L.S.T. Leung Chik Wai Memorial School
F.6 Chemistry
Chapter 9: Bonding Intermediate between Ionic and Covalent
Chapt 9: pg.1
BONDING 1NTERMEDIATE BETWEEN IONIC AND COVALENT
REVIEW
<1>
Pure ionic compound refers to the ionic compound formed by complete electron
transfer from a metallic atom to a non-metallic atom
<2> Pure covalent compound refers to the covalent compound formed by equal sharing of
electrons between the bonded atoms.
Compounds are rarely purely ionic or covalent. In fact, chemical bonds are mostly partially
ionic or partially covalent.
<3> Intermediate types of bond can he considered to arise from
(a) polarization of ionic bond — due to incomplete electron transfer
(b) polarization of covalent bond — due to electronegativity difference between the
bonded atoms.
I.
Incomplete Electron Transfer in Ionic Compound
(A)
Theoretical values of Lattice Enthalpy
Model for calculation of theoretical lattice enthalpy:
1.
2.
3.
(B)
The ions are spherical with charge uniformly’ distributed around them.
The cations and anions are just in contact with each other.
The calculations take into consideration the charges on the ions and the distance between
the ions.
Experimental Lattice Enthalpy Derived From Born-Haber Cycle
For M+(g) + X-(g) 
MX(s) H = lattice enthalpy
Lattice Enthalpy
= Hf [MX(s)] - Hatom[M(s)] - Hatom.[X] - HI.E. [M(g)] - HE.A.[X(g)]
L.S.T. Leung Chik Wai Memorial School
F.6 Chemistry
Chapter 9: Bonding Intermediate between Ionic and Covalent
Chapt 9: pg.2
(C) Comparing Theoretical And Experimental Lattice Enthalpy
Theoretical values of lattice enthalpy for ionic crystals can be compared with those obtained
indirectly from Born—Haber cycle.
Compound
Theoretical lattice
enthalpy/ kJ mol’
Experimental lattice enthalpy
(from Born-Haber cycle) / kJ mol’
NaCl
- 766
- 766
NaBr
-731
-735
NaI
-686
-688
KC1
-692
-698
KBr
-667
-673
KI
-631
-643
AgCl
-833
-890
AgBr
-808
-877
AgI
-774
-867
ZnS
-3427
-3615
Interpretations:
<1> For the halides of alkali metals, the theoretical values are in good agreement with experimental
values. This shows that
(a) The simple ionic model is suitable and applicable.
(b) The ions are spherical with uniformly distributed around
(c)The ionic crystals have strong ionic character.
<2> For silver halides and zinc sulphide, large discrepancies between the values exist. The
experimental values (from Born-Haber cycle) are greater than the theoretical values. This
shows that
(a) The simple ionic model is inaccurate for the AgX and ZnS. The ions are not
completely spherical and charge distribution is not uniform.
(h) The bonding is stronger than expected. This is because the bonding is partially covalent,
the electron density between the nuclei of the bonded ions is higher and extra bonding
exist. Electron transfer from metal to non-metal is incomplete.
L.S.T. Leung Chik Wai Memorial School
F.6 Chemistry
Chapter 9: Bonding Intermediate between Ionic and Covalent
Chapt 9: pg.3
In conclusion, when a comparison is made between the theoretical values and the
experimentally – determined Born-Haber value.
-
good agreement means that the crystals are essentially ionic in nature
-
poor agreement means that considerable covalent character exists in the ionic crystal,
i.e. the ionic bond is polarized.
(D) Formation of Polar Ionic Bond
<1> When cations and anions come together to form a crystal lattice, the net positive charge
on the cations can attract the electrons in the outershell of the anions.
<2> This causes a displacement; of electrons from the anion to the ca t ion.
<3> Electron transfer from the metallic atom to non-metallic atom is therefore incomplete.
