Section 4c
Section 4C
Intermediate type of bonding / Page 1
Intermediate Type of Bonding
Ionic and covalent bondings are two extreme cases. Most chemical bonds lie between these two
extremes.
Polarization of ions
The following are lattice energy of some compounds, the theoretical values are obtained from
calculation using the theoretical model described above. The experimental values are
determined through the Born-Haber Cycle.
Compound
Theoretical Value
Experimental Value
NaCl
- 766.1
- 776.4
NaBr
- 730.5
- 735.9
NaI
- 685.7
- 688.3
KCl
- 692.0
- 697.8
KBr
- 660.5
- 672.3
KI
- 630.9
- 631.8
AgCl
- 768.6
- 916.3
AgBr
- 758.5
- 907.9
AgI
- 735.9
- 865.4
ZnS
- 3427.0
- 3615
It was found that the two values for the alkali halides are in good agreement. Since the model
on which the theoretical values are based on the assumptions that an ionic crystal was made
up of discrete spherical ions, each with its electrostatic charge distributed evenly around it.
This agreement between the theoretical and experimental values give strong evidence that this
simple ionic model is a correct one.
But the values of silver halides and zinc sulphide do not agree with each other well. Thus
showing that the ionic model does not actually represent the bonding situation in those
compounds. The bonding was found to be stronger than that predicted by the ionic model.
In case of silver halides, the actual internuclear separation is smaller than the one obtained by
adding the ionic radius together. The shortening of the separation can only be achieved by a
higher concentration of electrons between the two nuclei. This implies a distortion of the electron
cloud was occurred, i.e. the electron distribution is no longer spherical but polarized. This
represent the start of transition from ionic bonding to covalent bonding.
N.B. The partly covalent nature of the ionic bond can be interpreted by saying that the
ionic bonds have been polarized or by suggesting that the electrons are incompletely
transferred during the formation of ions.
Types of ions which tend to produce polarization:
polarization power =
ability to distort the electron distribution of adjacent ion or atom
The polarization of cation is favoured by:
1.
2.
Higher charge
Smaller size (the positive nucleus can be closer to the negative ion.)
Types of ions most easily polarized (polarizability of ions):
The polarizability of anion is favoured by:
1.
2.
Higher charge (electron repulsion is greater)
Larger size
Hence Al3+, Mg2+ have a higher polarizing power while
polarized.
P3 -, O2- are more easily to be
Section 4c
.
1.
Intermediate type of bonding / Page 2
Explain why
(a) I`is more readily polarized than F - ;
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(b) O 2- is more readily polarized than F - ;
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(c) Li + has greater polarizing power than K + ;
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(d) Al 3+ has greater polarizing power than Li + .
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Electronegativity and Bond polarization
1. Electronegativity
This is the power of an atom in a molecule to attract the shared pair of electrons
itself.
to
This electron-attracting power of a particular atom will be different in different compounds
because it depends on the chemical environment which the atom under consideration is
attached.
Various methods were set up to quantify the electronegativity of atoms and by using
different scales (or different approaches) different values may be obtained. The
electronegativity values of the elements in Period II and III are given below:
Period II
Period III
Li
Be
B
C
N
O
F
1.0
1.5
2.0
2.5
3.0
3.5
4.0
Na
Mg
Al
Si
P
S
Cl
0.9
1.2
1.5
1.8
2.1
2.5
3.0
Two general periodic trends are found:
(i) the electronegativities of elements increase sharply across the period; and
(ii) the electronegativities of elements decrease gently when descending down a group.
When the electronegativity difference between the bonded atoms is large, such as in the
case for F and Li (electronegativity difference is 4.0 - 1.0 = 3.0), the bond formed will
predominantly be ionic.
When the electronegativity difference is small, the bonding by two atoms will be
predominantly covalent.
.
2.
Both electronegativity and electron affinity refer to the attraction by an atom for
electron. What is the essential difference between the two terms ?
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2. Bond polarization
If a covalent bond is formed between two different atoms than the attraction of their nuclei
for the bonding electrons must be somewhat different. And because of this, electrons will
be displaced towards the more electronegative atom. Therefore this end of the molecule is
relatively negative and the other end relatively positive. It represents the departure of the
bond from being purely covalent and it introduces some ionic character into the bond.
Example 1:
Hydrogen chloride
Section 4c
Intermediate type of bonding / Page 3
This unequal sharing of electron is known as bond polarization. Such a bond is said to
be polar. The extent of polarity in a bond depends on the electronegativity difference of
the two bonded atoms.
Any bond which has any degree of polarity will have a corresponding dipole moment given
by:
µ
=
distance (bond length)
x
charge
Dipole moments are vector quantities. Not every compound that has polar bonds possess a
dipole moment for the polarity of the molecule is the vector sum of the individual bond
moment. If the vector sum is zero, the dipole moment of the molecule is zero, and the
molecule is described as non-polar. The greater the resultant dipole moment, the more
polar the molecule is.
Example 2:
Ammonia and Boron trichloride
Example 3:
Water and Beryllium chloride
Dipole moments can provide important structural information about
example , the zero dipole moment of CO2 shows that the molecule must
the dipole moment of each C=O bond cancels each other in the vector
hand, the existence of a dipole moment for SO2 molecule indicates
contains polar bonds which are not linearly arranged.
.
3.
4.
a molecule. As an
be linear such that
sum. On the other
that the molecule
(a)
Draw a dot-and-cross diagram for a sulphur dichloride molecule,
SCl 2.
(b)
Given that the first ionization energies for S and Cl are
kJ mol-1 respectively. State which one is more likely to lose
(c)
Sketch the shape of the SCl2 molecule. Insert symbols to indicate the polarity
of the bonds and the direction of the overall dipole moment (if any).
1000 and
electrons.
1260
Tetrachloromethane, CCl4, is a non-polar liquid, but it has polar bonds. Explain
this statement, including a diagram in your answer. Has tetrachloromethane any
'ionic character' ?
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Section 4c
Intermediate type of bonding / Page 4
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A simple experiment to show the polarity of compounds
A fine jet of liquid is run from a burette into a beaker and
a charged rod is brought close to the stream of liquid. As
the polar liquid stream pass the charged rod, its
molecules are attracted towards the charged rod and the
jet is deflected.
If the liquid is not water, two conditions would be
necessary:
[i]
the apparatus must be dry.
[ii]
the experiment is performed in a moisture-free
cupboard.
Com pound
CHCl3
Trichloromethane
CCl4
Tetrachloromethane
H2O
Water
C6H6
Benzene
CH3(CH2)4CH3
Hexane
CH3(CH2)3OH
Butan-1-ol
deflection
charged rod
of
jet
of
liquid
As a summary of the above discussion, wholly ionic and wholly covalent bonds are only two
extreme types, bonds can be partially ionic and partially covalent in character.
For example, polarization of a covalent bond represents the existence of some ionic character in
a covalent bond while polarization of ions represents the existence of some covalent character
in ionic bonding.
Pure Covalent Bond
Polar Covalent Bond
Pure Ionic Bond
- Electron cloud is
symmetrical distributed
clouds
- Asymmetric distribution
of electron clouds
- No distortion of electron
The bonding between two atoms A and B would be mainly ionic in character if their
difference in electronegativity values is great. Similarly, the bonding would be mainly covalent
if their difference in electronegativity values is small.