Chapter 9Molecular Geometriesand Bonding Theories
Multiple Bonds
Formation of two π bonds in acetylene
Fig 9.26
Fig 9.27 Formation of σ and π bonds in formaldehyde, CH2O
Describing σ and π bonds in a molecule
formaldehyde σ
σ
σ
π
Sigma () and Pi Bonds ()
Single bond 1 sigma bond
Double bond 1 sigma bond and 1 pi bond
Triple bond 1 sigma bond and 2 pi bonds
How many and bonds are in the acetic acid(vinegar) molecule CH3COOH?
C
H
H
CH
O
O H bonds = 6 + 1 = 7
bonds = 1
Molecular Orbital (MO) Theory
• If waves interact constructively, the resulting orbital is lower in energy: a bonding molecular orbital.
• If waves interact destructively, the resulting orbital is higher in energy: an antibonding molecular orbital.
In MO theory, we invoke the wave nature of electrons
MO Theory
In H2 the two electrons go into the bonding molecular orbital.
The bond order is one half the difference between the number of bonding and antibonding electrons:
Bond order = ½ (no. of bonding e− – no. of antibonding e−)
Here: ½ (2-0) = 1
MO Theory
In the case of He2, the bond order would be:
• Therefore, MO theory predicts that He2 does not exist, which we know to be true.
Here: ½ (2-2) = 0
Fig 9.35
• In the case of He2+, the
bond order would be:
MO TheoryHe2
+
½ (2-1) = 1/2
• Therefore, MO theory predicts that He2+ does exist and it will be
relatively stable
MO Theory – Second-Row Diatomics
Consider only homonuclear diatomic molecules
• Number of MOs = number of AOs combined
• AOs combine most effectively with other AOs of similar energy
• The greater the overlap of AOs, the lower the energy of MO
• Each MO can hold max of 2 electrons (Pauli exclusion)
• Hund’s rule applies (same spin in degenerate orbitals)
MOs for Li2 and Be2
Fig 9.37 Energy-level diagram for the Li2 molecule
For atoms with both s and p orbitals, there are two types of interactions:
The p orbitals that are head to head overlap in fashion.
The other two sets of p orbitals overlap in fashion.
MOs from 2p Atomic OrbitalsFig 9.38
• There are both and bonding molecular orbitals and * and * antibonding molecular orbitals
• Diagram fits only O2 and F2
MO Theory – Second-Row DiatomicsFig 9.43
MO Theory• The smaller p-block elements in the
second period have a sizeable interaction between the s and p orbitals:
• This flips the order of the and molecular orbitals in O2 and F2
Fig 9.45
Fig 9.44
Fig 9.48 Paramagentism of O2
Figure 09.48
Fig 9.48 Paramagentism of O2
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