Jr|11th IIT-JEE MAIN|NEET|CHEMISTRY:VOL-I
“The attractive force that holds two or more “An atom or group of atoms which can exist
constituent atoms (or) oppositely charged ions independently at normal conditions is called
together in different chemical species is called Molecule”.
Chemical Bond”. The number of atoms present per molecule depends
In a Chemical Bond both attractive and repulsive on the combining capacity of the constituent
forces exist in equilibrium. elements.”This combining capacity is called valence”
Atoms are less stable and more energetic hence
Valence
they form molecules by losing some energy by
participating in Chemical Bond. The combining capacity of an atom with number of
H + H H2 + 434.72KJ hydrogen atoms or number of chlorine atoms or
Cl + Cl Cl2 + 239.1KJ double the number of oxygen atoms.
Eg: In NH 3 the valence of Nitrogen is 3
+ Repulsion
Potential Energy (KJ / mol)
Potential Envery Vs In CO2 the valence of carbon is 4
HH Internuclar Distance
Between Two Atoms
In Cl2O7 the valence of chlorine is 7
Bond length
H H
Kossel - Lewis Theory
Bond Energy
It is also called “Electronic Theory of Valence or
Chemical bond Theory ” or modern Theory
- Attraction
of Valence.
H H This was proposed on the basis of Bohr’s Atomic
Internuclear Distance (pm) theory
According to this, the outer most energy level in
Formation of bond is accompanied by decrease in an atom is known as valence shell and the electrons
potential energy. Chemical Bond formation is present in it are called Valence electrons. The
always Exothermic electrons present in the inner energy levels are
All Noble gases are very stable chemically due to known as core electrons.
the completely filled electronic configuration hence Nucleus plus inner electrons are called kernel.
all other elements participate in chemical bond in Elements with eight electrons in their valence
order to acquire stable configuration. shell are more stable than other elements .
Under normal conditions most of the elements do An atom must possess eight electrons in outermost
not exist as independent atoms.They occur as energy level for its stability (octet rule) Hence all
Molecules, because atoms can’t exist independently elements try to acquire eight electrons in their outer
at ordinary conditions.
CHEMICAL BONDING 113
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most orbit(octet configuration)
Eventhough ‘He’ has only two electrons in the No.of
valence shell,it is highly stable and chemically Lewis
Valence
inert(duplet Configuration) structure
Electrons
Atoms attain stable electronic configuration
(duplet and octet configuration) either by transfer 1 Hydrogen/Group IA H / Li
(or) by sharing of electrons. The Number of 2 Helium/ Group IIA Mg
electrons transferred per atom is called
Electrovalence and resultant bond is called 3 Group IIIA B
Electrovalent bond.The number of electrons shared
4 Group IVA C
per atom is called co-valence and the resultant bond
is called covalent bond. 5 Group VA N
Nature of the bond formed between two atoms
depends on electropositive and electronegative 6 Group VIA O
character of bonded atoms.
For example, the formation of NaCl from sodium They can be used to calculate the group valence
and chlorine, according to the above scheme, can of the element.
be explained as :
Na Na e H H H H
C C (or)
Ne 3s1 Ne C C
H 8e- 8e- H H H
Cl e Cl
C2H4 molecule
Ne 3s 2 3 p5 Ne 3s 2 3 p6 or Ar
Na Cl NaCl or Na Cl When combining atoms share three electron pairs
Similarly the formation of CaF2 may be shown as : as in the case of two nitrogen atoms in the molecule
and the two carbon atoms in the ethyne molecule,
Ca Ca 2 2e a triple bond is formed.
Ar 4s 2 Ar
N N (or) N N
F e F - -
8e 8e
He 2s 2 sp5 He 2s 2 sp 6 or Ne N2 molecule
Ca 2 2 F CaF2 or Ca 2 F
2 H C C H (or) H C C H
E .P E.N Ionic Bond 8e- 8e-
E .N E .N Covalent Bond or Dative Bond C2H2 molecule
E.P E.P Metallic Bond
Lewis introduced simple notations to represent Limitation of Octet Rule
valence electrons in an atom .They are called Lewis Octet rule is quite useful for understanding the
symbols. sturcture of most of the organic compounds and it
In these symbols valence electrons are represented applies mainly to elements of second period of the
by dots. periodic table.
