Classification of Elements
and Periodicity in
Properties
FAST TRACK : QUICK REVISION
• The first systematic classification of elements was provided by Russian
chemist D.I. Mendeleev.
1. Mendeleev's periodic law
“The physical and chemical properties of elements are periodic functions
of their atomic weight.”
2. It was modified to Modern Periodic law :
“The physical and chemical properties of elements are periodic functions
of their atomic numbers.”
It is the long form of periodic table :
7 Horizontal rows are called Periods and 18 Vertical columns are called
Group
Group-1 are called Alkali metals Group-2 are called Alkaline earth metals.
Group-15 are called Pnicogens Group-16 are called Chalcogens
Group-17 are called Halogens Group-18 are called Noble gases
3. 1st period – 2 elements 2nd and 3rd period – 8 elements
4th and 5th period – 18 elements 6th period – 32 elements
7th period – Incomplete (32 elements)
4. Groups
1 and 2 – ‘s’ block elements last electron entered in ‘s’ subshell [s1, s2]
3 to 12 – ‘d’ block elements last electrons entered in ‘d’ subshell [d1 to d10].
13 to 18 – ‘p’ block elements last electrons enter in ‘p ’ subshell [p1 to p6].
Two f-block series lanthanoids and actinoids are placed in the bottom of
periodic table.
Classification Of Elements And Periodicity In Properties 53
, 5. (A) In ‘s’ and ‘p’ block elements the electrons enters in outer most shell.
In ‘d’ block elements the electron enters in the penultimate shell (n – 1).
‘f ’ block elements last electron enter the antepenultimate shell (n – 2).
(B) ‘f ’ block elements are placed in between ‘d’ block elements.
‘f ’ block elements in 2 rows [4f lanthanoids, 5f actinoids]
6. General outer electronic configuration
‘s’ block : ns1, ns2 [Group 1 to 2]
‘p’ block : ns1np1 to ns2 np6 Group 13 to 18
‘d’ block : ns0–2 (n – 1) d1 to 10 Group 3 to 12
‘f ’ block : (n – 2)f 1 to 14 (n – 1)d0, 1 ns2
7. General periodic trends in properties of elements
• ATOMIC RADIUS
(A) Left to right decreases due to effect of successive increasing nuclear change
without addition of a new shell.
(B) From top to bottom atomic radius increases due to successive addition of
shell.
(C) Noble gases have large radius than group 17 due to complete filling of
electron in outer shell electron-electron repulsion mildy increases.
• COVALENT RADIUS
It is half of the distance between the centre of nuclei of two adjacent similar
atoms which are bonded to each other by single covalent bond.
• van der Waal's Radius
van der Waal's radius is defined as one-half the distance between the centres
of nuclei of two nearest like atoms belonging to two adjacent molecules of
the element in the solid state.
54 Chemistry Class XI
, • METALLIC RADIUS
Half of the distance between the centres of the nuclei of two adjacent atoms
in the metallic crystal. A comparison of the three atomic radii show that
van der Waal's radius is maximum while the covalent radius has the least value.
van der Waal's radius > Metallic radius > Covalent radius
• IONIC RADIUS
(A) Cation radius < Atomic radius – due to more no. of protons than number
of electron coloumbic force increases, size decreases.
[Mg2+ < Mg+ < Mg]
(B) Anion radius > Atomic radius – Due to more number of electron than
number of protons
[N3– > O2– > F–]
Electron-Electron repulsion increase, coloumbic force of attraction
decreases.
(C) For Isoelectronic species – More is the charge of cation lesser the size.
More is the charge of anion, more is the size.
(D) Order of size – O2– > F– > Na > Na+ > Mg2+
8. (A) Ionisation enthalpy :
The minimum amount of energy which is required to remove the most
loosely bound electron from an isolated atom in the gaseous state is called
Ionisation enthalpy.
M(g) + Energy — M+ + e–
IE3 > IE2 > IE1
(B) Variation of I.E along a period:
Ionisation enthalpy increase along the period because atomic radii
decrease and nuclear charge increase along the period.
I ionisation enthalpy Li < B < Be < C < O < N < F < Ar
II ionisation enthalpy Be < C < B < N < F < O < Ne
(C) Variation down the group:
Ionisation enthalpy decrease down the group because atomic radius
increase down the group.
Metallic behaviour : Decrease from left to right due to increase in
ionisation enthalpy.
