CHAPTER 4 - CHEMICAL BONDING

151

4.3 ORBITAL OVERLAP
AND HYBRIDISATION

LEARNING OUTCOME

At the end of this lesson, students should be able to

a) draw and describe the formation of sigma () and
pi () bonds from overlapping orbitals.

b) describe the formation of hybrid orbitals of central

atom : sp, sp2 ,sp3, sp3d and sp3d2

c) illustrate the hybridisation of the central atom and
the overlapping of orbitals in molecules.

153

Covalent Bond and Orbital Overlap

• Valence Bond Theory explains the formation of
covalent bonds and the molecular shape outlined
by the VSEPR.

• The theory states that a covalent bond is formed
when the neighboring atomic orbitals overlap.

154

• Orbitals overlapping may occur between:
a) orbitals with unpaired electrons
b) an orbital with paired electrons and another
empty orbitals (dative bond)

155

Example: The s-orbital of the
Hydrogen atom
HH

Change in electron density as
two hydrogen atoms
approach each other.

High electron density as the
orbitals overlap

(covalent bond formed)

156

Types of covalent bond
• Overlapping of orbitals produces two major

types:
a)  bond (sigma bond)
b)  bond (pi bond)

157

a) bond

• formed when orbitals overlap along its
internuclear axis (end to end overlapping)

• Atoms involved have free rotation about the
axis
Example:
i. overlapping s orbitals

s+ s ss

 bond

158

Overlap of two s orbitals

 bond

+ or

s orbitals

In  bond, the electron density is concentrated
between the nuclei of the bonding atoms

159

 bond

ii. Overlapping of s and p orbitals

s+ Px orbital x

x

 bond

160

Overlap of s and p orbitals  bond
+
or

 bond

161

 bond

iii. Overlapping of end to end p orbitals

x+ x x

 bond

162

Overlap of two p orbitals  bond
or
+
 bond

163

b)  bond

• Formed when two p-orbitals of the same
orientation overlap sideways

yy y  bond

+

164

 bond

+ or

In  bond, the electron density is concentrated
above and below the plane of the nuclei of the
bonding atoms

165

Formation of bonds in a molecule

• Covalent bonds may form by:
a) overlapping of pure orbitals
b) overlapping of hybrid orbitals

166

Overlapping of pure orbitals

Example :
Draw the orbital overlapping for the
following molecules.

i. H2
ii. HF
iii.F2
iv. O2
v. N2

167

Answer

i. H2
Valence orbital diagram:

H : 1s

Lewis structure : H―H

168

• Orbital overlap:

+

 bond

169

ii. HF

Valence orbital diagram;

H: 2p

1s

F:

2s

Lewis structure : H F

170

• Orbital overlap:

 bond

171

iii. F2
Valence orbital diagram;
F : 2s 2p

Lewis structure : F F

172

• Orbital overlap:

 bond

173

iv. O2
Valence orbital diagram;

O: Two unpaired electrons to be
used in bonding.
2s 2p

Lewis structure : O  O



174

Orbital Overlap:



Orbital overlapping occurs

pO σ Op side ways between the p-
orbitals of each atom form

p p a  bond

175

v. N2

Valence orbital diagram;

N : 2s 2p Three unpaired electrons to be
used in bonding.



N NLewis structure :


176

Orbital overlap :

 • 2 p-orbitals of each atom

overlapped side ways to
 form 2  bonds

pN Np

p p • 1 p-orbitals of each atom
p
p overlapped end to end
and form 1σ bonds

177

Hybrid Orbitals

• Valence bond theory is able to explain the
formation of bond pair electron and
observed molecular shape

• Equal repulsion of bonding pair electron
around the central atom suggests that the
orbitals are equal

• Therefore, atomic orbitals of the central
atom are assumed to mix and form new
orbitals called hybrid orbitals

178

• Hybridization is the mixing of different type
of atomic orbitals to form a set of equivalent
orbitals

• 5 types of orbital hybridizations are:
sp
sp2
sp3
sp3d
sp3d2

179

How do I predict the hybridization of the central atom?

Count the number of lone pairs AND the number of
atoms bonded to the central atom

No of Lone Pairs Hybridization Examples
+ sp BeCl2
sp2 BF3
No of Bonded Atoms sp3
2 sp3d CH4, NH3, H2O
PCl5
3 sp3d2 SF6 180

4

5

6

sp3 hybridization

• sp3 hybridization is the mixing of 1 s orbital
and 3 p orbitals to form 4 equivalent sp3
orbitals.

• The shape of the four hybrid orbitals is
tetrahedral with the angle of 109.5o

181

s px sp3 sp3

py pz sp3 sp3
one s + three p orbitals four sp3 orbitals

182

• simplified drawing of sp3 orbitals:

sp3

sp3

sp3 Shown together (large

sp3

lobes only)

183

1) CH4 Example:

H

• Lewis structure : H CH
H

• Valence orbital diagram ;

H : 1s Excitation: to have 4 unpaired electrons
C ground state :
2p
2s

C excited : 2s 2p 184
C hybrid :
3

sp3-Hybridized C atom in CH4

sp3

1s sp3 sp3 1s

sp3

1s

185

Example:

2) NH3 ..
• Lewis structure :
H NH

H

• Valence orbital diagram;

H ground state : 1s 2p
N ground state : 2s 2p
N excited state : 2s sp3

N hybrid : 186

Orbital Overlap:

s p3

 N s p3

H s p3 H

1s  sp3 1s

H

1s

Molecular Geometry : Trigonal pyramidal

187

3) H2O H
 Lewis structure : HO

 Valence orbital diagram;
O ground state :

2s 2p

O excited state :

2s 2p

O hybrid : sp3

H ground state : 1s 188

Answer

Orbital Overlap:

s p3

 O s p3

H s p3

1s  sp3

H

1s
Molecular Geometry : V- shaped

189

sp2 hybridization

• sp2 hybridization is the mixing of 1 s orbital
and 2 p orbitals to form 3 equivalent sp2
orbitals.

• The shape of the three hybrid orbitals is
trigonal planar with the angle of 120o

190

s sp2

px py sp2 sp2
one s orbital + two p orbitals three sp2 orbitals

191

• simplified drawing of sp2 orbitals:

sp2

Shown together (large
lobes only)

sp2 sp2

192

Example:

1) BF3 F F
• Lewis structure : B

F

• Valence orbital diagram;

F ground state : 2s 2p

B ground state : 2p

2s

B excited : 2s 2p

B hybrid : 193

sp2

p p



sp2 sp2

Shape: trigonal planar sp2



p

194

Example:

2) C2H2

• Lewis structure : H C CH
•
Valence orbital diagram;

C ground state :

2s 2p

C excited :

2s 2p

C hybrid :

sp 2p

H ground state :
1s

195

Orbital Overlap: Example:

C2H2 

H  H

1s 1s

s p2 s p2

C  C
sp2 
sp2 sp2
H 1s
 sp2

H

1s

196

 bonds

 bond

197

3) C6H6 Example:
H

H C C H
C

• Lewis structure : CC
HCH

H

• Valence orbital diagram;

C ground state : 2s 2p
C excited : 2s 2p

C hybrid : sp2 2p
H ground state : 1s

198

199

200