Transcript Slide 1

PowerPoint to accompany
Chapter 8
Part 2
Molecular
Geometry
and Bonding
Theories
Covalent Bonding and Orbital
Overlap

We think of covalent bonds forming through the
sharing of electrons by adjacent atoms.

In such an approach, this can only occur when
orbitals on the two atoms overlap.
Figure 8.14
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Covalent Bonding and Orbital
Overlap

Increased overlap
brings the electrons
and nuclei closer
together while
simultaneously
decreasing electronelectron repulsion.

However, if atoms get
too close, the
internuclear repulsion
greatly raises the
energy.
Figure 8.15
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Hybrid Orbitals

To explain geometries, we often
assume that the atomic orbitals of
an atom mix to form new orbitals
called hybrid orbitals.

The process of mixing orbitals as
atoms approach each other is called
hybridisation.
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sp Hybrid Orbitals
Consider beryllium:

In its ground electronic
state, it would not be able to
form bonds because it has
no singly-occupied orbitals.

But if it absorbs the small
amount of energy needed to
promote an electron from
the 2s to the 2p orbital, it
can form two bonds.
Brown, LeMay, Bursten, Murphy, Langford, Sagatys: Chemistry 2e © 2010 Pearson Australia
sp Hybrid Orbitals

Mixing the s and p orbitals yields two degenerate
orbitals that are hybrids of the two orbitals:


These sp hybrid orbitals have two lobes like a p orbital.
One of the lobes is larger and more rounded as is the s
orbital, this is the bonding lobe.
Figure 8.16
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sp Hybrid Orbitals

These two degenerate orbitals would align
themselves 180 from each other.

This is consistent with the observed geometry of
beryllium compounds: linear.
Figure 8.17
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sp Hybrid Orbitals

With hybrid orbitals, the orbital diagram
for beryllium would look like this.

The sp orbitals are higher in energy
than the 1s orbital but lower than the
2p.
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sp2 and sp3 Hybrid Orbitals
Using a similar model for boron leads to sp2
hybridisation, i.e.
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sp2 and sp3 Hybrid Orbitals
…three degenerate sp2 orbitals.
Figure 8.18
Brown, LeMay, Bursten, Murphy, Langford, Sagatys: Chemistry 2e © 2010 Pearson Australia
Brown, LeMay, Bursten, Murphy, Langford, Sagatys: Chemistry 2e © 2010 Pearson Australia
sp2 and sp3 Hybrid Orbitals
With carbon we get sp3 hybridisation, i.e. …
…four degenerate sp3 orbitals.
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Brown, LeMay, Bursten, Murphy, Langford, Sagatys: Chemistry 2e © 2010 Pearson Australia
Brown, LeMay, Bursten, Murphy, Langford, Sagatys: Chemistry 2e © 2010 Pearson Australia
Hybridisation involving d
orbitals
For geometries involving expanded octets on
the central atom, we must use d orbitals in our
hybrids.
What element is this?
Brown, LeMay, Bursten, Murphy, Langford, Sagatys: Chemistry 2e © 2010 Pearson Australia
Hybridisation involving d
orbitals
This leads to:
- five degenerate sp3d
orbitals
or
- six degenerate sp3d2
orbitals.
Brown, LeMay, Bursten, Murphy, Langford, Sagatys: Chemistry 2e © 2010 Pearson Australia
Hybrid Orbitals
Once the electrondomain geometry is
known, the
hybridisation state of
the atom is known.
Table 8.4
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Multiple Bonds
sigma,  overlap of
two orbitals
along the
internuclear
axis.
pi,  - sideways overlap of
two p orbitals.
Figure 8.21
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Molecular Orbital (MO) Theory
Though valence
bond theory
effectively conveys
most observed
properties of ions
and molecules,
there are some
concepts better
represented by
molecular orbitals.
Figure 8.22
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Molecular Orbital of H2

In H2 the two electrons
go into the bonding
molecular orbital.

