Transcript 2s - University of Lincoln

Chemical Bonding 2 MOLECULAR ORBITALS
University of Lincoln
presentation
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Molecular Orbitals
What you need to know…
•
In a covalent bond, atomic orbitals overlap to produce
MOLECULAR ORBITALS
•
Drawing molecular orbital diagrams for the
homonuclear diatomics:
H2, Li2, Be2, B2, C2, N2, O2, F2
•
Using molecular orbital diagrams to rationalise
observed trends in the properties of molecules
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The Covalent Bond – Recap
Non-bonded atoms –
NO OVERLAP of
atomic orbitals
Bonded atoms –
OVERLAP of atomic
orbitals
TWO ATOMS
ONE MOLECULE
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Which orbitals will overlap?
Rules:
• Only orbitals with the same symmetry (shape) will
overlap
– s-orbitals overlap with s-orbitals
– p-orbitals overlap with p-orbitals
• The more similar their energy, the better the
overlap (and hence, the better the bond)
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Extent of Overlap, S
No overlap
Weak overlap
Good overlap
No bond
Weak bond
Good bond
S is negligible
Same symmetry
Different energy
S is small
Same symmetry
Some difference
in energy
S is large
Same symmetry
Similar energy
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Molecular Orbital Theory
Where are the electrons most
likely to be found in a molecule?
Link to “Molecular
orbitals” video
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Remember…
• Heisenberg’s Uncertainty Principle “Electrons are
so small, it is impossible to be sure where they
are at any given time”
• Schrödinger
“It is possible to define volumes of space where the
electrons are most likely to be found”
– s p d and f atomic orbitals
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Consider 2 Hydrogen atoms, A and B
(1s1)
A
Electron is most likely to be found
within this volume
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If 2 H atoms BOND to form the H2
molecule…
Because A and B are bonded together,
the electrons are more likely to be found
in the shared space BETWEEN the nuclei
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Definitions
• ATOMIC ORBITALS are volumes of space in
which you are most likely to find an electron in
an atom
• MOLECULAR ORBITALS are volumes of space in
which you are most likely to find an electron in a
molecule
• MOLECULAR ORBITAL THEORY states that in a
molecule, all electrons are housed within
molecular orbitals
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Orbital summery
ATOMIC
Orbitals
H + H
MOLECULAR
Orbitals
H2
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How many molecular orbitals are
formed when two atoms overlap?
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TWO molecular orbitals are
formed for every two atomic
orbitals that overlap:
1. A bonding orbital (low energy)
2. An anti-bonding orbital (high energy)
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Remember
Only the occupied atomic orbitals are
relevant
(i.e. those containing electrons)
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ENERGY
Anti-bonding*
Atomic
orbitals
of Atom A
Bonding
MOLECULAR
ORBITALS
Atomic
orbitals of
Atom B
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Molecular Orbitals from
s-orbital Overlap
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When the overlap is directly in-line with the two
nuclei the resulting bond is called a SIGMA ()
bond
s-orbitals produce sigma bonds
Therefore, the two molecular orbitals are called:
 bonding
* anti-bonding
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(1s) Bonding and *(1s) Antibonding orbitals for the Hydrogen
Molecule
or
ψ bonding
ψ bonding
or
ψ antibonding
Electrons in an
antibonding
orbital try to pull
a bond apart, and
result in bond
weakening
ψ antibonding
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Bonding and antibonding video
Link to “Bonding and
antibonding” video
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Molecular Orbital Diagram
1s orbital overlap – H2
Ψ (antibonding)
Energy
σ*(1s)
Ψ (1s)A
Ψ (1s)B
σ*(1s)
Ψ (bonding)
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Molecular Orbital Diagram
2s orbital overlap – Li2
Energy
σ*(2s)
2s
2s
σ*(2s)
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Molecular Orbital Diagram
2s orbital overlap – Be2
Energy
σ*(2s)
2s
2s
Be2
Be
σ (2s)
Be
Be
Be2
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Molecular Orbitals from
p-orbital Overlap
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-Bonds with p-Orbitals
End-on overlap produces a -bond
z
2pZ
2pZ
z
σ(2pZ)
By convention, the z-axis always runs along the
main axis of the molecule
Pz-orbitals produce -bonds
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-Bonds with p-Orbitals
Sideways overlap results in a bond called a Pi
() bond
x
x
x
x
z
z
2px
2px
π(2px)
Px- and Py-orbitals produce -bonds
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Molecular Orbitals from p-Orbital
Overlap
• Pz-orbitals give -bonding and
*-antibonding molecular orbitals
• Px- and Py-orbitals give -bonding and
*-antibonding orbitals
Since -overlap is better than -overlap, the -bonding orbital is
the lowest in energy (most stable) and conversely the *antibonding is the highest in energy (least stable)
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Molecular Orbital Diagram
2p-orbital overlap
Energy
σ*(2pZ)
π*(2px) π*(2py)
2p
π(2px)
Sometimes the (2pz)
is higher in energy
π(2py) than the (2px) and
2p
(2py)
σ (2pZ)
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Energy
2s- and 2p-orbital overlap
3d
3p
N=3
3s
2p
N=2
N=1
2s
1s
The 2s atomic orbital
is lower in energy
than the 2p atomic
orbitals
Sometimes the (2pz)
is higher in energy
than the (2px) and
(2py)
Link to “Energy level
diagrams” video
A Diagram representing the energy solutions for n=1, 2 and 3 for the
Schrödinger equation of a multi-electron atom
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2s and 2p overlap MO diagram
Energy
σ*(2pZ)
2p
π*(2px)
π*(2py)
π(2px)
π(2py)
2p
σ (2pZ)
σ*(2s)
2s
2s
σ (2s)
A molecular orbital diagram showing the approximate molecular orbitals when combining
2s and 2p orbitals. Suitable when forming homonuclear diatomic molecules involving O and
F with the nuclai lying on the z-axis.
