CHAPTER 4 - CHEMICAL BONDING

Solution

(+1) (+1)

a. (-2) Even though Be is octet, formal charge of

Cl Be Cl Be is -2 , Be is an electropositive element.

(0) (0) Be is not octet, however formal
(0) charge of all atoms are zero, it is the
most plausible Lewis structure.
Cl Be Cl

• A plausible Lewis structure need to fulfilled
the appropriate formal charge of every atom.

• Thus, Be form incomplete octet

51

Solution

(0) (-1) (+1)

b. F B- F+ Even though B is octet, formal charge of B is

-1, B is an electropositive element.

F

(0)

(0) (0) (0) B is not octet, however formal charge of

F B F all atoms are zero, , it is the most
F plausible Lewis structure.

(0)

• A plausible Lewis structure need to fulfilled the
appropriate formal charge of every atom.

• Thus, B form incomplete octet

52

Solution

(0) (-1) (+1) Even though Al is octet, formal charge of Al
is -1, Al is an electropositive element.
c. Br Al- Br+

Br

(0)

(0) (0) (0)

F Al F Al is not octet, however formal charge

of all atoms are zero, it is the most

F plausible Lewis structure.

(0)

• A plausible Lewis structure need to fulfilled the
appropriate formal charge of every atom.

• Thus, Al form incomplete octet

53

b) Expanded Octet

 Occurs when central atom has more than 8
electrons.

 Atoms of the elements from period 3 or higher
can expand their valence shell due to the
existence of empty d orbitals.

 Examples of element that can form expanded
octet are phosphorous (P), sulphur (S), iodine
(I), selenium (Se)

54

Example:
Draw the Lewis structure for:

i) PCl5 Cl Cl
Cl P Cl

Cl

ii) SF6 FF

FSF
FF

55

c) Odd number electrons
• The central atom of some molecule may contain

an odd number of valence electrons.
• Nitrogen may form compounds that contain odd

number electrons.

Example:
Nitric oxide, NO
Nitrogen dioxide, NO2

56

Resonance

• Resonance structures are two or more plausible
Lewis structures for a molecule/ion whereby cannot
be represented accurately by only one Lewis
structure.

57

Resonance

• Resonance structures are two or more plausible
Lewis structures for a molecule/ion whereby cannot
be represented accurately by only one Lewis
structure.

Example: ozone (O3)

O OO OO O

 These structures have the same relative placement of
atoms but differ in the distribution of valence electrons.

• Each structure is called a resonance structure

• Double headed arrow (↔) indicates that the structures

are resonance to each other. 58

Example:
Draw the resonance structures of nitrate ion,
NO3

59

Solution

60



4.2 Molecular Shape
and Polarity

i. VSEPR theory
ii. Electron pair arrangements

and Molecular shape
iii. Bond and molecule’s polarity

Learning Outcomes

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

a) explain Valence Shell Electron Pair Repulsion
(VSEPR) theory.

b) draw the basic molecular shapes : linear, trigonal
planar, tetrahedral, trigonal bipyramidal and
octahedral.

c) predict and explain the shapes of molecule and
bond angles in a given species.

d) explain bond polarity and dipole moment.
e) deduce the polarity of molecules based on the

shapes and the resultant dipole moment.

63

Valence-Shell Electron-Pair Repulsion
(VSEPR) Theory

• Valence-shell electron-pair repulsion
(VSEPR) theory emphasizes that each group
of valence electrons around the central
atoms is located as far away as possible from
the others in order to minimize repulsions.

• Central atom is surrounded by bonding pair
electrons or/and lone pair electrons, thus create
repulsions between them.

64

• The electron repulsions at the central atom
are between lone pair – lone pair , lone pair-

bonding pair and bonding pair-bonding pair.

bonding pair – A lone pair
bonding pair X
repulsion lone pair – lone pair
repulsion
A
lone pair – bonding pair
bonding pair repulsion

A

65

• Lone-pair electrons spread out more than
bonding-pair electrons, so the repulsion of lone-
pair electrons is greater than other electron-
pairs.

• Therefore, the order of the repulsive forces
strength is

Lone pair-lone pair > lone pair- bonding pair >
bonding pair-bonding pair

66

• Due to these electron pair - electron pair
repulsion, molecules have different shapes
and bond angles.

• Thus, the VSEPR model is used to predict
the shapes of covalent molecules and
polyatomic ions.

67

Electron-pair arrangement and molecular
shape

• Electron-pair arrangement is the distribution
of electron-pair about the central atom.

• It is used to predict the molecular shape of a
molecule or ion.