The bond order is half
the difference between
the number of bonding
Figure 8.23
and antibonding
electrons, i.e.
bond order = ½(no. of bonding electrons - no. of
antibonding electrons)
Brown, LeMay, Bursten, Murphy, Langford, Sagatys: Chemistry 2e © 2010 Pearson Australia
Molecular Orbital of H2

For hydrogen, with
two electrons in the
bonding MO and
none in the
antibonding MO,
the bond order is:
Figure 8.23
1
2 (2 - 0) = 1
Brown, LeMay, Bursten, Murphy, Langford, Sagatys: Chemistry 2e © 2010 Pearson Australia
Molecular Orbital of He2

In the case of
He2, the bond
order would be:
1
2 (2 - 2) = 0
Figure 8.23

Therefore, He2
does not exist.
Brown, LeMay, Bursten, Murphy, Langford, Sagatys: Chemistry 2e © 2010 Pearson Australia
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Scotty!
There’s no dilithium!
Brown, LeMay, Bursten, Murphy, Langford, Sagatys: Chemistry 2e © 2010 Pearson Australia
Molecular Orbitals from 2p Orbitals
Figure 8.26

For atoms with both s
and p orbitals, there
are two types of
interactions:
The s and the p
orbitals that face each
other overlap in 
fashion.
 The other two sets of p
orbitals overlap in 
fashion.
Brown, LeMay, Bursten, Murphy, Langford, Sagatys: Chemistry 2e © 2010 Pearson Australia

This is the 2nd half
of the p-block
With paired p electons.
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Molecular Orbitals for Li2 and
Be2

The MO diagram
looks like this:

There are both 
and  bonding
molecular orbitals
and * and *
antibonding
molecular orbitals.
Figure 8.27
Brown, LeMay, Bursten, Murphy, Langford, Sagatys: Chemistry 2e © 2010 Pearson Australia
Brown, LeMay, Bursten, Murphy, Langford, Sagatys: Chemistry 2e © 2010 Pearson Australia
Brown, LeMay, Bursten, Murphy, Langford, Sagatys: Chemistry 2e © 2010 Pearson Australia
Brown, LeMay, Bursten, Murphy, Langford, Sagatys: Chemistry 2e © 2010 Pearson Australia
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Molecular Orbitals for B2 to
Ne2
Figure 8.31

The smaller p-block elements
in the second period have a
sizeable interaction between
the s and p orbitals.

This flips the order of the s
and p molecular orbitals in
these elements.
Figure 8.32
Brown, LeMay, Bursten, Murphy, Langford, Sagatys: Chemistry 2e © 2010 Pearson Australia
Molecular Orbitals of
Second-Row Diatomic
Molecules
Figure 8.33
Brown, LeMay, Bursten, Murphy, Langford, Sagatys: Chemistry 2e © 2010 Pearson Australia
Brown, LeMay, Bursten, Murphy, Langford, Sagatys: Chemistry 2e © 2010 Pearson Australia
Brown, LeMay, Bursten, Murphy, Langford, Sagatys: Chemistry 2e © 2010 Pearson Australia
Brown, LeMay, Bursten, Murphy, Langford, Sagatys: Chemistry 2e © 2010 Pearson Australia
Electron Configurations and
Molecular Properties
Paramagnetic molecules have one or
more unpaired electrons and are attracted
into a magnetic field.

Diamagnetic molecules have no
unpaired electrons and are weakly
repelled by a magnetic field. (It is a much
weaker effect than paramagnetism.)

Brown, LeMay, Bursten, Murphy, Langford, Sagatys: Chemistry 2e © 2010 Pearson Australia
Chapter 8 End of Part 2



Molecular Orbitals
Hybrid Orbitals
Special Molecular Properties
Brown, LeMay, Bursten, Murphy, Langford, Sagatys: Chemistry 2e © 2010 Pearson Australia