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Molecular Orbital Diagram F2
Energy
σ*(2pZ)
2p
π*(2px)
π*(2py)
π(2px)
π(2py)
2p
σ (2pZ)
σ*(2s)
2s
2s
F
σ (2s)
F
The formation of F2. The 1s atomic orbitals are emmited. The F nuclei lie
on the z-axis
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Molecular Orbital Diagram
O2
Energy
σ*(2pZ)
2p
π*(2px)
π*(2py)
π(2px)
π(2py)
2p
σ (2pZ)
σ*(2s)
2s
2s
O
σ (2s)
O
The formation of O2. The 1s atomic orbitals are emmited. The O nuclei lie on
the z-axis
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Using Molecular Orbital Diagrams to
Rationalise (explain) Observed Trends in
the Properties of Molecules
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Consider the homonuclear
diatomics B2, C2 andN2
Experimental FACTS:
• The vapour phase of B2 contains
PARAMAGNETIC B2 molecules
• The C2 molecule is a gas phase species and
is DIAMAGNETIC
• The N2 molecule is DIAMAGNETIC and has
a particularly high bond energy
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Definitions…
• A PARAMAGNETIC molecule contains one
or more unpaired electrons
• A DIAMAGNETIC molecule contains no
unpaired electrons
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Lets start with B2
FACT: The vapour phase of B2 contains
PARAMAGNETIC B2 molecules
The ATOM
B
Group 13 (3 valence electrons)
2 possible molecular
structures:
B
B
B≡B
Bond Order =3
B B
B–B
NOTE: Neither of
these structures have
unpaired electrons.
They are therefore
DIAMAGNETIC (not
correct)
Bond order = 1
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Consider the MO Diagram of B2
Energy
σ*(2pZ)
π*(2px)
2p
π*(2py)
2p
σ (2pZ)
π(2px)
π(2py)
σ*(2s)
2s
2s
B
σ (2s)
B
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What about the bonding?
MO diagrams can give us the
BOND ORDER:
BOND
ORDER
=½[(
Number of
bonding
electrons
)-(
)]
Number of antibonding
electrons
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Bond order in B2
No. bonding electrons = 4
No. anti-bonding electrons = 2
BOND ORDER = ½(4-2)= 1 (single
bond)
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Molecular Structure of B2
FACT: The vapour
phase of B2 contains
PARAMAGNETIC B2
molecules
B B
B–B
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What is the Molecular Structure of
C2?
FACT: The C2 molecule is a gas phase species and
is DIAMAGNETIC
The ATOM
Group 14 (4 valence electrons)
C
Possible molecular structure:
C
C=C
C
NOTE: This structure
has no unpaired
electrons and is
therefore
DIAMAGNETIC
(correct)
Bond Order =2
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MO Diagram of C2
Energy
σ*(2pZ)
π*(2px)
2p
π*(2py)
2p
σ (2pZ)
π(2px)
π(2py)
σ*(2s)
2s
2s
C
σ (2s)
C
No unpaired electrons – therefore DIAMAGNETIC. No. bonding electrons = 6 No. antibonding electrons = 2
BOND ORDER = ½(6-2)= 2
(double bond)
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Molecular Structure of C2
FACT: The C2 molecule
is a gas phase species
and is DIAMAGNETIC
C
C
C=C
Bond Order =2
In this case, the MO diagram agrees with
our initial structure
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What is the Molecular Structure of
N2 ?
FACT: The N2 molecule is DIAMAGNETIC and has a
particularly high bond energy
The ATOM
Group 15 (5 valence electrons)
N
Possible molecular structure:
N
N
N≡N
NOTE: This structure
has no unpaired
electrons and is
therefore
DIAMAGNETIC
(correct)
Bond Order =3
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MO Diagram of N2
Energy
σ*(2pZ)
π*(2px)
2p
π*(2py)
2p
σ (2pZ)
π(2px)
π(2py)
σ*(2s)
2s
2s
N
σ (2s)
N
No. bonding electrons = 8. No. anti-bonding electrons = 2 BOND ORDER = ½(8-2)= 3
(triple bond). No unpaired electrons – therefore DIAMAGNETIC
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Molecular Structure of N2
FACT: The N2 molecule
is DIAMAGNETIC and
has a particularly
high bond energy
N
N
N≡N
Bond Order =3
Triple bond is very strong –
hence would expect a high
bond energy
In this case, the MO diagram agrees with
our initial structure
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Summary
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Definitions
• Molecular Orbital Theory
• Molecular Orbitals
• Paramagnetism
• Diamagnetism
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What should you know?
• In a covalent bond, atomic orbitals overlap to
produce MOLECULAR ORBITALS
• How to draw MO diagrams:H2, Li2, Be2, B2, C2,
N2, O2, F2
• How to use the MO diagram to determine
molecular structure (including the bond order)
• How to use the MO diagram to rationalise
magnetic behaviour (paramagnetism or
diamagnetism)
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Acknowledgements
•
•
•
•
•
•
•
JISC
HEA
Centre for Educational Research and Development
School of natural and applied sciences
School of Journalism
SirenFM
http://tango.freedesktop.org
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