• Molecular shape is the geometric arrangement
of the atomic nuclei (the actual molecular
shape)

68

• The distribution of electron-pairs around the
central atom are two, three, four, five and six
electron pairs with certain bond angles.

• Thus, result five geometric patterns : linear,
trigonal planar, tetrahedral, trigonal
bipyramidal and octahedral respectively.

• The electron-pairs arrangements and the bond
angles are shown in the table below :

69

Number of Electron-pair Arrangement
electron-pairs

2

electron
pair

3

70

Number of Electron-pair Arrangement
electron-pairs

4

71

Number of Electron-pair Arrangement
electron-pairs

5

72

Number of Electron-pair Arrangement
electron-pairs

6

73

Molecular shape of molecule or ion

• It is important to identify the electron-pair
about the central atom to determine the
molecular shape

• Central atom are surrounded by :
i. only bonding pair electrons or
ii. bonding pair and lone pair electrons

74

• If the central atom has :

i. only bonding pair electrons - Assign the molecule
as ABm where A (central atom), B (terminal atom)
and m is intergers . The shape of the molecule
adopt the electron-pair arrangement and called the
basic shape.

ii. bonding pair and lone pair electrons – the
shape is different from the basic shape.

So, assign as ABmEn where A (central atom),
B (terminal atom), E (lone pair), m and n are

intergers 75

I) Molecular shape for central atom with only
bonding pair electrons (Basic Shape)

2 electron pairs in the valence shell of central atom:

Class of Number of Number of Shape
molecules bonding lone pairs
pairs 180°

AB2 2 0

Linear

76

3 electron pairs in the valence shell of central atom:

Class of Number of Number of Shape
molecules bonding lone pairs
pairs

AB3 120°

30

trigonal planar
77

4 electron pairs in the valence shell of central atom:

Class of Number of Number of Shape
molecules bonding lone pairs
pairs

AB4 4 0

Tetrahedral
78

5 electron pairs in the valence shell of central atom:

Class of Number of Number of Shape
molecules bonding lone pairs
pairs

90°

AB5 50 120°

Trigonal bipyramidal
79

6 electron pairs in the valence shell of central atom:

Class of Number of Number of Shape
molecules bonding lone pairs
pairs

AB6 90° 90°

60

Octahedral
80

II) Molecular shape for central atom with lone pair
and bonding pair electrons

3 electron pairs in the valence shell of central atom:

Class of Number of Number of Shape
molecules bonding lone pairs
pairs

AB2E 2 1

Bent / V-shaped

81

4 electron pairs in the valence shell of central atom:

Class of Number of Number of Shape
molecules bonding lone pairs
pairs

AB3E 3 1
Trigonal pyramidal

82

4 electron pairs in the valence shell of central atom:

Class of Number of Number of Shape
molecules bonding lone pairs
pairs

AB2E2 2 2

Bent / V-shaped

83

5 electron pairs in the valence shell of central atom:

Class of Number of Number of Shape
molecules bonding lone pairs
pairs

AB4E 4 1

Distorted tetrahedral
(see-saw)

84

5 electron pairs in the valence shell of central atom:

Class of Number of Number of Shape
molecules bonding lone pairs
pairs

AB3E2 3 2

T-shaped

85

5 electron pairs in the valence shell of central atom:

Class of Number of Number of Shape
molecules bonding lone pairs
pairs

AB2E3 2 3

Linear

86

6 electron pairs in the valence shell of central atom:

Class of Number of Number of Shape
molecules bonding lone pairs
pairs

AB5E 5 1

Square pyramidal

87

6 electron pairs in the valence shell of central atom:

Class of Number of Number of Shape
molecules bonding lone pairs
pairs

AB4E2 4 2

Square planar

88

As a conclusion :
Molecules of the same electron-pair
arrangement, but different repulsive force
will have different bond angles thus have
different molecular shape.

89

No. of Class of No. of bonding No. of Shape
electron molecule pair electron, lone pair,
linear
pair AB2 B E Trigonal planar
AB3 Bent / V-shaped
2 AB2E 2 0
3 AB4 3 0 Tetrahedral
4 AB3E 2 1 Trigonal pyramidal
AB2E2 4 0
5 AB5 3 1 Bent / V-shaped
AB4E 2 2 Trigonal bipyramidal
6 5 0 See-saw /distorted
AB3E2 4 1 tetrahedral
AB2E3
3 2 T-shaped
AB6 2 3 linear
AB5E 6 0
AB4E2 5 1 octahedral
4 2 Square pyramidal

Square planar 90

Steps to predict a molecular shape :

1) Draw the correct Lewis structure

2) Determine number of electron-pair

Each multiple bond in a molecule constitutes a single electron
domain (single bond)

3) Apply the VSEPR theory

4) Identify electron-pair arrangement.

