CH 3 classification of elements Class 11 Notes | EduRev

Class 11 : CH 3 classification of elements Class 11 Notes | EduRev

 Page 1


35 
 
CHAPTER-3  
CLASSIFICATION OF ELEMENTS AND PERIODICITY IN 
PROPERTIES OF ELEMENTS 
Mandeleev’s Periodic Law:- The properties of the elements are the periodic 
function of their atomic masses. 
Moseley, the English physicist showed that atomic number is more fundamental 
property of an element than its atomic mass. Therefore, the position of an element 
in the periodic table depends on its atomic number than its atomic mass. 
Modern Periodic Law: The physical and chemical properties of elements are the 
periodic functions of their atomic numbers. 
Types of Elements:  s-, p-, d- and f- blocks. 
MAIN GROUP ELEMENTS/ REPRESENTATIVE ELEMENTS: 
The s- and p- block elements are called main group elements or representative 
elements. 
s- block elements: Group-1 (Alkali metals) and Group-2 elements (Alkaline 
earth metals) which respectively have ns
1
 and ns
2
 outermost electronic 
configurations. 
p- Block elements: They  belongs to group- 13 to 18. The outer most electronic 
configuration is ns
2
 np
1-6
. He (1s
2
) is a s- block element but is positioned with the 
group 18 elements (ns
2
 np
6
) because it has completely filled valence shell and as 
a result, exhibits properties characteristic of other noble gases.  
d- block elements (Transition elements) are the elements of group 3 to 12 
having outer electronic configuration (n-1) d
1-10
 ns
1-2
. Four transition series are 
3d, 4d, 5d and 6d. The 6d- series is incomplete. Atomic radius generally 
decreases across a period and increases as we descend the group. 
f-Block elements (Inner- transition Series) 
Lanthanoids charecterised by the filling of4 f-orbitals, are the elements following 
lanthanum from 
58
Ce to 
71
Lu. Actinoids characterised by filling of 5f-orbitals, are 
the elements following actinium from 
70
Th
 to 
103
Lr. Characteristic outer electronic 
configuration is (n-2) f
1-14
 (n-1) d
0-1
 ns
2
. 
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Page 2


35 
 
CHAPTER-3  
CLASSIFICATION OF ELEMENTS AND PERIODICITY IN 
PROPERTIES OF ELEMENTS 
Mandeleev’s Periodic Law:- The properties of the elements are the periodic 
function of their atomic masses. 
Moseley, the English physicist showed that atomic number is more fundamental 
property of an element than its atomic mass. Therefore, the position of an element 
in the periodic table depends on its atomic number than its atomic mass. 
Modern Periodic Law: The physical and chemical properties of elements are the 
periodic functions of their atomic numbers. 
Types of Elements:  s-, p-, d- and f- blocks. 
MAIN GROUP ELEMENTS/ REPRESENTATIVE ELEMENTS: 
The s- and p- block elements are called main group elements or representative 
elements. 
s- block elements: Group-1 (Alkali metals) and Group-2 elements (Alkaline 
earth metals) which respectively have ns
1
 and ns
2
 outermost electronic 
configurations. 
p- Block elements: They  belongs to group- 13 to 18. The outer most electronic 
configuration is ns
2
 np
1-6
. He (1s
2
) is a s- block element but is positioned with the 
group 18 elements (ns
2
 np
6
) because it has completely filled valence shell and as 
a result, exhibits properties characteristic of other noble gases.  
d- block elements (Transition elements) are the elements of group 3 to 12 
having outer electronic configuration (n-1) d
1-10
 ns
1-2
. Four transition series are 
3d, 4d, 5d and 6d. The 6d- series is incomplete. Atomic radius generally 
decreases across a period and increases as we descend the group. 
f-Block elements (Inner- transition Series) 
Lanthanoids charecterised by the filling of4 f-orbitals, are the elements following 
lanthanum from 
58
Ce to 
71
Lu. Actinoids characterised by filling of 5f-orbitals, are 
the elements following actinium from 
70
Th
 to 
103
Lr. Characteristic outer electronic 
configuration is (n-2) f
1-14
 (n-1) d
0-1
 ns
2
. 
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Please Visit www.ncerthelp.com For Video lectures of all subjects Class 9 to 12 
36 
 
