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Title: Chemical bonding theories
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Chemical Bonding

89

Chapter

3

Chemical Bonding
Atoms of different elements excepting noble gases donot have
complete octet so they combine with other atoms to form chemical bond
...
It depends on the valency of atoms
...

2

2

2

2

3

2

(1) Conditions for formation of electrovalent bond

Cause and Modes of chemical combination

(i) The atom which changes into cation (+ ive ion) should possess 1,
2 or 3 valency electrons
...


Chemical bonding takes place due to acquire a state of minimum
energy and maximum stability and to convert atoms into molecule to
acquire stable configuration of the nearest noble gas
...
Electrovalent bond
is not possible between similar atoms
...
They have low ionisation potentials
...
They have
higher value of electronegativity
...

Different types of bonds are formed from these types of atoms
...


The energy changes involved in the formation of ionic compounds
from their constituent elements can be studied with the help of a
thermochemical cycle called Born Haber cycle
...


sub

   
  Cl  
   



or

Cl (g)

Na  Cl 

– EA

– e–

Na  (g)



1
Cl 2 (g)
2

+

Na(g)

An electrovalent bond is formed when a metal atom transfers one or
more electrons to a non-metal atom
...
The amount of energy released when free ions combine together to
K
form one mole of a crystal is called lattice energy (U)
...
e
...
For this an atom should have low value of Ionisation potential and
the other atom should have high value of electron affinity
...


H f  H Subl
...
 IE  EA  U
2

90 Chemical Bonding
(2) Characteristics of electrovalent compounds

2

(i) Electrovalent compounds are generally crystalline is nature
...

(ii) Electrovalent compounds possess high melting and boiling
points
...

(iv) Electrovalent solids do not conduct electricity
...

(v) Electrovalent compounds are fairly soluble in polar solvents and
insoluble in non-polar solvents
...
Thus
these compound do not show space isomerism e
...
geometrical or optical
isomerism
...
The chemical
reaction of these compounds are known as ionic reactions, which are fast
...

(ix) Cooling curve of an ionic compound is not smooth, it has two
break points corresponding to time of solidification
...


Covalent bond
Covalent bond was first proposed by Lewis in 1916
...
A covalent bond between two
similar atoms is non-polar covalent bond while it is polar between two
different atom having different electronegativities
...
We explain covalent bond formation by
Lewis octet rule
...
In the
formation of chlorine molecule, each chlorine atom contributes one electron
and the pair of electrons is shared between two atoms
...

**


*
 Cl   Cl *

**
( 2 , 8 , 7 ) ( 2, 8 , 7 )








 Cl * Cl 


( 2, 8 , 8 ) ( 2, 8 , 8 )

Some other examples are :

(ii) Diamond, Carborandum (SiC), Silica (SiO ), AlN etc
...

(iii) In general covalent substances are bad conductor of electricity
...
Graphite
can conduct electricity in solid state since electrons can pass from one layer
to the other
...
some covalent
compounds like alcohol, dissolve in water due to hydrogen bonding
...
These compounds,
thus show isomerism (structural and space)
...
The reaction
rates are usually low
...
Covalency
= 8 – [Number of the group to which element belongs]
...


(1) Conditions for formation of covalent bond
(i) The combining atoms should be short by 1, 2 or 3 electrons in
the valency shell in comparison to stable noble gas configuration
...

(iii) The approach of the atoms towards one another should be
accompanied by decrease of energy
...
Some covalent compounds exist as soft solids
...

The element such as P, S, Cl, Br, I have vacant d-orbitals in their
valency shell
...
The electrons from
paired orbitals get excited to vacant d-orbitals of the same shell
...
Thus, such an excitation is not possible and variable valency is
not shown by these elements
...

3

5

(3) The Lewis theory : The tendency of atoms to achieve eight
electrons in their outermost shell is known as lewis octet rule
...
e
...

BeF2 , BF3 , AlH3 are electron- deficients (Octet incomplete) hence
are Lewis acid
...
Sugden introduced singlet linkage in which one
electron is donated (Instead of one pair of electrons) to the electron
deficient atom so that octet rule is not violated
...
Thus, PCl5 and SF6 have structures as,

Cl

Cl
P

Cl

Cl

Cl

F

F
S

F

F

F

F

(5) Construction of structures for molecules and poly atomic ions :
The following method is applicable to species in which the octet rule is not
violated
...