Eg: L i
There are three types of exceptions to the Octet rule.
Be B etc ..
a) In some compounds, the number of electrons
Lewis Structure of Elements surrounding the central atom is less than eight.
(incomplete octet of the central atom)
114 CHEMICAL BONDING
,Jr|11th IIT-JEE MAIN|NEET|CHEMISTRY:VOL-I
Eg: LiCl , BeH 2 , BCl3 etc....
O
b) In some compounds, the number of electrons
surrounding the central atom is more than eight (ex- H O S O H
panded octet (or) super octet of central atom)
O
Eg: PF5 , SF6 , H 2 SO4 , IF7
H2SO4
c) In molecules with an odd number of electrons
like NO and NO2 , the octet rule is not satisfied for Note : Interestinglysulphur also forms manycompounds
all the atoms. in which the octet rule is obeyed. In sulphur dichloride,
the S atom has an octet of electrons around it
N O; O N O :
: Cl S Cl ; or : Cl : S Cl :
Ionic Bond (Electrovalent Bond)
Total No. of Valence “The strong electrostatic forces of attraction
Molecule Electrons in the between two oppositely charged ions which are
Central Atom formed due to transfer of electrons from one
CH4 C 8 (Octet) atom to another is called Ionic Bond (or) Elec-
NH3 N 8 (Octet) trovalent Bond”
H2O O 8 (Octet)
Generally Ionic Bond is formed between a Metal of
groups 1, 2 and Non metals of groups 16 and 17.
CO2 C 8 (Octet)
Formation of Ionic bond is a redox process,
C2H6 C (each) 8 (Octet) because oen atom undergoes oxidation and other
C2H4 C (each) 8 (Octet) one undergoes reduction.
C2H2 C (each) 8 (Octet) Mg Mg 2 2e2
BeCl2 Be 4 (Electron deficient)
2 F 2e 2 F
BCl3 B 6 (Electron deficient)
PCl5 P 10 (Expanded Octet)
Mg 2 2 F MgF2 or Mg 2 F
2
SF6 S 12 (Expanded Octet) The maximum electrovalency in the formation of
IF7 I 14 (Expanded Octet) Ionic Bond is 3.
In Mg F2 Electrovalence of Mg = 2, F = 1
In Na2O electrovalence of Na = 1, O = 2
Some of the example of such compounds are : Obviously ionic bonds will be formed more easily
PF3, SF6, H2SO4 and a number of coordination between elements with comparatively low ionization
compounds. enthalpies and elements with comparitively low
ionization enthalpies and elements with comparitively
high negative value of electron gain enthalpy.
F F Most ionic compounds have cations derived from
F F F metallic elements. The ammonium ion, NH 4 (made
F P P up of two non metallic elements) is an exception. It
F F F forms the cation of a number of ionic compounds.
F F Ionic compounds in the crystalline state consists of
orderly three dimensional arrangement of cations
PF5 SF6 and anions held together by coulombic interaction
energies. These compounds crystallise in different
crystal structures determined by the size of the ions,
their packing arrangements and other factors. The
CHEMICAL BONDING 115
, Jr|11th IIT-JEE MAIN|NEET|CHEMISTRY:VOL-I
crystal structure of sodium chloride, NaCl (rock Formation of cation having inert gas configuration
salt), for example is shown below. is very easy.
Eg: Of the two cations Zn+2(2, 8, 18) and Ca+2(2,
8, 8), Ca+2 is more readily formed and it is more
stable than Zn+2and gives compounds with more
ionic character.
b) Factors favorable for anion formation
High Electronegativity and Electron affinity
Atom having very high electron affinity / electro
negativity forms anion very easily.
F O N
Small atomic size
Small non metal atom forms anion very easily.
