1
Chapter 9
Orbitals and Covalent Bond

2
Atomic Orbitals Don’t Work
 to explain molecular geometry.
 In methane, CH
4 tetrahedral.
, the shape s
 The valence electrons of carbon should be two in s, and two in p.
 the p orbitals would have to be at right angles.
 The atomic orbitals change when making a molecule

3
Hybridization
 We blend the s and p orbitals of the valence electrons and end up with the tetrahedral geometry.
 We combine one s orbital and 3 p orbitals.
 sp
3 hybridization has tetrahedral geometry.

4
2s
In terms of energy
2p
Hybridization sp
3

5

How we get to hybridization
We know the geometry from experiment.
 We know the orbitals of the atom
 hybridizing atomic orbitals can explain the geometry.
 So if the geometry requires a tetrahedral shape, it is sp
3 hybridized
 This includes bent and trigonal pyramidal molecules because one of the sp 3 lobes holds the lone pair.

6 sp 2 hybridization


C
2
H
4
Double bond acts as one pair.
 trigonal planar
 Have to end up with three blended orbitals.
 Use one s and two p orbitals to make sp
2 orbitals.
 Leaves one p orbital perpendicular.

7

8

9
2s
In terms of energy
2p
Hybridization
2p sp
2

10
Where is the P orbital?
 Perpendicular
 The overlap of orbitals makes a sigma bond ( s bond)

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Two types of Bonds
 Sigma bonds from overlap of orbitals.


Between the atoms.
Pi bond ( p bond) above and below atoms
 Between adjacent p orbitals.
 The two bonds of a double bond.

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H
H
C C
H
H

13 sp 2 hybridization
 When three things come off atom.
 trigonal planar


120º on s one p bond

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What about two
 When two things come off.
 One s and one p hybridize.
 linear

15 sp hybridization

End up with two lobes 180º apart.

 p orbitals are at right angles
Makes room for two p bonds and two sigma bonds.
 A triple bond or two double bonds.

16
2s
In terms of energy
2p
Hybridization
2p sp

17
CO
2


C can make two s and two p
O can make one s and one p
O C O

18
Breaking the octet
 PCl
5
 The model predicts that we must use
 the d orbitals.
dsp
3 hybridization
 There is some controversy about how involved the d orbitals are.

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 Trigonal

 bipyrimidal can only s bond.
can’t p bond.
 basic shape for five things.
dsp 3

PCl
5
Can’t tell the hybridization of Cl
Assume sp
3 to minimize repulsion of electron pairs.
20

21 d 2 sp 3
 gets us to six things around
 octahedral

Molecular Orbital Model
 Localized Model we have learned explains much about bonding.

It doesn’t deal well with the ideal of resonance, unpaired electrons, and bond energy.
 The MO model is a parallel of the atomic orbital, using quantum mechanics.
 Each MO can hold two electrons with
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 opposite spins
Square of wave function tells probability

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What do you get?


Solve the equations for H
2
H
A
H
B
 get two orbitals
 MO
1
= 1s
A
- 1s
B
 MO
2
= 1s
A
+ 1s
B

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The Molecular Orbital Model

The molecular orbitals are centered on a line through the nuclei
– MO
1 the greatest probability is between the nuclei
– MO
2 it is on either side of the nuclei
– this shape is called a sigma molecular orbital

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The Molecular Orbital Model

In the molecule only the molecular orbitals exist, the atomic orbitals are gone

MO
1 is lower in energy than the 1s orbitals they came from.
– This favors molecule formation
– Called an bonding orbital
MO
2 is higher in energy
– This goes against bonding
– antibonding orbital

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The Molecular Orbital Model
MO
2
1 s
1 s
MO
1

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The Molecular Orbital Model

We use labels to indicate shapes, and whether the MO’s are bonding or antibonding.
– MO
1
– MO
2
= s
1s
= s
1s
* (* indicates antibonding)

Can write them the same way as atomic orbitals
– H
2
= s
1s
2

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The Molecular Orbital Model

Each MO can hold two electrons, but they must have opposite spins

Orbitals are conserved. The number of molecular orbitals must equal the number atomic orbitals that are used to make them.

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1 s
H
2
s
1s
*
1 s s
1s

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Bond Order
 The difference between the number of bonding electrons and the number of antibonding electrons divided by two
Bond Order =
# bonding-#antibonding
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