NCERT Textbook: Classification of Elements & Periodicity in Properties - Chemistry Class 11 - NEET

 Page 1


The Periodic Table is arguably the most important concept 
in chemistry, both in principle and in practice. It is the 
everyday support for students, it suggests new avenues 
of research to professionals, and it provides a succinct 
organization of the whole of chemistry. It is a remarkable 
demonstration of the fact that the chemical elements are 
not a random cluster of entities but instead display trends 
and lie together in families. An awareness of the Periodic 
Table is essential to anyone who wishes to disentangle 
the world and see how it is built up from the fundamental 
building blocks of the chemistry, the chemical elements.
 Glenn T. Seaborg
Unit 3
Classifi Cation of El EmEnts and 
PEriodi City in Pro PErti Es
After studying this Unit, you will be 
able to 
• 	 appreciate how the concept of 
grouping elements in accordance 
to their properties led to the 
development of Periodic Table.
• 	 understand  the Periodic Law; 
•	 understand 	 the 	 	 significance 	 of	
atomic number and electronic 
configuration as the basis for 
periodic 	classification;
• 	 name the elements with  
Z >100 according to IUPAC 
nomenclature; 
• 	 classify elements into s, p, d, 
f blocks and learn their main 
characteristics;
• 	 recognise the periodic trends in 
physical and chemical properties 
of elements;
• 	 compare the reactivity of elements 
and correlate it with their 
occurrence in nature;
• 	 explain the relationship  between 
ionization enthalpy and metallic 
character;
• 	 use  scientific  vocabulary 
appropriately to communicate 
ideas related to certain important 
properties of atoms e.g., 
atomic/ionic radii, ionization 
enthalpy, electron gain enthalpy, 
electronegativity, valence of 
elements. 
In this Unit, we will study the historical development of the 
Periodic Table as it stands today and the Modern Periodic 
Law.	 We 	 will	 also	 learn	 how	 the	 periodic	 classification 	
follows as a logical consequence of the electronic 
configuration 	 of 	 atoms. 	 Finally, 	 we 	 shall 	 examine 	 some 	 of	
the periodic trends in the physical and chemical properties 
of the elements.
3.1 WHy do WE nEEd to Classify El EmEnts ?
We know by now that the elements are the basic units of 
all types of matter. In 1800, only 31 elements were known. 
By 	 	 1865, 	 the 	 number 	 of 	 identified 	 elements 	 had 	 more 	 than	
doubled to 63. At present 114 elements are known. Of 
them, the recently discovered elements are man-made. 
Efforts to synthesise new elements are continuing. With 
such 	 a	 large	 number	 of	 elements	 it	 is	 very	 difficult 	 to	
study individually the chemistry of all these elements and 
their innumerable compounds individually. To ease out 
this problem, scientists searched for a systematic way to 
organise their knowledge by classifying the elements. Not 
only that it would rationalize known chemical facts about 
elements, but even predict new ones for undertaking 
further study. 
Unit 3.indd   74 9/9/2022   4:36:12 PM
Rationalised 2023-24
Page 2


The Periodic Table is arguably the most important concept 
in chemistry, both in principle and in practice. It is the 
everyday support for students, it suggests new avenues 
of research to professionals, and it provides a succinct 
organization of the whole of chemistry. It is a remarkable 
demonstration of the fact that the chemical elements are 
not a random cluster of entities but instead display trends 
and lie together in families. An awareness of the Periodic 
Table is essential to anyone who wishes to disentangle 
the world and see how it is built up from the fundamental 
building blocks of the chemistry, the chemical elements.
 Glenn T. Seaborg
Unit 3
Classifi Cation of El EmEnts and 
PEriodi City in Pro PErti Es
After studying this Unit, you will be 
able to 
• 	 appreciate how the concept of 
grouping elements in accordance 
to their properties led to the 
development of Periodic Table.
• 	 understand  the Periodic Law; 
•	 understand 	 the 	 	 significance 	 of	
atomic number and electronic 
configuration as the basis for 
periodic 	classification;
• 	 name the elements with  
Z >100 according to IUPAC 
nomenclature; 
• 	 classify elements into s, p, d, 
f blocks and learn their main 
characteristics;
• 	 recognise the periodic trends in 
physical and chemical properties 
of elements;
• 	 compare the reactivity of elements 
and correlate it with their 
occurrence in nature;
• 	 explain the relationship  between 
ionization enthalpy and metallic 
character;
• 	 use  scientific  vocabulary 
appropriately to communicate 
ideas related to certain important 
properties of atoms e.g., 
atomic/ionic radii, ionization 
enthalpy, electron gain enthalpy, 
electronegativity, valence of 
elements. 
