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