In ionic solids, the sum of the electorn gain enthalpy F Cl Br I
and the ionization enthalpy may be positive but still Low charge on the ion.
the crystal structure gets stabilized due to the energy Formation of anion carrying less negative charge is
released in the formation of the crystal lattice. FOr easy.
example the ionization enthalpy for Na+(g) formation
from Na(g) is 495.8KJ mol–1 : while the electron F O2 N3
gain enthalpy for the change Formation of anion having inert gas configuration
is very easy
Cl g e Cl g is, -348.7 KJ mol–1 only..
The sum of the two, 147.1 KJ mol–1 is more than Changes occuring
Nature of the
Element Element in the valence
compensated for by the enthalpy of lattice formation bond formed
A B electrons
of NaCl(s) (-788 KJ mol–1). Therefore, the energy
Low High Transfer of e Ionic bond
released in the processes is more than the energy
High High Sharing of e Covalent bond
absorbed. Thus a qualitative measure of the stability
of an ionic compound is provided by its enthalpy of Low Low Sea of e molecular Metallic bond
orbital bond
lattice formation and not simplify by achieving octet
of electrons around the ionic species in gaseous state. Among NaCl, KCl, RbCl and CsCl , CsCl is
readily formed.
Factors Favourable for the formation of Ionic
Among NaF, NaCl and NaBr, NaF is readily
Bond formed.
a) Factors favourable for cation formation Among NaF, CsF, MgO and CaO, CsF is readily
Low ionisation potential formed.
Atom having very low ionisation potential forms
the cation very easily. Potassium (IP=495.57 kJ/ Lattice Enthalpy
mole) forms the cation more readily than The amount of energy released when the required
sodium(IP=519.82 kJ/mole) number of oppositely charged gaseous ions present
Low charge on the ion at infinite distances come close and form one mole
Formation of cation carrying less positive of ionic crystalline solid is known as the lattice
charge is easy.( Al 3 Mg 2 Na ) energy of the compound.
Large atomic size Eg: Na g Cl g NaCl crystal
Atoms with large atomic size form cations easily U = -782 kJ mol-1
Cs Rb K Na Li 1) Lattice energy calculation from Born-Lande
Formation of cation with Inertgas
configuration.
116 CHEMICAL BONDING
“The attractive force that holds two or more “An atom or group of atoms which can exist
constituent atoms (or) oppositely charged ions independently at normal conditions is called
together in different chemical species is called Molecule”.
Chemical Bond”. The number of atoms present per molecule depends
In a Chemical Bond both attractive and repulsive on the combining capacity of the constituent
forces exist in equilibrium. elements.”This combining capacity is called valence”
Atoms are less stable and more energetic hence
Valence
they form molecules by losing some energy by
participating in Chemical Bond. The combining capacity of an atom with number of
H + H H2 + 434.72KJ hydrogen atoms or number of chlorine atoms or
Cl + Cl Cl2 + 239.1KJ double the number of oxygen atoms.
Eg: In NH 3 the valence of Nitrogen is 3
+ Repulsion
Potential Energy (KJ / mol)
Potential Envery Vs In CO2 the valence of carbon is 4
HH Internuclar Distance
Between Two Atoms
In Cl2O7 the valence of chlorine is 7
Bond length
H H
Kossel - Lewis Theory
Bond Energy
It is also called “Electronic Theory of Valence or
Chemical bond Theory ” or modern Theory
- Attraction
of Valence.
H H This was proposed on the basis of Bohr’s Atomic
Internuclear Distance (pm) theory
According to this, the outer most energy level in
Formation of bond is accompanied by decrease in an atom is known as valence shell and the electrons
potential energy. Chemical Bond formation is present in it are called Valence electrons. The
always Exothermic electrons present in the inner energy levels are
All Noble gases are very stable chemically due to known as core electrons.
the completely filled electronic configuration hence Nucleus plus inner electrons are called kernel.
all other elements participate in chemical bond in Elements with eight electrons in their valence
order to acquire stable configuration. shell are more stable than other elements .
Under normal conditions most of the elements do An atom must possess eight electrons in outermost
not exist as independent atoms.They occur as energy level for its stability (octet rule) Hence all
Molecules, because atoms can’t exist independently elements try to acquire eight electrons in their outer
at ordinary conditions.
