Lecture 13 Bond Polarity, Molecular Orbitals

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Lecture 13 Bond Polarity, Molecular Orbitals
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Dipole Moment, m
Q Q
r

• m Q r
• m has units of C m
• 1 D (Debye) 3.3 x 10-30 C m

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Bond Polarity
m 1.93 D
d-
d
F
H

• Dipole moment

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• Dipole
• Molecule Moment, D
• HF 1.93
• HCl 1.04
• HBr 0.80
• HI 0.38

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Water - polar

• d

m 1.85 D
H
2 d-
O
H
d
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Ammonia - polar
m 1.47 D
3 d-
d
N
H
H
d
H
d
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NF3 - zero dipole
N
F
F
F
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Carbon Dioxide
2 d
d-
d-
C
O
O

• No net dipole!

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Methane - no dipole
d
H
4 d
C
d
H
H
d
H
d
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• Valence Bond Theory
• localized overlap of atomic
• valence shell orbitals
• Hybrid Orbital Theory
• Localized overlap of hybrid
• and atomic orbitals
• MO theory
• orbitals delocalized over entire molecule

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Molecular Orbital Theory

• atomic orbitals atoms
• MO molecules

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MOs are...

• Linear Combinations of Atomic Orbitals (LCAO)

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positive overlap a bonding orbital

electron density
-
nuclei
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A bonding MO





s1s
1s
1s
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Negative overlap is also possible...
a node

electron density
-
nuclei
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An antibonding MO
-




s1s
1s
1s
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Bonding results in stability
1s - 1s


Energy
1s
1s


1s 1s
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add the MOs
s1s
s1s
H
H
H2
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put electrons in the MOs
s1s
s1s
H
H
H2
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put electrons in the MOs
s1s
s1s
H
H
H2
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Will the molecule hang together or fall apart?

• Calculate the Bond Order

no. of bonding electrons
no. of antibonding electrons
-
BO
2
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s1s

• for H2,

s1s
2 - 0
BO
1
2
BO gt 0, thus H2 is stable
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other molecules, e.g. H2-
s1s
s1s
H
H
H2-
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s1s

• for H2- ,

s1s
2 - 1
½
BO
2
BO gt 0, thus H2- is stable, (and has been formed
in the lab)
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e.g. H2-2
s1s
BO0
s1s
H2-2
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e.g. He2
s1s
BO0
s1s
He2
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e.g. He2
s1s
BO½, found in stars!
s1s
He2
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more complex molecules...
s2s
2s
2s
s2s
s1s
1s
1s
s1s
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Li2
BO 1
s2s
2s
2s
s2s
s1s
1s
1s
s1s
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Be2
BO 0
s2s
2s
2s
s2s
s1s
1s
1s
s1s
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2p
2p
s2s
2s
2s
s2s
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? bond
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s2p
2p
2p
s2p
s2s
2s
2s
s2s
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? bonds
? bond
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s2p
p2p
2p
2p
s2p
p2p
s2s
2s
2s
s2s
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N2, BO 3
s2p
p2p
s2p
p2p
s2s
s2s
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O2, BO 2
s2p
p2p
s2p
p2p
s2s
s2s
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F2, BO 1
s2p
p2p
s2p
p2p
s2s
s2s
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Ne2, BO 0
s2p
p2p
s2p
p2p
s2s
s2s
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MOs can explain observed physical properties

• e.g. bond energies,
• bond lengths,
• magnetic properties