Noble Gases: The gaseous elements of group 18 are called noble gases. The 
general outermost electronic configuration of noble gases (except He) is ns
2
 np
6
. 
He exceptionally has 1s
2
 configuration. Thus the outermost shell of noble gases is 
completely filled.  
PERIODICITY: The repetition of similar properties after regular intervals is 
called periodicity.   
Cause of Periodicity: The properties of elements are the periodic repetition of 
similar electronic configuration of elements as the atomic number increases. 
ATOMIC PROPERTIES: The physical characteristics of the atom of an 
element are called atomic properties. The properties such as atomic radius, ionic 
radius, ionisation energy, electro-negativity, electron affinity and valence etc., 
called atomic properties. 
ATOMIC RADIUS- The distance from the centre of the nucleus to the 
outermost shell of the electrons in the atom of any element is called its atomic 
radius. 
Periodicity- (a) In period- Atomic radius of elements decreases from left to right 
in a period. 
(b) In Group- Atomic radius of elements increases on moving top to bottom in a 
group. 
COVALENT RADIUS- Half the inter-nuclear distance between two similar 
atoms of any element which are covalently bonded to each other by a single 
covalent bond is called covalent radius. 
VAN DER WAALS’ RADIUS: Half the inter-nuclear separation between two 
similar adjacent atoms belonging to the two neighbouring molecules of the same 
substance in the solid state is called the van der waals’radius of that atom. 
METALLIC RADIUS: Half the distance between the nuclei of the two adjacent 
metal atoms in a close packed lattice of the metal is called its metallic radius. 
Van der Waals’radius > Metallic radius > Covalent radius 
IONIC RADIUS: The effective distance from the centre of the nucleus of an ion 
upto which it has an influence on its electron cloud is called its ionic radius.   
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Page 3


35 
 
CHAPTER-3  
CLASSIFICATION OF ELEMENTS AND PERIODICITY IN 
PROPERTIES OF ELEMENTS 
Mandeleev’s Periodic Law:- The properties of the elements are the periodic 
function of their atomic masses. 
Moseley, the English physicist showed that atomic number is more fundamental 
property of an element than its atomic mass. Therefore, the position of an element 
in the periodic table depends on its atomic number than its atomic mass. 
Modern Periodic Law: The physical and chemical properties of elements are the 
periodic functions of their atomic numbers. 
Types of Elements:  s-, p-, d- and f- blocks. 
MAIN GROUP ELEMENTS/ REPRESENTATIVE ELEMENTS: 
The s- and p- block elements are called main group elements or representative 
elements. 
s- block elements: Group-1 (Alkali metals) and Group-2 elements (Alkaline 
earth metals) which respectively have ns
1
 and ns
2
 outermost electronic 
configurations. 
p- Block elements: They  belongs to group- 13 to 18. The outer most electronic 
configuration is ns
2
 np
1-6
. He (1s
2
) is a s- block element but is positioned with the 
group 18 elements (ns
2
 np
6
) because it has completely filled valence shell and as 
a result, exhibits properties characteristic of other noble gases.  
d- block elements (Transition elements) are the elements of group 3 to 12 
having outer electronic configuration (n-1) d
1-10
 ns
1-2
. Four transition series are 
3d, 4d, 5d and 6d. The 6d- series is incomplete. Atomic radius generally 
decreases across a period and increases as we descend the group. 
f-Block elements (Inner- transition Series) 
Lanthanoids charecterised by the filling of4 f-orbitals, are the elements following 
lanthanum from 
58
Ce to 
71
Lu. Actinoids characterised by filling of 5f-orbitals, are 
the elements following actinium from 
70
Th
 to 
103
Lr. Characteristic outer electronic 
configuration is (n-2) f
1-14
 (n-1) d
0-1
 ns
2
. 
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CBSE sample papers, Question papers, Notes for Class 6 to 12 
Please Visit www.ncerthelp.com For Video lectures of all subjects Class 9 to 12 
36 
 
