1. Intermediate Type of Bonding9
Intermediate Type of Bonding
9.1 Incomplete Electron Transfer in Ionic Compounds
9.2 Electronegativity of Elements
9.3 Polarity of Covalent Bonds

2. Pure ionic and covalent bonds are only extremes of a continuum.
Most chemical bonds are intermediate between the two extremes.
Pure covalent Intermediate Pure ionic

3. Pure covalent Intermediate Pure ionic
Equal sharing of electrons
Symmetrical distribution of electron cloud
Non-polar molecule
Complete transfer of electrons
Spherical electron clouds
Electron cloud of D is not polarized by C+

4. 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

5. 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.

6. 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.

7. Pure ionic bond does not exist
Li+ F
LiF(g)
Electron clouds are not perfectly spherical
Slight distortion or sharing of electron cloud

8. Polarization of ionic bond - Incomplete Transfer of Electron

9. Determination of Lattice Enthalpy
1. Experimental method : -
from Born-Haber cycle

10. -349
-791.4

11. Determination of Lattice Enthalpy
1. Experimental method : -
from Born-Haber cycle
2. Theoretical calculation : -
based on an ionic model

12. r+ + r 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

13. 3. The crystal has certain assumed lattice structure.
4. The interaction between oppositely charged ions are electrostatic in nature.
5. Repulsive forces between oppositely charged ions at short distances are ignored.

14. Comparison of theoretical and experimental values of lattice enthalpy
Discrepancy : -
Reveals the nature of the bond in the compound

15. Lattice enthalpy (kJ mol-1)
0.14
-631.8
-630.9 KI
0.87
-672.3
-666.5
KBr
0.84
-697.8
-692.0
KCl
0.38
-688.3
-685.7
NaI
0.74
-733.0
-730.5
NaBr
0.04
-766.4
-766.1
NaCl
% deviation
Experimental
Theoretical
Lattice enthalpy (kJ mol-1)
Compound
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

16. Lattice enthalpy (kJ mol-1)
5.5
Zns 12
-867.0
-774.0
AgI
8.5
-877.0
-808.0
AgBr
6.8
-890.0
-833.0
AgCl
% deviation
Experimental
Theoretical
Lattice enthalpy (kJ mol-1)
Compound
Silver halides and zinc sulphide show large discrepancies between the two values.
 Silver halides and zinc sulphide are NOT purely ionic compounds

17. Lattice enthalpy (kJ mol-1)
5.5
Zns 12
-867.0
-774.0
AgI
8.5
-877.0
-808.0
AgBr
6.8
-890.0
-833.0
AgCl
% deviation
Experimental
Theoretical
Lattice enthalpy (kJ mol-1)
Compound
The experimental values are always more negative than the theoretical values
 Polarization of a chemical bond always results in a stronger bond.

18. Large % deviation of lattice enthalpy
The real picture of the polarized bond can be considered as a resonance hybrid of the two canonical forms.
E.g. Ag+ Cl  Ag–Cl
Purely ionic
Purely covalent
Large % deviation of lattice enthalpy
 greater b and more covalent character

19. Small % deviation of lattice enthalpy
The real picture of the polarized bond can be considered as a resonance hybrid of the two canonical forms.
E.g. Ag+ Cl  Ag–Cl
Purely ionic
Purely covalent
Small % deviation of lattice enthalpy
 smaller b and less covalent character

20. 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 of the cation 

21. Q.50(a)
Charge : Al3+ > Mg2+ > Na+
Size : Al3+ < Mg2+ < Na+
Al3+ > Mg2+ > Na+
Polarizing power : Al3+ > Mg2+ > Na+

22. Q.50(b)
Charge : Li+ = Na+ = K+
Size : Li+ < Na+ < K+
Li+ > Na+ > K+
Polarizing power : Li+ > Na+ > K+

23. 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 

24. I > Br > Cl > F S2 > O2
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

25. Lattice enthalpy (kJ mol-1)
5.5
ZnS 12
-867.0
-774.0
AgI
8.5
-877.0
-808.0
AgBr
6.8
-890.0
-833.0
AgCl
% deviation
Experimental
Theoretical
Lattice enthalpy (kJ mol-1)
Compound
Polarizability :
I > Br > Cl
% deviation : AgI > AgBr > AgCl
Covalent character : AgI > AgBr > AgCl

26. Lattice enthalpy (kJ mol-1)
5.5
ZnS 12
-867.0
-774.0
AgI
8.5
-877.0
-808.0
AgBr
6.8
-890.0
-833.0
AgCl
% deviation
Experimental
Theoretical
Lattice enthalpy (kJ mol-1)
Compound
Great % deviation of ZnS due to high polarizability of the large S2 ion