(ii) Determine n = [2 × (number of H atoms) + 8 × (number of
other atoms)]
...
No
...

(iv) Determine the number of non-bonding electrons, n , which
equals n – n
...
of lone pairs equals n /2
...
Then complete octets using n /2 lone pairs
...

3

1

91

Examples : CO, N O, H O , N O , N O , N O , HNO , NO 3 , SO , SO ,

2

2

H SO ,
2

2

2

2

3

2

4

2

5

3

SO 42  , SO 22  ,

3

4

2

3

H 3 PO4 , H 4 P2O7 ,

4

1

3

4

3

4

(vii) Formal Charge = [valence electrons in atom) – (no
...
of unshared electrons)]
(viii) Other aspects like resonance etc
...

Illustrative examples
(i) CO 32  ; n1  4  (6  3)  2  24 [2 added for net charge]
n 2 = (2 × 0) + (8 × 4) = 32 (no
...


Since carbon is the central atom, 3 oxygen atoms are to be arranged
around it, thus,

H 3 PO3 , Al2Cl6 (Anhydrous), O3 , SO 2Cl2 , SOCl 2 , HIO3 , HClO4 ,

HClO3 , CH 3 NC , N 2 H 5 , CH 3 NO 2 , NH 4 , [Cu(NH 3 )4 ]2 etc
...

(2) These are sparingly soluble in polar solvent like water but readily
soluble in non-polar solvents
...
Their solutions or fused masses do not allow the passage to
electricity
...
Thus, coordinate compounds
show isomerism
...

 = Electric charge  bond length
As q is in the order of 10 esu and d is in the order of 10 cm,  is in
the order of 10 esu cm
...

1D  10 18 esu cm = 3
...
I
...

–10

–8

–18

O
|

O  C  O , but total bonds are equal to 4
...
Now, arrange lone pairs to complete

...
O :
|


...


Dipole moment is indicated by an arrow having a symbol (
)
pointing towards the negative end
...


Symmetrical polyatomic molecules are not polar so they do not have
any value of dipole moment
...


(ii)

O

CO 2 ; n = 4 + (6 × 2 ) = 16

C

O

C

B

1

n = (2 × 0) + (8 × 3) = 24
n = 24 – 16 = 8, hence 4 bonds
n = 16 – 8 = 8, hence 4 lone-pairs
Since C is the central atom, the two oxygen atoms are around to be
arranged it thus the structure would be; O – C – O, but total no
...
H O, CH Cl, NH , etc are
polar molecules as they have some positive values of dipole moments

...
After arrangement of lone pairs to complete

...



...
The atom
which contributes the electrons is called the donor (Lewis base) while the
other which only shares the electron pair is known as acceptor (Lewis acid)
...

BF molecule, boron is short of two electrons
...

3

H

 




F 

 

H * N **  B
 

H




 

F 



H




 F


 




 H * N




H




 

H

Water

F 

B




 

F

 



H
|

F

General
formula

|

|

H

F

H
H

Molecular geometry

AX

Linear

AX 2

AX 3

|

F   H  N  B  F





C
H

H

H
Ammonia
Methyl chloride
 = 1
...
86D
  0 due to unsymmetry
(1) Dipole moment is an important factor in determining the
geometry of molecules
...
1 Molecular geometry and dipole moment


 

N
H

H

 = 1
...


Dipole
moment

Example

HF, HCl

Linear
Bent or V-shape

May
be
non zero
Zero
Non zero

Triangular planar
Pyramidal
T-shape

Zero
Non zero
Non zero

BF3

Tetrahedral

Zero

CO 2 ,CS 2

H 2O, NO 2
NH 3 , PCl3
ClF3

Formation of a co-ordinate
bond between NH3 and BF3

AX 4

CH 4 ,CCl 4

92 Chemical Bonding
Square planar
See saw

Zero
Non zero

Trigonal bipyramidal
Square pyramidal

Zero
Non zero

PCl5

AX 6

Octahedral
Distorted octahedral

Zero
Non zero

SF6

AX 7

Pentagonal bipyramidal

Zero

IF7

AX 5

XeF4
SF4 ,TeCl 4
BrCl5

XeF6

(2) Every ionic compound having some percentage of covalent
character according to Fajan's rule
...