Noble Gases: The gaseous elements of group 18 are called noble gases. The 
general outermost electronic configuration of noble gases (except He) is ns
2
 np
6
. 
He exceptionally has 1s
2
 configuration. Thus the outermost shell of noble gases is 
completely filled.  
PERIODICITY: The repetition of similar properties after regular intervals is 
called periodicity.   
Cause of Periodicity: The properties of elements are the periodic repetition of 
similar electronic configuration of elements as the atomic number increases. 
ATOMIC PROPERTIES: The physical characteristics of the atom of an 
element are called atomic properties. The properties such as atomic radius, ionic 
radius, ionisation energy, electro-negativity, electron affinity and valence etc., 
called atomic properties. 
ATOMIC RADIUS- The distance from the centre of the nucleus to the 
outermost shell of the electrons in the atom of any element is called its atomic 
radius. 
Periodicity- (a) In period- Atomic radius of elements decreases from left to right 
in a period. 
(b) In Group- Atomic radius of elements increases on moving top to bottom in a 
group. 
COVALENT RADIUS- Half the inter-nuclear distance between two similar 
atoms of any element which are covalently bonded to each other by a single 
covalent bond is called covalent radius. 
VAN DER WAALS’ RADIUS: Half the inter-nuclear separation between two 
similar adjacent atoms belonging to the two neighbouring molecules of the same 
substance in the solid state is called the van der waals’radius of that atom. 
METALLIC RADIUS: Half the distance between the nuclei of the two adjacent 
metal atoms in a close packed lattice of the metal is called its metallic radius. 
Van der Waals’radius > Metallic radius > Covalent radius 
IONIC RADIUS: The effective distance from the centre of the nucleus of an ion 
upto which it has an influence on its electron cloud is called its ionic radius.   
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37 
 
A cation is smaller but the anion is larger than the parent atom. In case of iso-
electronic species, the cation with greater positive charge has smaller radius but 
anion with greater negative charge has the larger radii. 
IONISATION ENTHALPY: The ionisation enthalpy is the molar enthalpy 
change accompanying the removal of an electron from a gaseous phase atom or 
ion in its ground state. Thus enthalpy change for the reaction; M
(g)
? M
+
(g)
 + e
-
 
Is the ionisation enthalpy of the element M. Like ionisation energies for 
successive ionisation, the successive ionisation enthalpy may also be termed as 
2
nd
 ionisation enthalpy (?
r
H
2
), third ionisation enthalpy  (?
r
H
3
) etc. The term 
ionisation enthalpy is taken for the first ionisation enthalpy, (?
r
H
1
) is expressed 
in kg mol
-
 or in eV. 
Periodicity: 
i) Generally the ionisation enthalpies follow the order ( there are few 
exceptions): 
(?
r
H
1
)  < (?
r
H
2
) < (?
r
H
3
) 
ii) The ionisation enthalpy decreases on moving top to bottom in a group. 
iii) The ionisation enthalpy increases on moving from left to right in a period. 
ELECTRON GAIN ENTHALPY: The electron gain enthalpy ((?
eg
H) is the 
molar enthalpy change when an isolated gaseous atom or ion in its ground state 
adds an electron to form the corresponding anion thus the enthalpy change for the 
reaction; X
(g)
 +  e
-
 ? X
-
(g)
 
Is called the electron gain enthalpy (?
eg
 H) of the element X. The?
eg
 H may be 
positive or negative. 
The successive values for the addition of second, third etc. Electron, these are 
called second, third etc. electron gain enthalpies. For example, 
X
(g)
 +   e
- 
 ?  X
-
(g)
  ?H= ?
eg
 H
1
 is called first electron gain 
enthalpy 
X
-
(g)
 + e
-
 ? X
2-
(g)
  ?H= ?
eg
 H
2
 is called second electron gain 
enthalpy 
X
2-
(g)
 + e
-
 ? X
3-
(g)
  ?H= ?
eg
 H
3
 is called third electron gain 
enthalpy Usually the term electron gain enthalpy (?
eg
H) means the first electron 
gain enthalpy. 
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CBSE sample papers, Question papers, Notes for Class 6 to 12 
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Page 4