In this Unit, we will study the historical development of the 
Periodic Table as it stands today and the Modern Periodic 
Law.	 We 	 will	 also	 learn	 how	 the	 periodic	 classification 	
follows as a logical consequence of the electronic 
configuration 	 of 	 atoms. 	 Finally, 	 we 	 shall 	 examine 	 some 	 of	
the periodic trends in the physical and chemical properties 
of the elements.
3.1 WHy do WE nEEd to Classify El EmEnts ?
We know by now that the elements are the basic units of 
all types of matter. In 1800, only 31 elements were known. 
By 	 	 1865, 	 the 	 number 	 of 	 identified 	 elements 	 had 	 more 	 than	
doubled to 63. At present 114 elements are known. Of 
them, the recently discovered elements are man-made. 
Efforts to synthesise new elements are continuing. With 
such 	 a	 large	 number	 of	 elements	 it	 is	 very	 difficult 	 to	
study individually the chemistry of all these elements and 
their innumerable compounds individually. To ease out 
this problem, scientists searched for a systematic way to 
organise their knowledge by classifying the elements. Not 
only that it would rationalize known chemical facts about 
elements, but even predict new ones for undertaking 
further study. 
Unit 3.indd   74 9/9/2022   4:36:12 PM
Rationalised 2023-24
75 Classifi Cation of El Em Ents and P Eriodi City in Pro PErti Es 3.2 GEnEsis of PEriodi C 
Classifi Cation
Classification 	 of 	 elements 	 into 	 groups 	 and	
development of Periodic Law and Periodic 
Table are the consequences of systematising 
the knowledge gained by a number of 
scientists through their observations and 
experiments. The German chemist, Johann 
Dobereiner 	 in 	 early 	 1800’s 	 was 	 the 	 first 	 to 	
consider the idea of trends among properties 
of elements. By 1829 he noted a similarity 
among the physical and chemical properties 
of several groups of three elements (t riads). In 
each case, he noticed that the middle  element  
of each of the t riads had an atomic weight 
about half way between the atomic weights of 
the other two (Table 3.1). Also the properties 
of the middle element were in between those 
of the other two members. Since Dobereiner’s 
the periodic recurrence of properties. This 
also did not attract much attention. The 
English chemist, John Alexander Newlands 
in 1865 profounded the l aw of o ctaves. He 
arranged the elements in increasing order 
of their atomic weights and noted that every 
eighth element had properties similar to the 
first 	 element 	 (Table 	 3.2). 	 The 	 relationship 	 was	
just like every eighth note that resembles the 
first 	 in	 octaves 	 of	 music.	 Newlands’s	 Law	 of	
Octaves seemed to be true only for elements 
up to calcium. Although his idea was not 
widely accepted at that time, he, for his work, 
was later awarded Davy Medal in 1887 by the 
Royal Society, London.
The Periodic Law, as we know it today 
owes its development to the Russian chemist, 
Dmitri Mendeleev (1834-1907) and the 
German chemist, Lothar Meyer (1830-1895). 
t able 3.1  dobereiner’s t riads
Element
atomic 
weight
Element
atomic 
weight
Element
atomic 
weight
l i
na
K
7
23
39
Ca
sr
Ba
40
88
137
Cl
Br
i
35.5
80
127
  t able 3.2  newlands’ o ctaves
Element l i Be B C n o f
At. wt. 7 9 11 12 14 16 19
Element na mg al si P s Cl
At. wt. 23 24 27 29 31 32 35.5
Element K Ca
At. wt. 39 40
relationship, referred to as the l aw of t riads, 
seemed to work only for a few elements, it was  
dismissed as coincidence. The next reported 
attempt to classify elements was made by a 
French 	 geologist,	 A.E.B. 	 de	 Chancourtois 	 in	
1862. He arranged the then known elements 
in order of increasing atomic weights and 
made a cylindrical table of elements to display 
Working independently, both the chemists in 
1869 proposed that on arranging elements in  
the increasing order of their atomic weights, 
similarities appear in physical and chemical 
properties at regular intervals. Lothar Meyer 
plotted the physical properties such as 
atomic volume, melting point and boiling 
point against atomic weight and obtained 
Unit 3.indd   75 9/9/2022   4:36:12 PM
Rationalised 2023-24
Page 3


The Periodic Table is arguably the most important concept 
in chemistry, both in principle and in practice. It is the 
everyday support for students, it suggests new avenues 
of research to professionals, and it provides a succinct 
organization of the whole of chemistry. It is a remarkable 
demonstration of the fact that the chemical elements are 
not a random cluster of entities but instead display trends 
and lie together in families. An awareness of the Periodic 
Table is essential to anyone who wishes to disentangle 
the world and see how it is built up from the fundamental 
building blocks of the chemistry, the chemical elements.