5) State the class of molecule.

6) Draw the shape and state the bond angle
based on VSEPR

91

Example
Predict the molecular shape of
i. CCl4
ii. SeF4
iii. XeF4

92

Solution 5) VSEPR theory :
repulsion of bonding pair
i. CCl4 electrons are equal

1) Lewis structure : Cl 6) The shape :
Valence e- C = 4 Cl C Cl
4Cl = 28
32

bonding - 8 Cl Cl 109.5o

24 C
Cl
2) No of electron pair: Cl

4 electron pairs and all are bonding Cl

pair electrons

3) Class of molecule : AB4 tetrahedral
Bond angle Cl-C-Cl: 109.5 °
4) Electron-pair arrangement :
Tetrahedral

93

Solution

ii. SeF4 3) Class of molecule : AB4E

1) Lewis structure :

Valence e- Se = 6 4) Electron-pair arrangement :
Trigonal bipyramidal
4F = 28

34

bonding - 8 5) VSEPR theory :
repulsion of bonding pair -lone pair
26 electrons > bonding pair- bonding
pair electrons
F -24
F Se F 2

F 6) The shape :

2) No of electron pair: FF see- saw shaped
5 electron pairs: Se Bond angle F-Se-F :
4 bonding pair electrons
1 lone pair electron F <120° , < 90°
F
94

Solution

iii. XeF4 3) Class of molecule : AB4E2

1) Lewis structure :

Valence e- Xe = 8 4) Electron-pair arrangement :
octahedral
4F = 28
5) VSEPR theory :
36 repulsion of lone pair-lone pair
electron > bonding pair -lone pair
bonding - 8 electrons > bonding pair- bonding
pair electrons
26
6) The shape :
F -24
F Xe F 4

F

2) No of electron pair: FF Square planar shaped
6 electron pairs: Xe Bond angle F-Xe-F :
4 bonding pair electrons 90°
2 lone pair electron FF
95

Example:

Draw the Lewis structure and determine its shape for
each of the following:

1. CH4 9. SF4 96
2. BeCl2 10. ClF3
3. BF3 11. BrF5
4. NH3 12. I3-
5. H2O 13. NO2-
6. SO2 14. XeF2
7. PCl5 15. IF5
8. SF6 16. CH2Cl2

Solution 5) VSEPR theory :
repulsion of bonding
1. CH4 pair-bonding pair
electrons are equal.
1) Lewis structure :

Valence e- C = 4 H

4H=4 HC H 6) The shape :
8
H
bonding - 4 H

4 109.5

2) No of electron pair: C

4 electrons pair and all are bonding HH H

pair electrons

3) Class of molecule : AB4 tetrahedral
Bond angle H-C-H: 109.5°

4) Electron-pair arrangement :
Tetrahedral

97

Solution 5) VSEPR theory :
repulsion of bonding
2. BeCl2 pair-bonding pair
electrons are equal.
1) Lewis structure :

Valence e- Be = 2

2Cl = 14 Cl Be Cl

16 6) The shape :

bonding -4

12 180o

2) No of electron pair: Cl Be Cl
2 electrons pair and all are bonding
pair electrons

3) Class of molecule : AB2 linear
Bond angle Cl-Be-Cl: 180°

4) Electron-pair arrangement :
linear

98

Solution 5) VSEPR theory :
repulsion of bonding
3. BF3 pair-bonding pair
electrons are equal.
1) Lewis structure :
6) The shape :
Valence e- B = 3 FB F
F F 120o
3 F = 21
BF
24 F

bonding -6 Trigonal planar
Bond angle F-B-F: 120°
18
99
2) No of electron pair:
3 electrons pair and all are bonding
pair electrons

3) Class of molecule : AB3

4) Electron-pair arrangement :
trigonal planar

Solution 5) VSEPR theory :
repulsion of lone pair –
4. NH3 bonding pair electrons
>bonding pair-bonding pair
1) Lewis structure : electrons.

Valence e- N = 5 HN H 6) The shape :
H
3H=3 N
HH
8
H 107.5o
bonding -6
Trigonal pyramidal
2 Bond angle H-N-H: 107.5°

2) No of electron pair: 100
4 electrons pair and 3 bonding pair
electrons and 1 lone pair electron

3) Class of molecule : AB3E

4) Electron-pair arrangement :
tetrahedral