27. 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

28. Lattice enthalpy (kJ mol-1)12
-867.0
-774.0
AgI
8.5
-808.0
AgBr
6.8
-890.0
-833.0
AgCl
0.38
-688.3
-685.7
NaI
0.74
-733.0
-730.5
NaBr
0.04
-766.4
-766.1
NaCl
% deviation
Experimental
Theoretical
Lattice enthalpy (kJ mol-1)
Compound
Ionic radius : Ag+ > Na+
Why are AgX more covalent than NaX ?

29. Ag+ = [Kr] 5s1 4d9 Na+ = Ne Polarizing power : Ag+ > Na+
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+

30. Ag+ = [Kr] 5s1 4d9 Na+ = Ne Polarizing power : Ag+ > Na+
Noble gas configuration of the cation produces better shielding effect and less polarizing power
Polarizing power : Ag+ > Na+

31. 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

32. 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

33. 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

34. Example 9-1 Check Point 9-1 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

35. Fajans’ rules – A summary 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

36. Apart from those compounds mentioned on p
Apart from those compounds mentioned on p.63, list THREE ionic compounds with high covalent character.
AlCl3 , MgI2 , CuCO3

37. Polarization of Covalent Bond : – Unequal Sharing of electrons
Evidence : -
Deflection of a jet of a polar liquid(e.g. H2O) in a non-uniform electrostatic field
Breakdown of additivity rule of covalent radii
Breakdown of additivity rule of bond enthalpies

38. Contains
polar
molecules
Liquid shows
deflection
Contains
non-polar
molecules
Liquid shows
no deflection

39. a charged rod
deflection
of water
Deflection of a polar liquid (water) under the influence of a charged rod.

40. a positively charged rod
a polar molecule
a positively charged rod
Orientation of polar molecules towards a positively charged rod.
Demonstration

41. Solvents showing no deflection Solvents showing a marked deflection
Trichloromethane, CHCl3
Ethanol,CH3CH2OH
Propanone
Water, H2O
Tetrachloromethane
Cyclohexane
Benzene
Carbon disulphide

42. A stream of water is attracted (deflected) to a charged rod, regardless of the sign of the charges on the rod. Explain. 
          +

43. Additivity rule of covalent radii
Assumption : Electrons are equally shared between A and B
Pure covalent bond

44. Experimental value/nm 0.1940 0.1320 0.1430 0.1160
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
0.1275
0.1510
0.1480
0.1910
% deviation
9.91%
5.59%
12.12%
-1.54%
Failure of additivity rule indicates formation of
covalent bond with ionic character due to polarization of shared electron cloud to the more electronegative atom.

45. Experimental value/nm 0.1940 0.1320 0.1430 0.1160
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
0.1275
0.1510
0.1480
0.1910
% deviation
9.91%
5.59%
12.12%
-1.54%
Polarization of a covalent bond always results in the formation a stronger bond with shorter bond length. + 

46. Equal sharing of electrons
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
A.M.
G.M.
E(H – F) = 565 kJ mol1 >> A.M. or G.M.

47. Pauling Scale of Electronegativity (1932)
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)

48. For the molecule A–X
nA and nX are the electronegativity values of A and X respectively
nF = 4.0

49. 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
nH = 2.2
nCl = 3.3

50. 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)

51. 1. The difference in electronegativity between
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

52. 1 Debye (D) = 3.3361030 Coulomb meter
2. The electric dipole moment of diatomic molecule (Quantitative)
 = q  d
SI units : - Coulomb meter
1 Debye (D) = 3.3361030 Coulomb meter

53. Centre of postive charge
Electric dipole moment is a vector pointing from the positive pole to the negative pole

54. Dipole moment (Coulomb meter)
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

55. 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

56. 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
70.0
41.1
11.3
14.8
2.87

57. 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
83.9
80.1
82.2
79.3

58. Good correlation between two methods
Calculated from dipole moment
Good correlation between two methods
nA – nX 

59. Ionic with covalent character
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 MgCl2 AlCl3 SiCl4 PCl5 SCl2 Cl2
Ionic with covalent character
Polar covalent
Purely Ionic
Purely covalent

60. Ionic with covalent character
 difference in electronegativity values
 difference in electronegativity values
NaCl MgCl2 AlCl3 SiCl4 PCl5 SCl2 Cl2
Ionic with covalent character
Polar covalent
Purely Ionic
Purely covalent

