the electron’s atomic orbitals are involved in bonding
Need a theory that combines the idea of two electron bonds
Chem 101 9
with the theory of atomic orbitals
Valence Bond Theory
valence electrons are in the localized atomic orbitals of isolated atoms
bond is formed from overlap of half-filled valence orbitals, spin-pairing of valence electrons
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shape of molecule determined by geometry of overlapping orbitals
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For HCl
and H2S
Chem 101 11
consider CH4
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Make new orbitals
orbitals are (wave) functions can make math combinations to form new orbitalsnew orbitals
Hybrid orbitals
- have shapes that match actual electron distribution in bonded atoms
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- number of hybrid orbitals
- central or interior atoms have
Reconsider CH4
need a hybrid orbital on central atom to enable 4 equal bonds
E
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Chem 101 15
hybridized orbitals
2 electron domains – sp hybrid orbitals
E
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2 electron domains
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3 electron domains – sp2 hybrid orbitals
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3 electron domains
5 electron domains – sp3d hybrid orbitals
5 electron domains
Chem 101 18
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6 electron domains – sp3d2 hybrid orbitals
6 electron domains
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Bonding schemes:
1. Draw the Lewis structure
2. Determine the electron domain geometry using VSEPR
3 Choose hybrid orbitals for central/interior atoms based on VSEPR shape3. Choose hybrid orbitals for central/interior atoms based on VSEPR shape
example: XeF2
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Multiple Bonds
The sp and sp2 hybrid orbitals have unused p orbitals on the central atom
These p orbitals are
The p orbitals can
Chem 101 21
overlap along line between nuclei
overlap above and below the line between nuclei
single bonds are
double bonds are
triple bonds are
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Multiple bonds are rigid
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Resonance in valence bond theory
two or more resonance structures with pi bonds can not
th i b di i t t ithe pi bonding in resonance structures is
all atoms with delocalized π bonding must be in the same plane
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Bonding schemes:
1. Draw the Lewis structure
2. Determine the electron domain geometry using VSEPR
3 Choose hybrid orbitals for central/interior atoms based on VSEPR shape3. Choose hybrid orbitals for central/interior atoms based on VSEPR shape
example: CH3-CN
Chem 101 25
Molecular orbital (MO) theory (only section 9.7)
VSEPR and Valence bond theory are good at predicting geometry of molecules
Need better theory to understand excited states and the yproperties of some molecules
MO theory describes the electrons in moleculeswith wave functions called
wave function over entire molecule is constructed from the atomic orbitals of all the atoms in the molecule
Chem 101 26
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consider H2, molecule hastwo 1s atomic orbitals one on each atom
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each MO holds 2 electrons
MO diagram (energy level diagram)
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Bond order
stability of the bond depends on the relative number of bonding and antibonding electrons
Bond order =
Chem 101 29
Metal bonding (Sections 23.5 and 12.2)
Distinctive properties of metals
luster
high heat conductivity
high electrical conductivity
malleable, ductile
Need a bonding theory that explains these properties
metals have a close
Chem 101 30
metals have too few electrons in their valence shell
metals have a close packed structure with
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Electron sea model
array of metal cations in a ‘sea’ of electrons
electrons are
model explains conductivity, malleability and ductility
Chem 101 31
MO model
combines atomic orbitals tomake MO over entire molecule
each MO can hold 2 electrons
number of MO = number of atomic orbitals combined
an infinite chain of atoms
Chem 101 32
in general:
as no. of atoms increase
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MO model for metals - band structure
each valence subshell (s, d, p) has its own band
the bands overlap in energy
electrons occupy the lowest energy levels regardless of band (blue area)
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Metallic properties are explained by the band structure
bands are not independent and can be represented as one set of energy levels
Bonding MOs are called theBonding MOs are called the
Antibonding MOs are called the
Conduction arises when electrons move into unoccupied energy orbitals
Chem 101 35
Bonding strengthens as
valence electron configurations
3B
6B
1B
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Metals, insulators and semiconductors
differ in the size of the gap between the valence and conduction bands
t l
Chem 101 37
metals
insulators
semiconductors