(E)
Polarizing Power of Cations and Polarizability of Anions
In general:
1. cation have low polarizabilities but high polarizing power
2. anions have high polarizabilities hut low polarizing power
Note
<1> Anion polarization is the distortion of the shape of a polarizable anion caused by the
attraction of a polarizing cation.
<2> Polarizing power of a cation increases as its charges increase and as the ionic radius
decreases
e.g. polarizing power Al3+ > Mg2+ > Na+
<3> Polarizability of an anion increases as its charge increases
(e.g. N3- > O2- > F- ) and as its radius increases (e. .g. I >Br >Cl > F )
An ionic compound will have an appreciable covalent character if
<1> The anion is large : more polarizable
<2> The cation is small more polarizing
<3> Either the cation or anion is highly charged
L.S.T. Leung Chik Wai Memorial School
F.6 Chemistry
Chapter 9: Bonding Intermediate between Ionic and Covalent
Chapt 9: pg.4
Note: Covalent bonding if more probable and ionic bond less probable if
(i)
(ii)
the ions carry multiple charges acid
the atoms produce small cations and large anions
CONCLUSION:
For ionic compounds with large anion and small cation, the electron cloud of the anion may be
distorted by the high charge density of the cation.
This leads to polarization of the ionic bond. Electron transfer from metal to non-metal is
incomplete. As a result, the electrons in one ion are not only confined to the charge cloud of
the ion, but shared with the neighbouring ions. The ionic bond is said to have cova1ent
character.
Exercise 1
Comment on the statement
“The bond in sodium chloride is purely ionic.”
‘Exercise 2
Describe the ionic model based on which the theoretical value of lattice enthalpy is calculated.
Comment on the differences between experimentally derived and theoretical values of lattice
enthalpies of sodium bromide and silver bromide, with reference to the ionic model.
Lattice enthalpy/ kJ mol-1
NaBr
AgBr
Theoretical value
-731
-808
Experimental value
-735
-877
L.S.T. Leung Chik Wai Memorial School
F.6 Chemistry
Chapter 9: Bonding Intermediate between Ionic and Covalent
(F)
Chapt 9: pg.5
Properties of Ionic Compounds with Covalent Character
<1> Ionic compounds with a high degree of polarization may be soluble in organic solvent.
e.g.
LiCl is very soluble in ethanol and diethyl ether, while NaC1 (nearly purely ionic)
is insoluble.
<2> Ionic compounds with large degree of polarization such as A1C13 are similar to simple
molecules instead of giant covalent lattice. They have low boiling points and melting
points.
II.
Electroneciativity of Elements
Electronegativity is an arbitrary measrue of an atom’s tendency in a molecule to attract
the electrons shared between the bonded atoms.
<1> Electronegativity values (Paulang s scale) for some elements are listed in the table below:
H
2.1
Li
Be
B
C
N
O
F
1.0
1.5
2.0
2.5
3.0
3.5
4.0
Na
0.9
Mg
1.2
Al
1.5
Si
1.8
P
2.1
S
2.5
Cl
3.0
K
0.8
Ca
1.0
Ga
1.6
Ge
1.8
As
2.0
Se
2.4
Br
2.8
Rb
0.8
Sr
1.0
In
1.5
Sn
1.8
Sb
1.9
Te
2.1
I
2.5
<2> In Pauling’s scale, a value of 4.0 is assigned to the most electronegative element, fluorine.
Other elements have lower values.
The higher the electronegativit.y value, the higher the electron attracting ability is.
<3> Trend of electronegativities
(a) Increases on going from left to right across a period.
Reason :
(b)
Decrease on going down a group.
Reason :
L.S.T. Leung Chik Wai Memorial School
F.6 Chemistry
Chapter 9: Bonding Intermediate between Ionic and Covalent
Chapt 9: pg.6
<4> Metallic elements tend to lose electrons to form positive ions. They have low electronegativity
values and are said to be electropositive.
Exercise
Compare electronegativity and electron affinity
III.