The % ionic character 

Observed 
 100
...
Bond energy :
2s  2s < 2s  2 p < 2 p  2 p
(9) s -orbitals are spherically symmetrical and thus show only head
on overlapping
...
Overlapping of different type gives sigma () and pi () bond
...

Stronger
Bond energy 80 kcals
More stable
Less reactive
Can exist independently

It result from the sidewise (lateral)
overlapping of two p-orbitals
...


Fajan’s rule
The magnitude of polarization or increased covalent character
depends upon a number of factors
...
e
...

(2) Large size of anion : Larger the size of anion greater is its
polarizing power i
...
greater will be the covalent nature of the bond
...

(4) Electronic configuration of the cation : For the two ions of the
same size and charge, one with a pseudo noble gas configuration (i
...
18
electrons in the outermost shell) will be more polarizing than a cation with
noble gas configuration (i
...
, 8 electron in outer most shell)
...

(1) To form a covalent bond, two atoms must come close to each
other so that orbitals of one overlaps with the other
...

(3) After overlapping a new localized bond orbital is formed which
has maximum probability of finding electrons
...

(5) Greater is the overlapping, lesser will be the bond length, more

will be attraction and more will be bond energy and the stability of bond
will also be high
...

(7) More closer the valence shells are to the nucleus, more will be
the overlapping and the bond energy will also be high
...


Hybridization
The concept of hybridization was introduced by Pauling and Slater
...
The new orbitals so formed
are known as hybrid orbitals
...

(2) Hybridization takes place only in orbitals, electrons are not
involved in it
...

(4) In the excited state, the number of unpaired electrons must
correspond to the oxidation state of the central atom in the molecule
...

(6) Hybrid orbitals form only sigma bonds
...

(8) Hybridization never takes place in an isolated atom but it occurs
only at the time of bond formation
...

(10) Hybridized orbitals provide efficient overlapping than
overlapping by pure s, p and d-orbitals
...


How to determine type of hybridization : The structure of any
molecule can be predicted on the basis of hybridization which in turn can
be known by the following general formulation,
H 

1
(V  M  C  A)
2

Chemical Bonding
Where H = Number of orbitals involved in hybridization viz
...

2

3

3

3

2

3

3

V = Number electrons in valence shell of the central atom,
M = Number of monovalent atom
C = Charge on cation,

Bond order 

A = Charge on anion

The phenomenon of resonance was put forward by Heisenberg to
explain the properties of certain molecules
...
The molecule is then supposed to have
many structures, each of which can explain most of the properties of the
molecule but none can explain all the properties of the molecule
...
This phenomenon is called
resonance
...
Its structure
can be written as

...


...
O
...


...


...


...


...

O

...


...
O
...


...

(a )
(b )
As a resonance hybrid of above two structures (a) and (b
...


Resonance is shown by benzene, toluene, O , allenes

(>C = C =

3

C<), CO, CO , CO 3 , SO , NO, NO while it is not shown by H O , H O, NH ,
CH , SiO
...
33
3

Bond characteristics

Resonance

2

O
|
C
// \
O
O

93

3

2

2

2

2

3

(1) Bond length
“The average distance between the centre of the nuclei of the two
bonded atoms is called bond length”
...

In an ionic compound, the bond length is the sum of their ionic
radii ( d  r  r ) and in a covalent compound, it is the sum of their
covalent radii (e
...
, for HCl, d  rH  rCl )
...

For example, bond length of H  X
are in the order,
HI  HBr  HCl  HF
...

Thus, bond length of carbon–carbon bonds are in the order,
C CC CC –C
...

For example, sp 3 C – H  sp 2 C – H  sp C – H

2

As a result of resonance, the bond lengths of single and double bond
in a molecule become equal e
...
O–O bond lengths in ozone or C–O bond
lengths in CO 32 – ion
...

