9
1
Intermediate Type
of Bonding
9.1
Incomplete Electron Transfer in Ionic
Compounds
9.2
Electronegativity of Elements
9.3
Polarity of Covalent Bonds
Pure ionic and covalent bonds are only extremes
of a continuum.
Most chemical bonds are intermediate between
the two extremes.
Pure covalent
2
Intermediate
Pure ionic
Pure covalent
Equal sharing of
electrons
Symmetrical
distribution of
electron cloud
Non-polar
molecule
3
Intermediate
Pure ionic
Complete
transfer of
electrons
Spherical
electron clouds
Electron cloud of
D is not
polarized by C+
Pure covalent
Intermediate
Pure ionic
Incomplete transfer of electrons
Or
Unequal sharing of electrons
Polar molecule with partial –ve charge on B
and partial +ve charge on A
4
Polarization of a covalent bond means the
displacement of shared electron cloud towards
the more electronegative atom (Cl).
Polarization of a covalent bond results in a
covalent bond with ionic character.
5
Polarization of an ionic bond
means the distortion of the
electron cloud of an anion
towards a cation by the
influence of the electric field
of the cation.
Polarization of an ionic bond
results in an ionic bond with
covalent character.
6
Pure ionic bond does not exist
LiF(g)
Li+
7
Electron clouds
are not
perfectly
spherical
F
Slight distortion
or sharing of
electron cloud
Polarization of ionic bond Incomplete Transfer of
Electron
8
Determination of Lattice Enthalpy
1. Experimental method : from Born-Haber cycle
9
-349
-791.4
10
Determination of Lattice Enthalpy
1. Experimental method : from Born-Haber cycle
2. Theoretical calculation : based on an ionic model
11
Ionic model : Assumptions
1. Ions are spherical and have no distortion
of electron cloud, I.e. 100% ionic.
2. Oppositely charged ions are in direct
contact with each other.
r+ + r 
12
3. The crystal has certain assumed lattice
structure.
4. The interaction between oppositely
charged ions are electrostatic in nature.
ΔH
o
lattice
MLQQ

4π0 (r  r )
5. Repulsive forces between oppositely
charged ions at short distances are
ignored.
13
Comparison of theoretical and
experimental values of lattice enthalpy
Discrepancy : Reveals the nature of the bond in the
compound
14
Compound
NaCl
NaBr
NaI
KCl
KBr
KI
Lattice enthalpy (kJ mol-1)
Theoretical Experimental % deviation
-766.1
-766.4
0.04
-730.5
-733.0
0.74
-685.7
-688.3
0.38
-692.0
-697.8
0.84
-666.5
-672.3
0.87
-630.9
-631.8
0.14
Good agreement between the two values for alkali halides
 The simple ionic model used for calculating the
theoretical value holds true
 All alkali halides are typical ionic compounds
15
Compound
AgCl
AgBr
AgI
Zns
Lattice enthalpy (kJ mol-1)
Theoretical Experimental % deviation
-833.0
-890.0
6.8
-808.0
-877.0
8.5
-774.0
-867.0
12
-3427.0
-3615.0
5.5
Silver halides and zinc sulphide show large discrepancies
between the two values.
 Silver halides and zinc sulphide are NOT purely
ionic compounds
16
Compound
AgCl
AgBr
AgI
Zns
Lattice enthalpy (kJ mol-1)
Theoretical Experimental % deviation
-833.0
-890.0
6.8
-808.0
-877.0
8.5
-774.0
-867.0
12
-3427.0
-3615.0
5.5
The experimental values are always more negative than
the theoretical values
 Polarization of a chemical bond always results in a
stronger bond.
17
The real picture of the polarized bond can be
considered as a resonance hybrid of the two
canonical forms.
E.g.
Ag+ Cl
Purely
ionic
Ag–Cl
Purely
covalent

 AgCl  a Ag Cl  b AgCl


Large % deviation of lattice enthalpy
 greater b and more covalent character
18
The real picture of the polarized bond can be
considered as a resonance hybrid of the two
canonical forms.
E.g.
Ag+ Cl
Purely
ionic
Ag–Cl
Purely
covalent