35 
 
CHAPTER-3  
CLASSIFICATION OF ELEMENTS AND PERIODICITY IN 
PROPERTIES OF ELEMENTS 
Mandeleev’s Periodic Law:- The properties of the elements are the periodic 
function of their atomic masses. 
Moseley, the English physicist showed that atomic number is more fundamental 
property of an element than its atomic mass. Therefore, the position of an element 
in the periodic table depends on its atomic number than its atomic mass. 
Modern Periodic Law: The physical and chemical properties of elements are the 
periodic functions of their atomic numbers. 
Types of Elements:  s-, p-, d- and f- blocks. 
MAIN GROUP ELEMENTS/ REPRESENTATIVE ELEMENTS: 
The s- and p- block elements are called main group elements or representative 
elements. 
s- block elements: Group-1 (Alkali metals) and Group-2 elements (Alkaline 
earth metals) which respectively have ns
1
 and ns
2
 outermost electronic 
configurations. 
p- Block elements: They  belongs to group- 13 to 18. The outer most electronic 
configuration is ns
2
 np
1-6
. He (1s
2
) is a s- block element but is positioned with the 
group 18 elements (ns
2
 np
6
) because it has completely filled valence shell and as 
a result, exhibits properties characteristic of other noble gases.  
d- block elements (Transition elements) are the elements of group 3 to 12 
having outer electronic configuration (n-1) d
1-10
 ns
1-2
. Four transition series are 
3d, 4d, 5d and 6d. The 6d- series is incomplete. Atomic radius generally 
decreases across a period and increases as we descend the group. 
f-Block elements (Inner- transition Series) 
Lanthanoids charecterised by the filling of4 f-orbitals, are the elements following 
lanthanum from 
58
Ce to 
71
Lu. Actinoids characterised by filling of 5f-orbitals, are 
the elements following actinium from 
70
Th
 to 
103
Lr. Characteristic outer electronic 
configuration is (n-2) f
1-14
 (n-1) d
0-1
 ns
2
. 
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CBSE sample papers, Question papers, Notes for Class 6 to 12 
Please Visit www.ncerthelp.com For Video lectures of all subjects Class 9 to 12 
36 
 
Noble Gases: The gaseous elements of group 18 are called noble gases. The 
general outermost electronic configuration of noble gases (except He) is ns
2
 np
6
. 
He exceptionally has 1s
2
 configuration. Thus the outermost shell of noble gases is 
completely filled.  
PERIODICITY: The repetition of similar properties after regular intervals is 
called periodicity.   
Cause of Periodicity: The properties of elements are the periodic repetition of 
similar electronic configuration of elements as the atomic number increases. 
ATOMIC PROPERTIES: The physical characteristics of the atom of an 
element are called atomic properties. The properties such as atomic radius, ionic 
radius, ionisation energy, electro-negativity, electron affinity and valence etc., 
called atomic properties. 
ATOMIC RADIUS- The distance from the centre of the nucleus to the 
outermost shell of the electrons in the atom of any element is called its atomic 
radius. 
Periodicity- (a) In period- Atomic radius of elements decreases from left to right 
in a period. 
(b) In Group- Atomic radius of elements increases on moving top to bottom in a 
group. 
COVALENT RADIUS- Half the inter-nuclear distance between two similar 
atoms of any element which are covalently bonded to each other by a single 
covalent bond is called covalent radius. 
VAN DER WAALS’ RADIUS: Half the inter-nuclear separation between two 
similar adjacent atoms belonging to the two neighbouring molecules of the same 
substance in the solid state is called the van der waals’radius of that atom. 
METALLIC RADIUS: Half the distance between the nuclei of the two adjacent 
metal atoms in a close packed lattice of the metal is called its metallic radius. 
Van der Waals’radius > Metallic radius > Covalent radius 
IONIC RADIUS: The effective distance from the centre of the nucleus of an ion 
upto which it has an influence on its electron cloud is called its ionic radius.   
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CBSE sample papers, Question papers, Notes for Class 6 to 12 
Please Visit www.ncerthelp.com For Video lectures of all subjects Class 9 to 12 
37 
 
A cation is smaller but the anion is larger than the parent atom. In case of iso-
electronic species, the cation with greater positive charge has smaller radius but 
anion with greater negative charge has the larger radii. 
IONISATION ENTHALPY: The ionisation enthalpy is the molar enthalpy 
change accompanying the removal of an electron from a gaseous phase atom or 
ion in its ground state. Thus enthalpy change for the reaction; M
(g)
? M
+
(g)
 + e
-
 
