s and p Bonds
MO Theory
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Remember back 5
rows of even side
are no seating zones
ANSWERING QUESTIONS
• Review Question: How do you know when to move the order of the elements
when listing the ionization energy?
• Does one of them have a stable electron configuration (half filled or fully filled)?
• Why does the anti bonding orbital has an additional node between the nuclei
and is called the pi*2p orbital?
• Are nodes and lobes the same thing?
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MO THEORY: S ORBITALS
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s orbitals form s and s* molecular orbitals
Add and subtract orbitals to get the bonding and anti-bonding orbitals
Start with 2 atomic s orbitals end with 2 molecular orbitals.
Fill in electrons from low to high energy, just like in atomic orbital diagrams




1s-1s
Anti-bonding MO, σ*1s
E
isolated H
atoms
1s+1s
Bonding MO, σ1s
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MO THEORY: P ORBITALS

p orbitals form s and s* molecular
orbitals or p and p* orbitals depending
on type of overlap

Add and subtract orbitals to get the
bonding and anti-bonding orbitals

Start with 6 atomic orbitals end with 6
molecular orbitals

Fill in electrons from low to high energy,
just like in atomic orbital diagrams
End to end
overlap
Side to side
overlap
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PRACTICE
• Draw the molecular orbital diagram for
nitrogen. Is it paramagnetic or diamagnetic?
Diamagnetic
N2
N
N
2p
• What is the bond order?
Bond Order: ½(8-2)=3
• Hint: ½(bonding electron-antibonding electrons)
• What would happen to the bond order if you
added an electron? The bond length?
• What would happen to the bond order if you
subtracted an electron? The bond length?
E
Adding electron: adds to
antibonding orbital so it would
subtract from the bond order
making it longer.
subtracting electron: subtracts
from bonding orbital, lowering
bond order, making the bond
longer
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PRACTICE
• Draw the molecular orbital diagram for fluorine.
• Is it paramagnetic or diamagnetic?
diamagnetic
Bond Order: ½(8-6)=1
• What is the bond order?
• Hint: ½(bonding electron-antibonding electrons)
• What does adding an electron do to the bond
order? To the bond length?
• What does subtracting an electron do to the
bond order? To the bond length?
E
Adding electron: adds to
antibonding orbital so it would
subtract from the bond order
making the bond longer.
subtracting electron: subtracts
from an anti-bonding orbital,
increasing the bond order,
making the bond shorter
Charge= -2
Carbon 4+4=8 valence electrons is neutral
10 present so -2 charge
PRACTICE
• The ground-state electron configuration of the
ion C2n- is s2s2s2s*2p2p4s2p2.
C-
C2p
• What is the charge on the anion?
• Draw the MO diagram.
• Is it paramagnetic or diamagnetic?
• What is the bond order?
Diamagnetic
C22-
E
Bond Order: ½(8-2)=3
Adding electron: adds to
antibonding orbital so it would
subtract from the bond order
• Hint: ½(bonding electron-antibonding electrons)
• Would adding an electron add or subtract from
the bond order?
• Would subtracting an electron make the bond
longer or shorter?
subtracting electron: subtracts
from bonding orbital, lowering
bond order, making the bond
longer
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Heteronuclear
Practice
CN-
CN
C
C
N
N
2p
2p
• For CN the energy of the molecular
orbitals follows the order of carbon
(a.k.a. p2p<s2p)
• Draw the MO diagram.
• Give the valence shell electron
configurations of CN and CN-.
E
E
• Are they paramagnetic or
diamagnetic?
• Tell the bond order of each.
• Which has the stronger bond?
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s2s2s2s*2p2p4s2p1
paramagnetic
Bond order= ½ (7-2)=2.5
s2s2s2s*2p2p4s2p2
diamagnetic
Bond order= ½ (8-2)=3
Stronger!
PRACTICE
• Consider the hypothetical species HeH. What
charge, if any, should be present on this
combination of atoms to produce the most
stable molecule or ion possible?
• Draw the MO diagram for the neutral species
first.
• Decide what ion gives largest bond order
• What is the bond order on the neutral species
and what bond order, if changed, is on the ion
you chose?
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H
He
HeH
Neutral
B.O.= ½(2-1)=0.5
Maximum bond
order if you
remove electron
from antibonding
orbital.
Gives +1 ion
PRACTICE
He
H
• If the charge on the neutral
species were increased or
decreased by one what would be
the affect on the bonding of the
atom.
• If the charge on the species you
chose were increased or
decreased by one what would be
the affect on the bonding of the
atom.
HeH
Neutral
B.O.= ½(2-1)=0.5
H
He
HeH
+1
B.O.= ½ (2)=1
Neutral:
If charge was increased: remove
from antibonding giving higher
bond order.
+1:
If charge was increased: remove
from bonding giving lower bond
order.
If charge decreased: add
electron to antibonding giving
lower bond order.
If charge decreased: add
electron to antibonding giving
lower bond order.
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