61. Ionic with covalent character
 extent of polarization of ionic bond
 extent of polarization of covalent bond
NaCl MgCl2 AlCl3 SiCl4 PCl5 SCl2 Cl2
Ionic with covalent character
Polar covalent
Purely Ionic
Purely covalent

62. Polarity of Molecules
depends on : -
Polarity of bonds
 nA – nX  or dipole moment
Geometry of molecules
Symmetrical molecules are usually non-polar
due to symmetrical arrangements of dipole moments

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

64. The overall dipole moment of a molecule is the vector sum of dipole moments of individual bonds and lone pairs.
Net dipole moment (the vector sum) is zero
 Non-polar

65. The overall dipole moment of a molecule is the vector sum of dipole moments of individual bonds and lone pairs.

66. The overall dipole moment of a molecule is the vector sum of dipole moments of individual bonds and lone pairs.
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.
Net dipole moment (the vector sum) is zero
 Non-polar

68. The overall dipole moment of a molecule is the vector sum of dipole moments of individual bonds
and lone pairs.

69. Q.54 +  or

70. Q.55
Non-polar
Polar

71. Symmetrical  Non-polar
Q.55
Symmetrical  Non-polar

72. Xe F
Dipole moments of the two lone pairs point in opposite directions
Non-polar

73. Q.55
Non-polar

74. Q.55
Non-zero vector sum
 Polar molecule

75. Q.56(a)
>

76. Q.56(b)
>

77. Explain the following phenomena:
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.

78. Explain the following phenomena:
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.

79. Explain the following phenomena:
(b) Both NBr3 and NF3 are polar but their molecules align differently in a non-uniform electrostatic field.

80. (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:

81. 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.

82. Cancelling out of dipole moments
Non-polar molecules
Tetrahedral
Trigonal planar
Linear
Cancelling out of dipole moments
Molecule
Shape

83. Cancelling out of dipole moments
Non-polar molecules
Octahedral
Trigonal bipyramidal
Cancelling out of dipole moments
Molecule
Shape

84. Polar molecules Tetrahedral Trigonal pyramidal V-shaped ( or bent)
Net resultant dipole moment
Dipole moment of individual polar bonds
Molecule
Shape

85. Use of dipole moments
Provide important structural information about molecules

86. 9.1 Incomplete electron transfer in ionic compounds (SB p.250)
Example 9-1
The following gives the theoretical and experimental values of the lattice enthalpies of two metal bromides. X+Br- and Y+Br-.
(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?
Compound
Theoretical lattice enthalpy (kJ mol-1)
Experimental lattice enthalpy (kJ mol-1)
X+Br-(s)
-665
-670
Y+Br-(s)
-758
-890
Answer

87. 9.1 Incomplete electron transfer in ionic compounds (SB p.250)
Example 9-1
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.

88. 9.1 Incomplete electron transfer in ionic compounds (SB p.250)
Example 9-1
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

89. 9.3 Polarity of covalent bonds (SB p.252)
Let's Think 1
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

90. Ionic with covalent character
9.3 Polarity of covalent bonds (SB p.252)
Back
Check Point 9-3A
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 MgCl2 AlCl3 SiCl4 PCl5 SCl2 Cl2
Purely Ionic
Ionic with covalent character
Polar covalent
Purely covalent

91. 9.3 Polarity of covalent bonds (SB p.257)
Example 9-3
Explain the variation in dipole moment of the following molecules.
Molecule
Dipole moment (D)
CH4
NH3
0.35
H2O
0.65 HF
1.07
Answer

92. Example 9-3 9.3 Polarity of covalent bonds (SB p.257)
The dipole moment of a molecule is based on two factors:
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.
H C N O F

93. Example 9-3 9.3 Polarity of covalent bonds (SB p.257) The geometry
If the molecule have symmetrical arrangements of polar bonds, the dipole moments of the bonds will cancel out each other.
CH4 NH3
No net dipole moment Net dipole moment resulted

94. Example 9-3 Back 9.3 Polarity of covalent bonds (SB p.257) H2O HF
Net dipole moment resulted 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.

95. 9.3 Polarity of covalent bonds (SB p.257)
Check Point 9-3B
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
Answer

96. Check Point 9-3B Back 9.3 Polarity of covalent bonds (SB p.257) Polar
Tetrahedral
(c) CHCl3
Trigonal pyramidal
(b) NH3
Non-polar
Trigonal planar
(a) BCl3
Polar or non-polar
Structural formula
Shape
Molecule