Is the ionisation enthalpy of the element M. Like ionisation energies for 
successive ionisation, the successive ionisation enthalpy may also be termed as 
2
nd
 ionisation enthalpy (?
r
H
2
), third ionisation enthalpy  (?
r
H
3
) etc. The term 
ionisation enthalpy is taken for the first ionisation enthalpy, (?
r
H
1
) is expressed 
in kg mol
-
 or in eV. 
Periodicity: 
i) Generally the ionisation enthalpies follow the order ( there are few 
exceptions): 
(?
r
H
1
)  < (?
r
H
2
) < (?
r
H
3
) 
ii) The ionisation enthalpy decreases on moving top to bottom in a group. 
iii) The ionisation enthalpy increases on moving from left to right in a period. 
ELECTRON GAIN ENTHALPY: The electron gain enthalpy ((?
eg
H) is the 
molar enthalpy change when an isolated gaseous atom or ion in its ground state 
adds an electron to form the corresponding anion thus the enthalpy change for the 
reaction; X
(g)
 +  e
-
 ? X
-
(g)
 
Is called the electron gain enthalpy (?
eg
 H) of the element X. The?
eg
 H may be 
positive or negative. 
The successive values for the addition of second, third etc. Electron, these are 
called second, third etc. electron gain enthalpies. For example, 
X
(g)
 +   e
- 
 ?  X
-
(g)
  ?H= ?
eg
 H
1
 is called first electron gain 
enthalpy 
X
-
(g)
 + e
-
 ? X
2-
(g)
  ?H= ?
eg
 H
2
 is called second electron gain 
enthalpy 
X
2-
(g)
 + e
-
 ? X
3-
(g)
  ?H= ?
eg
 H
3
 is called third electron gain 
enthalpy Usually the term electron gain enthalpy (?
eg
H) means the first electron 
gain enthalpy. 
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CBSE sample papers, Question papers, Notes for Class 6 to 12 
Please Visit www.ncerthelp.com For Video lectures of all subjects Class 9 to 12 
38 
 
Periodicity: 
(i) In period- The electron gain enthalpy increases from left to right in a 
period. 
(ii) In group- The electron gain enthalpy decreases from top to bottom in a 
group. 
ELECTRONEGATIVITY: “The relative tendency of an atom in a molecule to 
attract the shared pair of electrons towards itself is termed as its electro-
negativity.” 
Periodicity: 
(i) In period- The electro-negativity increases from left to right in a period. 
(ii) In group- The electro-negativity decreases from top to bottom in a group. 
VALENCE ELECTRONS: The electrons present in outermost shell are called 
as valence electron. Because the electrons in the outermost shell determine the 
valency of an element. 
VALENCY OF AN ELEMENT: The number of hydrogen or halogen atom or 
double the number of oxygen atom, which combin with one atom of the element 
is taken as its valency. According to the electronic concept of valency, “ the 
number of electrons which an atom loses or gains or shares with other atom to 
attain the noble gas configuration is termed as its valency.” 
 Periodicity: 
(i) In period- The valency first increases then decreases from left to right in a 
period. 
(ii) In group- The valency remains constant from top to bottom in a group. 
ELECTROPOSITIVE OR METALLIC CHARACTER: The tendency of an 
element to lose electrons and forms positive ions (cations) is called 
electropositive or metallic character. The elements having lower ionisation 
energies have higher tendency to lose electrons, thus they are electropositive or 
metallic in their behaviour.  
Alkali metals are the most highly electropositive elements. 
Periodicity: In period- The electropositive or metallic characters decreases from 
left to right in a period. 
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CBSE sample papers, Question papers, Notes for Class 6 to 12 
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Page 5


