Bond Order

1
Bond Order

• The difference between the number of bonding
  electrons and the number of antibonding electrons
  divided by two

2
Only outer orbitals bond

• The 1s orbital is much smaller than the 2s
  orbital
• When only the 2s orbitals are involved in
  bonding
• Dont use the s1s or s1s for Li2
• Li2 (s2s)2
• In order to participate in bonds the orbitals
  must overlap in space.

3
Bonding in Homonuclear Diatomic Molecules

• Need to use Homonuclear so that we know the
  relative energies.
• Li2-
• (s2s)2 (s2s)1
• Be2
• (s2s)2 (s2s)2
• What about the p orbitals? How do they form
  orbitals?
• Remember that orbitals must be conserved.

4
B2
5
B2
s2p
s2p
p2p
p2p
6
Expected Energy Diagram
s2p
p2p
p2p
2p
2p
p2p
p2p
s2p
Energy
s2s
2s
2s
s2s
7
B2
2p
2p
Energy
2s
2s
8
B2

• (s2s)2(s2s)2 (s2p)2
• Bond order (4-2) / 2
• Should be stable.
• This assumes there is no interaction between the
  s and p orbitals.
• Hard to believe since they overlap
• proof comes from magnetism.

9
Magnetism

• Magnetism has to do with electrons.
• Paramagnetism attracted by a magnet.
• associated with unpaired electrons.
• Diamagnetism attracted by a magnet.
• associated with paired electrons.
• B2 is paramagnetic.

10
Magnetism

• The energies of of the p2p and the s2p are
  reversed by p and s interacting
• The s2s and the s2s are no longer equally
  spaced.
• Heres what it looks like.

11
Correct energy diagram
s2p
p2p
p2p
2p
2p
s2p
p2p
p2p
s2s
2s
2s
s2s
12
B2
s2p
p2p
2p
2p
s2p
p2p
s2s
2s
2s
s2s
13
Patterns

• As bond order increases, bond energy increases.
• As bond order increases, bond length decreases.
• Supports basis of MO model.
• There is not a direct correlation of bond order
  to bond energy.
• O2 is known to be paramagnetic.
• Movie.

14
Magnetism

• Ferromagnetic strongly attracted
• Paramagnetic weakly attracted
• Liquid Oxygen
• Diamagnetic weakly repelled
• Graphite
• Water Frog

15
Examples

• C2
• N2
• O2
• F2
• P2

16
Heteronuclear Diatomic Species

• Simple type has them in the same energy level, so
  can use the orbitals we already know.
• Slight energy differences.
• NO

17
NO
2p
2p
2s
2s
18
You try

• NO
• CN-
• What if they come from completely different
  orbitals and energy?
• HF
• Simplify first by assuming that F only uses one
  if its 2p orbitals.
• F holds onto its electrons, so they have low
  energy

19
s
1s
2p
s
20
Consequences

• Paramagnetic
• Since 2p is lower in energy, favored by
  electrons.
• Electrons spend time closer to fluorine.
• Compatible with polarity and electronegativity.

21
Names

• sp orbitals are called the Localized electron
  model
• s and p Molecular orbital model
• Localized is good for geometry, doesnt deal well
  with resonance.
• seeing s bonds as localized works well
• It is the p bonds in the resonance structures
  that can move.

22
p delocalized bonding

• C6H6

23
C2H6
24
NO3-