 AgCl  a Ag Cl  b AgCl


Small % deviation of lattice enthalpy
 smaller b and less covalent character
19
Factors that Favour Polarization of Ionic
Bond – Fajans’ Rules
For cations
Polarizing power :
- The ability of a cation to polarize the
electron cloud of an anion.
Polarizing power 
as the
20
charge
si ze
of the cation 
Q.50(a)
Charge : Al3+ > Mg2+ > Na+
Size :
Al3+ < Mg2+ < Na+
Charge
: Al3+ > Mg2+ > Na+
Size
Polarizing power : Al3+ > Mg2+ > Na+
21
Q.50(b)
Charge : Li+ = Na+ = K+
Size :
Li+ < Na+ < K+
Charge
: Li+ > Na+ > K+
Size
Polarizing power : Li+ > Na+ > K+
22
For anions
Polarizability : A measure of how easily the electron cloud
of an anion can be distorted or polarized by
a cation.
Polarizability  as the size of the anion 
Polarizability  as the charge of the anion 
23
Polarizability  as the size of the anion 
Larger size of anion
 outer electrons are further
away from the nucleus
 electrons are less firmly
held by the nucleus and are
more easily polarized by
cations
I > Br > Cl > F
S2 > O2
24
Compound
AgCl
AgBr
AgI
ZnS
Lattice enthalpy (kJ mol-1)
Theoretical Experimental % deviation
-833.0
-890.0
6.8
-808.0
-877.0
8.5
-774.0
-867.0
12
-3427.0
-3615.0
5.5
Polarizability : I > Br > Cl
% deviation : AgI > AgBr > AgCl
Covalent character : AgI > AgBr > AgCl
25
Compound
AgCl
AgBr
AgI
ZnS
Lattice enthalpy (kJ mol-1)
Theoretical Experimental % deviation
-833.0
-890.0
6.8
-808.0
-877.0
8.5
-774.0
-867.0
12
-3427.0
-3615.0
5.5
Great % deviation of ZnS due to high
polarizability of the large S2 ion
26
Polarizability  as the charge of the anion 
Higher charge in the anion results in
greater repulsion between electrons
 electrons are less firmly held by the
nucleus and are more easily polarized
by cations
27
Compound
NaCl
NaBr
NaI
AgCl
AgBr
AgI
Lattice enthalpy (kJ mol-1)
Theoretical Experimental % deviation
-766.1
-766.4
0.04
-730.5
-733.0
0.74
-685.7
-688.3
0.38
-833.0
-890.0
6.8
-808.0
-867.0
8.5
-774.0
-867.0
12
Ionic radius : Ag+ > Na+
Why are AgX more covalent than NaX ?
28
Ag+ = [Kr] 5s1 4d9
Na+ = Ne
The valence 4d electrons are less penetrating
 They shield less effectively the electron cloud
of the anion from the nuclear attraction of
the cation
 The electron cloud of the anion experiences a
stronger nuclear attraction
Ag+ has a higher ENC than Na+
Polarizing power : Ag+ > Na+
29
Ag+ = [Kr] 5s1 4d9
Na+ = Ne
Noble gas configuration of the cation produces
better shielding effect and less polarizing power
Polarizing power : Ag+ > Na+
30
Q.51(a)
Solubility in water : NaX >> AgX
AgX has more covalent character due to higher
extent of bond polarization.
Thus, it is less soluble in water
31
Q.51(b)
Solubility in water : AgF > AgCl > AgBr > AgI
Polarizability : F < Cl < Br < I
Extent of polarization : F < Cl < Br < I
Ionic character : AgF > AgCl > AgBr > AgI
32
Q.51(c)
Solubility in water : Gp I carbonates >> other carbonates
Carbonate ions are large and carry two negative
charges. Thus, they can be easily polarized by
cations to exhibit more covalent character.
However, ions of group I metals have very small
charge/size ratio and thus are much less
polarizing than other metal ions.
Gp I carbonates have less covalent character
33
Q.51(d)
Solubility in water : LiX << other Gp I halide
Li+ is very small and thus is highly polarizing.
LiX has more covalent character
Example 9-1
Check Point 9-1
34
Fajans’ rules – A summary
35
Ionic
Covalent
Low charge on ions
High charge on ions
Large cation
Small cation
Small anion
Large anion
Noble gas
configuration
Valence shell electron
configuration with
incomplete d/f subshell
Apart from those compounds mentioned on p.63,
list THREE ionic compounds with high covalent
character.
AlCl3 , MgI2 , CuCO3
36
Polarization of Covalent Bond : –
Unequal Sharing of electrons
Evidence : 1. Deflection of a jet of a polar liquid(e.g. H2O) in
a non-uniform electrostatic field
2. Breakdown of additivity rule of covalent radii
3. Breakdown of additivity rule of bond enthalpies
37
Liquid shows
deflection
Liquid shows
no deflection
38
Contains
polar
molecules
Contains
non-polar
molecules
a charged
rod
deflection Deflection of a polar
of water liquid (water) under the
influence of a charged rod.
39
a positively charged rod
a polar molecule
Orientation of polar molecules towards a positively
charged rod.
Demonstration
40
Solvents showing a
marked deflection
Trichloromethane, CHCl3
Ethanol,CH3CH2OH
Propanone
Water, H2O
41
Solvents showing no
deflection
Tetrachloromethane
Cyclohexane
Benzene
Carbon disulphide
A stream of water is attracted (deflected) to a
charged rod, regardless of the sign of the
charges on the rod. Explain.
H