35 
 
CHAPTER-3  
CLASSIFICATION OF ELEMENTS AND PERIODICITY IN 
PROPERTIES OF ELEMENTS 
Mandeleev’s Periodic Law:- The properties of the elements are the periodic 
function of their atomic masses. 
Moseley, the English physicist showed that atomic number is more fundamental 
property of an element than its atomic mass. Therefore, the position of an element 
in the periodic table depends on its atomic number than its atomic mass. 
Modern Periodic Law: The physical and chemical properties of elements are the 
periodic functions of their atomic numbers. 
Types of Elements:  s-, p-, d- and f- blocks. 
MAIN GROUP ELEMENTS/ REPRESENTATIVE ELEMENTS: 
The s- and p- block elements are called main group elements or representative 
elements. 
s- block elements: Group-1 (Alkali metals) and Group-2 elements (Alkaline 
earth metals) which respectively have ns
1
 and ns
2
 outermost electronic 
configurations. 
p- Block elements: They  belongs to group- 13 to 18. The outer most electronic 
configuration is ns
2
 np
1-6
. He (1s
2
) is a s- block element but is positioned with the 
group 18 elements (ns
2
 np
6
) because it has completely filled valence shell and as 
a result, exhibits properties characteristic of other noble gases.  
d- block elements (Transition elements) are the elements of group 3 to 12 
having outer electronic configuration (n-1) d
1-10
 ns
1-2
. Four transition series are 
3d, 4d, 5d and 6d. The 6d- series is incomplete. Atomic radius generally 
decreases across a period and increases as we descend the group. 
f-Block elements (Inner- transition Series) 
Lanthanoids charecterised by the filling of4 f-orbitals, are the elements following 
lanthanum from 
58
Ce to 
71
Lu. Actinoids characterised by filling of 5f-orbitals, are 
the elements following actinium from 
70
Th
 to 
103
Lr. Characteristic outer electronic 
configuration is (n-2) f
1-14
 (n-1) d
0-1
 ns
2
. 
Visit www.ncerthelp.com For All NCERT solutions, 
CBSE sample papers, Question papers, Notes for Class 6 to 12 
Please Visit www.ncerthelp.com For Video lectures of all subjects Class 9 to 12 
36 
 
Noble Gases: The gaseous elements of group 18 are called noble gases. The 
general outermost electronic configuration of noble gases (except He) is ns
2
 np
6
. 
He exceptionally has 1s
2
 configuration. Thus the outermost shell of noble gases is 
completely filled.  
PERIODICITY: The repetition of similar properties after regular intervals is 
called periodicity.   
Cause of Periodicity: The properties of elements are the periodic repetition of 
similar electronic configuration of elements as the atomic number increases. 
ATOMIC PROPERTIES: The physical characteristics of the atom of an 
element are called atomic properties. The properties such as atomic radius, ionic 
radius, ionisation energy, electro-negativity, electron affinity and valence etc., 
called atomic properties. 
ATOMIC RADIUS- The distance from the centre of the nucleus to the 
outermost shell of the electrons in the atom of any element is called its atomic 
radius. 
Periodicity- (a) In period- Atomic radius of elements decreases from left to right 
in a period. 
(b) In Group- Atomic radius of elements increases on moving top to bottom in a 
group. 
COVALENT RADIUS- Half the inter-nuclear distance between two similar 
atoms of any element which are covalently bonded to each other by a single 
covalent bond is called covalent radius. 
VAN DER WAALS’ RADIUS: Half the inter-nuclear separation between two 
similar adjacent atoms belonging to the two neighbouring molecules of the same 
substance in the solid state is called the van der waals’radius of that atom. 
METALLIC RADIUS: Half the distance between the nuclei of the two adjacent 
metal atoms in a close packed lattice of the metal is called its metallic radius. 
Van der Waals’radius > Metallic radius > Covalent radius 
IONIC RADIUS: The effective distance from the centre of the nucleus of an ion 
upto which it has an influence on its electron cloud is called its ionic radius.   
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CBSE sample papers, Question papers, Notes for Class 6 to 12 
Please Visit www.ncerthelp.com For Video lectures of all subjects Class 9 to 12 
37 
 
A cation is smaller but the anion is larger than the parent atom. In case of iso-
electronic species, the cation with greater positive charge has smaller radius but 
anion with greater negative charge has the larger radii. 
IONISATION ENTHALPY: The ionisation enthalpy is the molar enthalpy 
change accompanying the removal of an electron from a gaseous phase atom or 
ion in its ground state. Thus enthalpy change for the reaction; M
(g)
? M
+
(g)
 + e
-
 