Polarity of Covalent Bond
Covalent bond formed between different atoms is not purely covalent because
Displacement of an electron cloud leads to the formation of a polar bond.. A covalent bond with
polarization is known as a polar covalent bond.
(A)
Polarity in Terms of Electronegativity Difference between Bonded atoms
Unequal sharing of bonded electron pair(s) can he explained ~n terms of the electrongativity
difference bonded atoms.
<1> A pure covalent bond is formed between identical atoms or atoms of very similar
electronegativity Polar covalent bond will be formed between atoms with different electronegativity
The one wIth higher electronegativity attracts the bonded electron pair more. As a result of
electron displacement.,
The less electronegative atom acquires a partial positive charge (+), whereas
the more electronegative atom acquires a partial negative charge (-)
e.g. A polar covalent bond is formed in a hydrogen chloride molecule
H—Cl
<2> Extent of Polarity
When the electronegativity difference between the bonded atom is high.
• the bond is more polar, and
• the ionic character of the bond is high.
L.S.T. Leung Chik Wai Memorial School
F.6 Chemistry
Chapter 9: Bonding Intermediate between Ionic and Covalent
Chapt 9: pg.7
Consider the following table
Note : When the difference in electçopegativity is
(a) large: the bond is predominantly ionic. e.g. LiF
(b) small: the bond is predominantly covalent, e.g.
(B) Polarity in Terms of Dipole Moment
In a polar covalent bond, there is a displacement of the electron cloud from the less
electronegative atom towards the more electronegative atom. This results in the formation of a
dipole with two equal and opposite charges (+q and -q) separated by a distance d.
Center of charge
L.S.T. Leung Chik Wai Memorial School
F.6 Chemistry
Chapter 9: Bonding Intermediate between Ionic and Covalent
Chapt 9: pg.8
The extent of bond polarization is measured in terms of dipole moment .
<1>
p=qxd
<2> Dipole moments are expressed in Debye (D) unit.
In HCl,  = 1.1D
<3> Dipole moments are vector quantities.
Dipole moment = vector sum of the dipole moment of individual polar bonds.
If the sum is zero, the molecule is non-polar
The greater the resultant dipole moment, the more polar the molecule is.
<4> Ionic compounds have large dipole moments because the separation of charge is large.
<5> Polar molecules
Molecule with permanent dipole moment is known as polar molecule.
Examples : water molecule
ammonia molecule
Non-polar molecule
A molecule is non-polar when it
•
•
it has no polar bonds or
the individual dipole moments cancel out.
Examples : carbon dioxide
tetrachloromethane
Exercise 1
Draw the 3 dimensional structures of PCl5 and BF3 and describe their structures. Comment on
the polarity of PCl5 and BF3.
L.S.T. Leung Chik Wai Memorial School
F.6 Chemistry
Chapter 9: Bonding Intermediate between Ionic and Covalent
IV.
Chapt 9: pg.9
Investigating the effect of A Non-uniform Electric Field on A Jet Of Liquid
The polarity (whether polar or non-polar) of a liquid can be predicted by investigating the effect
of a non-uniform electrostatic field on a jet of liquid.
Method and procedure
<1> Fine jets of liquid is run from a burette.
<2> A positively charged glass rod is brought near to the liquid to see whether
it is deflected or not.
Examples : Water : deflected
Reason:
Tetrachloromethane : Not deflected
Reason :
Exercise : Predict the behaviour of various liquid streams towards a charged rod.
(A)
Properties of Covalent Compounds with ionic charge
Polar molecules usually have higher melting and boiling points than non-polar molecules with
similar molecular mass. This is due to the dipole-dipole attractions between the molecules.
In conclusion,
The greater the difference in electronegativities of two bonded atoms, or the greater the
resultant dipole moment of a covalent molecule,
• the more the electron cloud is displaced ,and
• the more polar the covalent bond between the two bonded atoms is.