CHEMICAL BONDING 113
, Jr|11th IIT-JEE MAIN|NEET|CHEMISTRY:VOL-I
most orbit(octet configuration)
Eventhough ‘He’ has only two electrons in the No.of
valence shell,it is highly stable and chemically Lewis
Valence
inert(duplet Configuration) structure
Electrons
Atoms attain stable electronic configuration
(duplet and octet configuration) either by transfer 1 Hydrogen/Group IA H / Li
(or) by sharing of electrons. The Number of 2 Helium/ Group IIA Mg
electrons transferred per atom is called
Electrovalence and resultant bond is called 3 Group IIIA B
Electrovalent bond.The number of electrons shared
4 Group IVA C
per atom is called co-valence and the resultant bond
is called covalent bond. 5 Group VA N
Nature of the bond formed between two atoms
depends on electropositive and electronegative 6 Group VIA O
character of bonded atoms.
For example, the formation of NaCl from sodium They can be used to calculate the group valence
and chlorine, according to the above scheme, can of the element.
be explained as :
Na Na e H H H H
C C (or)
Ne 3s1 Ne C C
H 8e- 8e- H H H
Cl e Cl
C2H4 molecule
Ne 3s 2 3 p5 Ne 3s 2 3 p6 or Ar
Na Cl NaCl or Na Cl When combining atoms share three electron pairs
Similarly the formation of CaF2 may be shown as : as in the case of two nitrogen atoms in the molecule
and the two carbon atoms in the ethyne molecule,
Ca Ca 2 2e a triple bond is formed.
Ar 4s 2 Ar
N N (or) N N
F e F - -
8e 8e
He 2s 2 sp5 He 2s 2 sp 6 or Ne N2 molecule
Ca 2 2 F CaF2 or Ca 2 F
2 H C C H (or) H C C H
E .P E.N Ionic Bond 8e- 8e-
E .N E .N Covalent Bond or Dative Bond C2H2 molecule
E.P E.P Metallic Bond
Lewis introduced simple notations to represent Limitation of Octet Rule
valence electrons in an atom .They are called Lewis Octet rule is quite useful for understanding the
symbols. sturcture of most of the organic compounds and it
In these symbols valence electrons are represented applies mainly to elements of second period of the
by dots. periodic table.
Eg: L i
There are three types of exceptions to the Octet rule.
Be B etc ..
a) In some compounds, the number of electrons
Lewis Structure of Elements surrounding the central atom is less than eight.
(incomplete octet of the central atom)
114 CHEMICAL BONDING
,Jr|11th IIT-JEE MAIN|NEET|CHEMISTRY:VOL-I
Eg: LiCl , BeH 2 , BCl3 etc....
O
b) In some compounds, the number of electrons
surrounding the central atom is more than eight (ex- H O S O H
panded octet (or) super octet of central atom)
O
Eg: PF5 , SF6 , H 2 SO4 , IF7
H2SO4
c) In molecules with an odd number of electrons
like NO and NO2 , the octet rule is not satisfied for Note : Interestinglysulphur also forms manycompounds
all the atoms. in which the octet rule is obeyed. In sulphur dichloride,
the S atom has an octet of electrons around it
N O; O N O :
: Cl S Cl ; or : Cl : S Cl :
Ionic Bond (Electrovalent Bond)
Total No. of Valence “The strong electrostatic forces of attraction
Molecule Electrons in the between two oppositely charged ions which are
Central Atom formed due to transfer of electrons from one
CH4 C 8 (Octet) atom to another is called Ionic Bond (or) Elec-
NH3 N 8 (Octet) trovalent Bond”
H2O O 8 (Octet)
Generally Ionic Bond is formed between a Metal of
groups 1, 2 and Non metals of groups 16 and 17.
CO2 C 8 (Octet)
Formation of Ionic bond is a redox process,
C2H6 C (each) 8 (Octet) because oen atom undergoes oxidation and other
C2H4 C (each) 8 (Octet) one undergoes reduction.