Is the ionisation enthalpy of the element M. Like ionisation energies for 
successive ionisation, the successive ionisation enthalpy may also be termed as 
2
nd
 ionisation enthalpy (?
r
H
2
), third ionisation enthalpy  (?
r
H
3
) etc. The term 
ionisation enthalpy is taken for the first ionisation enthalpy, (?
r
H
1
) is expressed 
in kg mol
-
 or in eV. 
Periodicity: 
i) Generally the ionisation enthalpies follow the order ( there are few 
exceptions): 
(?
r
H
1
)  < (?
r
H
2
) < (?
r
H
3
) 
ii) The ionisation enthalpy decreases on moving top to bottom in a group. 
iii) The ionisation enthalpy increases on moving from left to right in a period. 
ELECTRON GAIN ENTHALPY: The electron gain enthalpy ((?
eg
H) is the 
molar enthalpy change when an isolated gaseous atom or ion in its ground state 
adds an electron to form the corresponding anion thus the enthalpy change for the 
reaction; X
(g)
 +  e
-
 ? X
-
(g)
 
Is called the electron gain enthalpy (?
eg
 H) of the element X. The?
eg
 H may be 
positive or negative. 
The successive values for the addition of second, third etc. Electron, these are 
called second, third etc. electron gain enthalpies. For example, 
X
(g)
 +   e
- 
 ?  X
-
(g)
  ?H= ?
eg
 H
1
 is called first electron gain 
enthalpy 
X
-
(g)
 + e
-
 ? X
2-
(g)
  ?H= ?
eg
 H
2
 is called second electron gain 
enthalpy 
X
2-
(g)
 + e
-
 ? X
3-
(g)
  ?H= ?
eg
 H
3
 is called third electron gain 
enthalpy Usually the term electron gain enthalpy (?
eg
H) means the first electron 
gain enthalpy. 
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38 
 
Periodicity: 
(i) In period- The electron gain enthalpy increases from left to right in a 
period. 
(ii) In group- The electron gain enthalpy decreases from top to bottom in a 
group. 
ELECTRONEGATIVITY: “The relative tendency of an atom in a molecule to 
attract the shared pair of electrons towards itself is termed as its electro-
negativity.” 
Periodicity: 
(i) In period- The electro-negativity increases from left to right in a period. 
(ii) In group- The electro-negativity decreases from top to bottom in a group. 
VALENCE ELECTRONS: The electrons present in outermost shell are called 
as valence electron. Because the electrons in the outermost shell determine the 
valency of an element. 
VALENCY OF AN ELEMENT: The number of hydrogen or halogen atom or 
double the number of oxygen atom, which combin with one atom of the element 
is taken as its valency. According to the electronic concept of valency, “ the 
number of electrons which an atom loses or gains or shares with other atom to 
attain the noble gas configuration is termed as its valency.” 
 Periodicity: 
(i) In period- The valency first increases then decreases from left to right in a 
period. 
(ii) In group- The valency remains constant from top to bottom in a group. 
ELECTROPOSITIVE OR METALLIC CHARACTER: The tendency of an 
element to lose electrons and forms positive ions (cations) is called 
electropositive or metallic character. The elements having lower ionisation 
energies have higher tendency to lose electrons, thus they are electropositive or 
metallic in their behaviour.  
Alkali metals are the most highly electropositive elements. 
Periodicity: In period- The electropositive or metallic characters decreases from 
left to right in a period. 
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39 
 
In group- The electropositive or metallic characters increases from top 
to bottom in a group. 
ELECTRO-NEGATIVE OR NON- METALLIC CHARACTERS: the 
tendency of an element to accept electrons to form an anion is called its non 
metallic or electronegative character. The elements having high electro-negativity 
have higher tendency to gain electrons and forms anion. So, the elements in the 
upper right hand portion of the periodic table are electro-negative or non-metallic 
in nature. 
Periodicity: 
(i) In period- The electro-negative or non- metallic characters increases from 
left to right in a period. 
(ii) In group- The electro-negative or non-metallic characters decreases from 
top to bottom in a group. 
 
REACTIVITY OF METALS:  
Periodicity: 
(i) In period- The tendency of an element to lose electrons decreases in a 
period. So the reactivity  
     of metals decreases from left to right in a period. 
(ii) In group- The tendency of an element to lose electrons increases in a 
period. So the reactivity  
   of metals increases from top to bottom in a group. 
 
REACTIVITY OF NON- METALS: 
 
(i) In period- The tendency of an element to gain electrons increases in a 
period. So the reactivity  
     of non-metals increases from left to right in a period. 
(ii) In group- The tendency of an element to gain electrons decreases in a 
group. So the reactivity  
   of non-metals increases from top to bottom in a group. 
 
 
 
 
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