O
H
        
 
42
 
  
 




H
+
O
H +
Additivity rule of covalent radii
Assumption : Electrons are equally shared between A and B
Pure covalent bond
43
Bond
CBr in
CBr4
CF in
CF4
CO in
CH3OH
CO in
CO2
Experimental
value/nm
0.1940
0.1320
0.1430
0.1160
Estimated bond
length/nm
0.1910
0.1480
0.1510
0.1275
% deviation
-1.54%
12.12%
5.59%
9.91%
Failure of additivity rule indicates formation of
covalent bond with ionic character due to polarization of
shared electron cloud to the more electronegative atom.
44
Bond
CBr in
CBr4
CF in
CF4
CO in
CH3OH
CO in
CO2
Experimental
value/nm
0.1940
0.1320
0.1430
0.1160
Estimated bond
length/nm
0.1910
0.1480
0.1510
0.1275
% deviation
-1.54%
12.12%
5.59%
9.91%
Polarization of a covalent bond always results in the
formation a stronger bond with shorter bond length.
+

C
45
F
Breakdown of additivity rule of bond enthalpy
E(H – H) = 436 kJ mol1
E(F – F) = 158 kJ mol1
Equal sharing of
electrons
E(H  H)  E(F  F)
 297 kJ mol1
2
A.M.
E(H  H)  E(F  F)  262 kJ mol1 G.M.
E(H – F) = 565 kJ mol1 >> A.M. or G.M.
46
E(H – F) = 565 kJ mol1 >> A.M. or G.M.
Greater difference
 Higher extent of bond polarization
 Greater difference in electronegativity values
of bonding atoms
Pauling Scale of Electronegativity (1932)
47
For the molecule A–X
96nA  nX   E(A  A)  E(X  X)  E(A  X)
2
nA and nX are the electronegativity values of A
and X respectively
nF = 4.0
48
Q.52
Given : E(H–H)  436 kJ mol1 , E(F–F)  158 kJ mol1 ,
E(H–F)  565 kJ mol1 , E(Cl–Cl)  242 kJ mol1 ,
E(H–Cl)  431 kJ mol1
Calculate the electronegativity values of H and Cl.
More electronegative
96(4.0 nH )2 
436 158  565
nH = 2.2
96(nCl  2.2)2 
nCl = 3.3
49
436 242  431
Estimation of Ionic Character of Chemical Bonds
Two methods : 1. The difference in electronegativity between
the bonding atoms nA – nX  (Qualitative)
2. The electric dipole moment of diatomic
molecule (Quantitative)
50
1. The difference in electronegativity between
the bonding atoms nA – nX  (Qualitative)
nA – nX  2.0
ionic or nearly ionic bond
e.g. Li – F bond (4.0 – 1.0) = 3.0
nA – nX  0.4
covalent or nearly covalent bond
e.g. C – H bond (2.5 – 2.1) = 0.4
0.4  nA – nX  2.0
covalent bond with ionic character or
ionic bond with covalent character
51
2. The electric dipole moment of diatomic
molecule (Quantitative)
 =qd
SI units : Coulomb meter
1 Debye (D) = 3.3361030 Coulomb meter
52
Centre of postive charge
Electric dipole moment is a vector pointing from
the positive pole to the negative pole
53
Estimating the % ionic character of H–Cl bond by
dipole moment
Molecule
Dipole moment
(Coulomb meter)
Bond length
meter
H–Cl
3.6891030