Greater is the number of canonical forms especially with nearly
same energy, greater is the stability of the molecule
...
Thus,

Resonance energy = Energy of resonance hybrid – Energy of the
most stable of resonating structure
...
of bonds between two atoms in all the structures

In benzene

Total no
...
5
2
2
In carbonate ion

(iv) Polar bond length is usually smaller than the theoretical nonpolar bond length
...
Greater is the bond energy,
stronger is the bond
...

Factors affecting bond energy
(i) Greater the size of the atom, greater is the bond length and less
is the bond dissociation energy i
...
less is the bond strength
...

(iii) Greater the number of lone pairs of electrons present on the
bonded atoms, greater is the repulsion between the atoms and hence less is
the bond dissociation energy
...
Thus, bond energy decreases in the

following order, sp  sp 2  sp 3
(v) Greater the electronegativity difference, greater is the bond
polarity and hence greater will be the bond strength i
...
, bond energy,
H  F  H  Cl  H  Br  H  I ,

94 Chemical Bonding
(vi) Among halogens Cl – Cl > F – F > Br – Br > I – I, (Decreasing
order of bond energy) Resonance increases bond energy
...


(3) Bond angle
In case of molecules made up of three or more atoms, the average

angle between the bonded orbitals (i
...
, between the two covalent bonds) is
known as bond angle 
...

(ii) In hybridisation as the s character of the s hybrid bond
increases, the bond angle increases
...
5 o 102 o 98
...
5 o 101o

Valence shell electron pair repulsion theory (VSEPR )
The basic concept of the theory was suggested by Sidgwick and
Powell (1940)
...
The concept tells that, the arrangement of bonds around the
central atom depends upon the repulsion’s operating between electron
pairs(bonded or non bonded) around the central atom
...

The main postulates of VSEPR theory are

sp

2

Bond angle

(1) For polyatomic molecules containing 3 or more atoms, one of the
180°

atoms is called the central atom to which other atoms are linked
...
5%
...

Bond angle

H 2O
104
...
2 o

 H 2 Se
91
...
5 o

(2) The geometry of a molecule depends upon the total number of
valence shell electron pairs (bonded or not bonded) present around the
central atom and their repulsion due to relative sizes and shapes
...
It gives the
symmetrical shape to the molecule
...

(5) The relative order of repulsion between electron pairs is as

follows : lp – lp > lp – bp > bp – bp
...
As such repulsion becomes
greater when a lone pair is involved
...
2 Geometry of Molecules/Ions having bond pair as well as lone pair of electrons
Type of
mole-cule

No
...
of lone
pairs of
electrons

Hybridization

Bond angle

Expected
geometry

AX 3

2

1

sp 2

< 120

Trigonal planar

AX 4

2

2

sp 3

< 109 28

Tetrahedral

AX 4

3

1

sp 3

< 109 28

AX 5

4

1

sp 3 d

< 109 28

AX 5

3

2

sp 3 d

90

AX 5

2

3

sp 3 d

180

AX 6

5

1

sp 3 d 2

< 90

2

3

2

3

3

AX 6

AX

7

4
6

1

sp d
sp d

Actual geometry

Examples

V-shape, Bent,
Angular
V-shape,
Angular

H O, H S, SCl , OF , NH , ClO

Tetrahedral

Pyramidal

NH , NF , PCl, PH, AsH, ClO , H O

Trigonal
bipyramidal
Trigonal
bipyramidal
Trigonal
bipyramidal

Irregular
tetrahedron

SF , SCl , TeCl

T-shaped

ICl , IF , ClF

Linear

XeF , I , ICl

Octahedral

Square
pyramidal

ICl , BrF , IF



Octahedral

Square planar

XeF , ICl



Pentagonal
pyramidal

Distorted
octahedral

XeF

o

o

o

o

o

o

o

Molecular orbital theory or MOT
Molecular orbital theory was given by Hund and Mulliken in 1932
...
The new orbitals thus formed are called
molecular orbitals
...