Classification Of Elements And Periodicity In Properties 55
and Periodicity in
Properties
FAST TRACK : QUICK REVISION
• The first systematic classification of elements was provided by Russian
chemist D.I. Mendeleev.
1. Mendeleev's periodic law
“The physical and chemical properties of elements are periodic functions
of their atomic weight.”
2. It was modified to Modern Periodic law :
“The physical and chemical properties of elements are periodic functions
of their atomic numbers.”
It is the long form of periodic table :
7 Horizontal rows are called Periods and 18 Vertical columns are called
Group
Group-1 are called Alkali metals Group-2 are called Alkaline earth metals.
Group-15 are called Pnicogens Group-16 are called Chalcogens
Group-17 are called Halogens Group-18 are called Noble gases
3. 1st period – 2 elements 2nd and 3rd period – 8 elements
4th and 5th period – 18 elements 6th period – 32 elements
7th period – Incomplete (32 elements)
4. Groups
1 and 2 – ‘s’ block elements last electron entered in ‘s’ subshell [s1, s2]
3 to 12 – ‘d’ block elements last electrons entered in ‘d’ subshell [d1 to d10].
13 to 18 – ‘p’ block elements last electrons enter in ‘p ’ subshell [p1 to p6].
Two f-block series lanthanoids and actinoids are placed in the bottom of
periodic table.
Classification Of Elements And Periodicity In Properties 53
, 5. (A) In ‘s’ and ‘p’ block elements the electrons enters in outer most shell.
In ‘d’ block elements the electron enters in the penultimate shell (n – 1).
‘f ’ block elements last electron enter the antepenultimate shell (n – 2).
(B) ‘f ’ block elements are placed in between ‘d’ block elements.
‘f ’ block elements in 2 rows [4f lanthanoids, 5f actinoids]
6. General outer electronic configuration
‘s’ block : ns1, ns2 [Group 1 to 2]
‘p’ block : ns1np1 to ns2 np6 Group 13 to 18
‘d’ block : ns0–2 (n – 1) d1 to 10 Group 3 to 12
‘f ’ block : (n – 2)f 1 to 14 (n – 1)d0, 1 ns2
7. General periodic trends in properties of elements
• ATOMIC RADIUS
(A) Left to right decreases due to effect of successive increasing nuclear change
without addition of a new shell.
(B) From top to bottom atomic radius increases due to successive addition of
shell.
(C) Noble gases have large radius than group 17 due to complete filling of
electron in outer shell electron-electron repulsion mildy increases.
• COVALENT RADIUS
It is half of the distance between the centre of nuclei of two adjacent similar
atoms which are bonded to each other by single covalent bond.
• van der Waal's Radius
van der Waal's radius is defined as one-half the distance between the centres
of nuclei of two nearest like atoms belonging to two adjacent molecules of
the element in the solid state.
54 Chemistry Class XI
, • METALLIC RADIUS
Half of the distance between the centres of the nuclei of two adjacent atoms
in the metallic crystal. A comparison of the three atomic radii show that
van der Waal's radius is maximum while the covalent radius has the least value.
van der Waal's radius > Metallic radius > Covalent radius
• IONIC RADIUS
(A) Cation radius < Atomic radius – due to more no. of protons than number
of electron coloumbic force increases, size decreases.
[Mg2+ < Mg+ < Mg]
(B) Anion radius > Atomic radius – Due to more number of electron than
number of protons
[N3– > O2– > F–]
Electron-Electron repulsion increase, coloumbic force of attraction
decreases.
(C) For Isoelectronic species – More is the charge of cation lesser the size.
More is the charge of anion, more is the size.
(D) Order of size – O2– > F– > Na > Na+ > Mg2+
8. (A) Ionisation enthalpy :
The minimum amount of energy which is required to remove the most
loosely bound electron from an isolated atom in the gaseous state is called
Ionisation enthalpy.
M(g) + Energy — M+ + e–
IE3 > IE2 > IE1
(B) Variation of I.E along a period:
Ionisation enthalpy increase along the period because atomic radii
decrease and nuclear charge increase along the period.
I ionisation enthalpy Li < B < Be < C < O < N < F < Ar
II ionisation enthalpy Be < C < B < N < F < O < Ne
(C) Variation down the group:
Ionisation enthalpy decrease down the group because atomic radius
increase down the group.
Metallic behaviour : Decrease from left to right due to increase in
ionisation enthalpy.
Classification Of Elements And Periodicity In Properties 55