1.2841010
Electronic charge, e  1.6021019 Coulomb
54
If H–Cl is 100% ionic,
dipole moment
 1.6021019 Coulomb1.2841010 meter
 2.0571029 Cm
The measured dipole moment of H–Cl
 3.6891030 Cm
3.689 1030 Cm
% ioniccharacter 
 100%  17.9%
29
2.057 10 Cm
55
Q.53 Electronic charge, e  1.6021019 Coulomb
Molecule
NO
HI
ClF
HF
CsF
Bond
length(Å)
1.154
1.620
1.632
0.926
2.347
Dipole
moment(
D)
0.159
0.448
0.888
1.827
7.884
% ionic
character
2.87
14.8
11.3
41.1
70.0
56
Q.53 Electronic charge, e  1.6021019 Coulomb
Molecule
NaCl
KF
KCl
LiF
Bond
length(Å)
2.365
2.176
2.671
1.570
Dipole
moment(
D)
9.001
8.593 10.269 6.327
% ionic
character
79.3
57
82.2
80.1
83.9
Calculated from
dipole moment
Good correlation
between two methods
nA – nX 
58
How do you expect the bond type to change for the
chlorides of the third period elements, NaCl, MgCl2, AlCl3,
SiCl4, PCl5, SCl2 and Cl2, going from left to right?
Explain the change in the bond type.
NaCl
Purely
Ionic
59
MgCl2
AlCl3
Ionic with
covalent
character
SiCl4
PCl5
SCl2
Polar covalent
Cl2
Purely
covalent
 difference in
electronegativity
values
NaCl
Purely
Ionic
60
MgCl2
AlCl3
Ionic with
covalent
character
 difference in
electronegativity
values
SiCl4
PCl5
SCl2
Polar covalent
Cl2
Purely
covalent
 extent of
polarization of
ionic bond
NaCl
Purely
Ionic
61
MgCl2
AlCl3
Ionic with
covalent
character
 extent of
polarization of
covalent bond
SiCl4
PCl5
SCl2
Polar covalent
Cl2
Purely
covalent
Polarity of Molecules
depends on : 1. Polarity of bonds
 nA – nX  or dipole moment
2. Geometry of molecules
Symmetrical molecules are usually non-polar
due to symmetrical arrangements of dipole
moments
62
63
Bond polarity
Geometry of
molecule
Polarity of
molecule
Polar
Asymmetrical
Polar
Polar
Symmetrical
Non-polar
Non-polar
Asymmetrical
Non-polar
Non-polar
Symmetrical
Non-polar
The overall dipole moment of a molecule is the
vector sum of dipole moments of individual bonds
and lone pairs.
O
C
O
Net dipole moment (the vector sum) is zero
 Non-polar
64
The overall dipole moment of a molecule is the
vector sum of dipole moments of individual bonds
and lone pairs.
F
F
65
B
F
The overall dipole moment of a molecule is the
vector sum of dipole moments of individual bonds
and lone pairs.
F
F
B
F
Net dipole moment (the vector sum) is zero
 Non-polar
66
The overall dipole moment of a molecule is the
vector sum of dipole moments of individual bonds
and lone pairs.
Cl
Cl
C
Cl
Cl
Net dipole moment (the vector sum) is zero
 Non-polar
67
The overall dipole moment of a molecule is the
vector sum of dipole moments of individual bonds
and lone pairs.
68
Q.54
H
+
N
+
H
H