(3) In terms of probability distribution, a molecular orbital gives
the electron probability distribution around a group of nuclei just as an
atomic orbital gives the electron probability distribution around the single
nucleus
...


Chemical Bonding
(5) The number of molecular orbitals formed is equal to the number
of combining atomic orbitals
...

(7) The bonding molecular orbital has lower energy and hence
greater stability than the corresponding antibonding molecular orbital
...

N A  number of electrons in antibonding MO
...

Bond length
(14) If all the electrons in a molecule are paired then the substance
is a diamagnetic on the other hand if there are unpaired electrons in the
molecule, then the substance is paramagnetic
...

*
 (2pz)

*(2pz)
2p
2p

*(2py)

2p

*(2px)

bond

 N  NA 
Bond order   B

2



by  * ,  * etc
...

(10) The filling of molecular orbitals in a molecule takes place in
accordance with Aufbau principle, Pauli's exclusion principle and Hund's
rule
...


95

*(2py)

2p

*(2px)

 (2py)  (2px)

 (2pz)
2s

2s

*(2s)

Increasing energy

Increasing energy

 (2pz)
 (2py)  (2px)
2s

 (2s)

 (2s)
1s

1s

1s

*(1s)

Molecular
orbitals

1s

*(1s)

 (1s)

 (1s)
Atomic
orbitals

2s

*(2s)

Atomic
orbitals

Molecular orbital energy level diagram
(Applicable for elements with Z > 7)

Hydrogen bonding
In 1920, Latimer and Rodebush introduced the idea of “hydrogen
bond”
...

Types of hydrogen bonding
(1) Intermolecular hydrogen bond : Intermolecular hydrogen bond is
formed between two different molecules of the same or different
substances
...


Atomic
orbitals

Molecular
orbitals

Atomic
orbitals

Molecular orbital energy level diagram obtained by the overlap of 2s and 2pz
atomic orbitals after mixing
(Applicable for elements with Z < 7)

(ii) Hydrogen bond in alcohol and water molecules
(2) Intramolecular hydrogen bond (Chelation)
Intramolecular hydrogen bond is formed between the hydrogen atom
and the highly electronegative atom (F, O or N) present in the same
molecule
...
Consequently, the effect of
intramolecular hydrogen bond on the physical properties is negligible
...

O
H
|
|
C
C
O
O
H
O
H
O
O
H
N
N
O
|
|
Salicyldehyde
O
O
(o-hydroxy benzaldehyde)
Ortho nitrophenol

Ortho nitrobenzoic acid

96 Chemical Bonding

The extent of both intramolecular and intermolecular hydrogen
bonding depends on temperature
...
Some of the properties affected by H-bond are given below,
(1) Dissociation : In aqueous solution, hydrogen fluoride dissociates
and gives the difluoride ion (HF2 ) instead of fluoride ion (F  )
...
This explains the existence of KHF2
...
g
...
99 Å but H-bond = 1
...

2

(2) Association : The molecules of carboxylic acids exist as dimers
because of the hydrogen bonding
...

For example, molecular mass of acetic acid is found to be 120
...

(6) The substances which contain hydrogen bonding have higher
viscosity and high surface tension
...

As a matter of fact, each water molecule is linked tetrahedrally to four other
water molecules
...
That is why ice floats on water
...
This goes
on upto 277K
...


breaking of
H-bonds
...
Hence density of water

The high melting points and boiling points of the compounds
(H 2 O, HF and NH 3 ) containing hydrogen bonds is due to the fact that

is maximum 277 K
...
For example, lower

alcohols are soluble in water because of the hydrogen bonding which can
take place between water and alcohol molecules as shown below,








H

0
...




H

H

H

The intermolecular hydrogen bonding increases solubility of the
compound in water while, the intramolecular hydrogen bonding decreases
...
H  O
...
77 Å
(177 pm)

O

H

O
H

H
O

H

H
O

H

H

H

O

O

N
|
O

H – O …… H – O – H

H
O

o- Nitrophenol
Due to chelation, – OH group is not
available to form hydrogen bond with
water hence it is sparingly soluble in
water
...



Title: Chemical bonding theories
Description: Now you may practice your mcquestions with the help of this theory