C2H2 C (each) 8 (Octet) Mg Mg 2 2e2
BeCl2 Be 4 (Electron deficient)
2 F 2e 2 F
BCl3 B 6 (Electron deficient)
PCl5 P 10 (Expanded Octet)
Mg 2 2 F MgF2 or Mg 2 F
2
SF6 S 12 (Expanded Octet) The maximum electrovalency in the formation of
IF7 I 14 (Expanded Octet) Ionic Bond is 3.
In Mg F2 Electrovalence of Mg = 2, F = 1
In Na2O electrovalence of Na = 1, O = 2
Some of the example of such compounds are : Obviously ionic bonds will be formed more easily
PF3, SF6, H2SO4 and a number of coordination between elements with comparatively low ionization
compounds. enthalpies and elements with comparitively low
ionization enthalpies and elements with comparitively
high negative value of electron gain enthalpy.
F F Most ionic compounds have cations derived from
F F F metallic elements. The ammonium ion, NH 4 (made
F P P up of two non metallic elements) is an exception. It
F F F forms the cation of a number of ionic compounds.
F F Ionic compounds in the crystalline state consists of
orderly three dimensional arrangement of cations
PF5 SF6 and anions held together by coulombic interaction
energies. These compounds crystallise in different
crystal structures determined by the size of the ions,
their packing arrangements and other factors. The
CHEMICAL BONDING 115
, Jr|11th IIT-JEE MAIN|NEET|CHEMISTRY:VOL-I
crystal structure of sodium chloride, NaCl (rock Formation of cation having inert gas configuration
salt), for example is shown below. is very easy.
Eg: Of the two cations Zn+2(2, 8, 18) and Ca+2(2,
8, 8), Ca+2 is more readily formed and it is more
stable than Zn+2and gives compounds with more
ionic character.
b) Factors favorable for anion formation
High Electronegativity and Electron affinity
Atom having very high electron affinity / electro
negativity forms anion very easily.
F O N
Small atomic size
Small non metal atom forms anion very easily.
In ionic solids, the sum of the electorn gain enthalpy F Cl Br I
and the ionization enthalpy may be positive but still Low charge on the ion.
the crystal structure gets stabilized due to the energy Formation of anion carrying less negative charge is
released in the formation of the crystal lattice. FOr easy.
example the ionization enthalpy for Na+(g) formation
from Na(g) is 495.8KJ mol–1 : while the electron F O2 N3
gain enthalpy for the change Formation of anion having inert gas configuration
is very easy
Cl g e Cl g is, -348.7 KJ mol–1 only..
The sum of the two, 147.1 KJ mol–1 is more than Changes occuring
Nature of the
Element Element in the valence
compensated for by the enthalpy of lattice formation bond formed
A B electrons
of NaCl(s) (-788 KJ mol–1). Therefore, the energy
Low High Transfer of e Ionic bond
released in the processes is more than the energy
High High Sharing of e Covalent bond
absorbed. Thus a qualitative measure of the stability
of an ionic compound is provided by its enthalpy of Low Low Sea of e molecular Metallic bond
orbital bond
lattice formation and not simplify by achieving octet
of electrons around the ionic species in gaseous state. Among NaCl, KCl, RbCl and CsCl , CsCl is
readily formed.
Factors Favourable for the formation of Ionic
Among NaF, NaCl and NaBr, NaF is readily
Bond formed.
a) Factors favourable for cation formation Among NaF, CsF, MgO and CaO, CsF is readily
Low ionisation potential formed.
Atom having very low ionisation potential forms
the cation very easily. Potassium (IP=495.57 kJ/ Lattice Enthalpy
mole) forms the cation more readily than The amount of energy released when the required
sodium(IP=519.82 kJ/mole) number of oppositely charged gaseous ions present
Low charge on the ion at infinite distances come close and form one mole
Formation of cation carrying less positive of ionic crystalline solid is known as the lattice
charge is easy.( Al 3 Mg 2 Na ) energy of the compound.
Large atomic size Eg: Na g Cl g NaCl crystal
Atoms with large atomic size form cations easily U = -782 kJ mol-1
Cs Rb K Na Li 1) Lattice energy calculation from Born-Lande
Formation of cation with Inertgas
configuration.
116 CHEMICAL BONDING