F
N
F

+
or
69

F
Q.55
O
O
S
O
Non-polar
70
O
S
Polar
O
Q.55
F
F
F
S
F
F
F
Symmetrical  Non-polar
71
F
F
F
F
F
Xe
F
F
72
Xe
Dipole moments
of the two lone
pairs point in
opposite
directions
Non-polar
F
Q.55
H
H
C
C
H
H
Non-polar
73
Q.55
Non-zero vector sum
 Polar molecule
74
Q.56(a)
Cl
H
75
C
Br
H
H
>
H
C
I
H
H
>
H
C
H
H
Q.56(b)
Cl
Cl
Cl
Cl
>
>
Cl
Cl
76
Explain the following phenomena:
(a) PCl3 is polar but BCl3 is non-polar.
BCl3 has three polar B−Cl bonds and is trigonal
planar in shape. As the dipole moments of the
three polar bonds cancel out each other, the
molecule is non-polar.
Cl
B
Cl
77
Cl
Explain the following phenomena:
(a) PCl3 is polar but BCl3 is non-polar.
PCl3 has three polar P−Cl bonds and is trigonal
pyramidal in shape. As there is a resultant
dipole moment arising from the three polar
bonds, the molecule is polar.
P
Cl
78
Cl
Cl
Explain the following phenomena:
(b) Both NBr3 and NF3 are polar but their
molecules align differently in a non-uniform
electrostatic field.
79
(b) As the order of electronegativity is F > N > Br,
the resultant dipole moments of NBr3 and NF3
are pointing to different directions. The
situations are shown below:
80
In a non-uniform electrostatic field, the
nitrogen end of NBr3 will point to the positive
pole while the nitrogen end of NF3 will point to
the negative pole.
81
Non-polar molecules
Shape
Linear
Trigonal
planar
Tetrahedral
82
Molecule
Cancelling out of
dipole moments
Non-polar molecules
Shape
Trigonal
bipyramidal
Octahedral
83
Molecule
Cancelling out of
dipole moments
Polar molecules
Shape
V-shaped
( or bent)
Trigonal
pyramidal
Tetrahedral
84
Molecule
Dipole
Net resultant
moment of
dipole
individual
moment
polar bonds
Use of dipole moments
• Provide important structural information
about molecules
85
9.1 Incomplete electron transfer in ionic compounds (SB p.250)
The following gives the theoretical and experimental values
of the lattice enthalpies of two metal bromides. X+Br- and
Y+Br-.
Compound
X+Br-(s)
Theoretical
lattice enthalpy
(kJ mol-1)
-665
Experimental
lattice enthalpy
(kJ mol-1)
-670
Y+Br-(s)
-758
-890
(a) There is a high degree of agreement between the
theoretical and experimental values in the case of X+Br-(s)
but a large discrepancy in the case of Y+Br-(s). What can
you tell about the bond type of the two compounds?
86
Answer
9.1 Incomplete electron transfer in ionic compounds (SB p.250)
(a) Since the theoretical value of the lattice enthalpy is calculated
based on a simple ionic model, the good agreement for X+Br-(s)
suggests that the compound is nearly purely ionic. The ions are
nearly spherical with nearly uniform distribution of charges. The
bond type in the compound is thus nearly purely ionic.
For Y+Br-(s), the large discrepancy suggests that the simple ionic
model does not hold due to the distortion of the electron cloud of
the anion. Thus the bond type in this compound has a certain
degree of covalent character.
87
9.1 Incomplete electron transfer in ionic compounds (SB p.250)
(b) To which group in the Periodic Table does metal X
belong? Explain your answer.
Answer
(b) As X+ ion must have a low polarizing power, its
charge to size ratio should be small. X is a
Group I metal.
Back
88
9.3 Polarity of covalent bonds (SB p.252)
Pure ionic bond and pure covalent bond are two extreme
bond types. Why?
Answer
In pure ionic bonding, the bonded atoms are so different that one
or more electrons are transferred to form oppositely charged ions.
Two identical atoms share electrons equally in pure covalent
bonding. This type of bonding results from the mutual attraction of
the two nuclei for the shared electrons. Between these extremes
are intermediate cases in which the atoms are not so different that
electrons are incompletely transferred and unequal sharing results,
forming polar covalent bond.
Back
89
9.3 Polarity of covalent bonds (SB p.252)
Back
How do you expect the bond type to change for the
chlorides of the third period elements, NaCl, MgCl2, AlCl3,
SiCl4, PCl5, SCl2 and Cl2, going from left to right?
Explain the change in the bond type.
NaCl
Purely
Ionic
90
MgCl2
AlCl3
Ionic with
covalent
character
SiCl4
PCl5
SCl2
Polar covalent
Cl2
Purely
covalen
t
9.3 Polarity of covalent bonds (SB p.257)
Explain the variation in dipole moment of the following
molecules.
Molecule
Dipole moment (D)
CH4
NH3
0
0.35
H2 O
HF
0.65
1.07
Answer
91
9.3 Polarity of covalent bonds (SB p.257)
The dipole moment of a molecule is based on two factors:
1. Bond polarity
This depends on the electronegativity of the atoms involved in a
bond. A bond is said to be polar if there is a difference in
electronegativity between two bonded atoms. The larger the
difference, the more polar is the bond.
92
H
C
N
O
F
2.1
2.5
3.0
3.5
4.0
9.3 Polarity of covalent bonds (SB p.257)
2. The geometry
If the molecule have symmetrical arrangements of polar bonds,
the dipole moments of the bonds will cancel out each other.
93
CH4
NH3
No net dipole moment
Net dipole moment resulted
9.3 Polarity of covalent bonds (SB p.257)
Back
H 2O
Net dipole moment resulted
HF
Net dipole moment resulted
(Note: Lone pair(s) is/are not shown in the above diagrams)
Hence, zero dipole moment is only observed in CH4. HF has the
largest dipole moment since the difference in electronegativity
between the hydrogen atom and the fluorine atom is the largest. H2O
comes the second, followed by NH3.
94
9.3 Polarity of covalent bonds (SB p.257)
Give the shapes and structural formulae of the following
molecules. State whether each molecule is polar or non-polar.
(a) BCl3
(b) NH3
(c) CHCl3
95
Answer
9.3 Polarity of covalent bonds (SB p.257)
Back
96
Molecule
Shape
Structural
formula
Polar or nonpolar
(a) BCl3
Trigonal planar
Non-polar
(b) NH3
Trigonal
pyramidal
Polar
(c) CHCl3
Tetrahedral
Polar