 Glenn T. Seaborg
Unit 3
Classifi Cation of El EmEnts and 
PEriodi City in Pro PErti Es
After studying this Unit, you will be 
able to 
• 	 appreciate how the concept of 
grouping elements in accordance 
to their properties led to the 
development of Periodic Table.
• 	 understand  the Periodic Law; 
•	 understand 	 the 	 	 significance 	 of	
atomic number and electronic 
configuration as the basis for 
periodic 	classification;
• 	 name the elements with  
Z >100 according to IUPAC 
nomenclature; 
• 	 classify elements into s, p, d, 
f blocks and learn their main 
characteristics;
• 	 recognise the periodic trends in 
physical and chemical properties 
of elements;
• 	 compare the reactivity of elements 
and correlate it with their 
occurrence in nature;
• 	 explain the relationship  between 
ionization enthalpy and metallic 
character;
• 	 use  scientific  vocabulary 
appropriately to communicate 
ideas related to certain important 
properties of atoms e.g., 
atomic/ionic radii, ionization 
enthalpy, electron gain enthalpy, 
electronegativity, valence of 
elements. 
In this Unit, we will study the historical development of the 
Periodic Table as it stands today and the Modern Periodic 
Law.	 We 	 will	 also	 learn	 how	 the	 periodic	 classification 	
follows as a logical consequence of the electronic 
configuration 	 of 	 atoms. 	 Finally, 	 we 	 shall 	 examine 	 some 	 of	
the periodic trends in the physical and chemical properties 
of the elements.
3.1 WHy do WE nEEd to Classify El EmEnts ?
We know by now that the elements are the basic units of 
all types of matter. In 1800, only 31 elements were known. 
By 	 	 1865, 	 the 	 number 	 of 	 identified 	 elements 	 had 	 more 	 than	
doubled to 63. At present 114 elements are known. Of 
them, the recently discovered elements are man-made. 
Efforts to synthesise new elements are continuing. With 
such 	 a	 large	 number	 of	 elements	 it	 is	 very	 difficult 	 to	
study individually the chemistry of all these elements and 
their innumerable compounds individually. To ease out 
this problem, scientists searched for a systematic way to 
organise their knowledge by classifying the elements. Not 
only that it would rationalize known chemical facts about 
elements, but even predict new ones for undertaking 
further study. 
Unit 3.indd   74 9/9/2022   4:36:12 PM
Rationalised 2023-24
75 Classifi Cation of El Em Ents and P Eriodi City in Pro PErti Es 3.2 GEnEsis of PEriodi C 
Classifi Cation
Classification 	 of 	 elements 	 into 	 groups 	 and	
development of Periodic Law and Periodic 
Table are the consequences of systematising 
the knowledge gained by a number of 
scientists through their observations and 
experiments. The German chemist, Johann 
Dobereiner 	 in 	 early 	 1800’s 	 was 	 the 	 first 	 to 	
consider the idea of trends among properties 
of elements. By 1829 he noted a similarity 
among the physical and chemical properties 
of several groups of three elements (t riads). In 
each case, he noticed that the middle  element  
of each of the t riads had an atomic weight 
about half way between the atomic weights of 
the other two (Table 3.1). Also the properties 
of the middle element were in between those 
of the other two members. Since Dobereiner’s 
the periodic recurrence of properties. This 
also did not attract much attention. The 
English chemist, John Alexander Newlands 
in 1865 profounded the l aw of o ctaves. He 
arranged the elements in increasing order 
of their atomic weights and noted that every 
eighth element had properties similar to the 
first 	 element 	 (Table 	 3.2). 	 The 	 relationship 	 was	
just like every eighth note that resembles the 
first 	 in	 octaves 	 of	 music.	 Newlands’s	 Law	 of	
Octaves seemed to be true only for elements 
up to calcium. Although his idea was not 
widely accepted at that time, he, for his work, 
was later awarded Davy Medal in 1887 by the 
Royal Society, London.
The Periodic Law, as we know it today 
owes its development to the Russian chemist, 
Dmitri Mendeleev (1834-1907) and the 
German chemist, Lothar Meyer (1830-1895). 
t able 3.1  dobereiner’s t riads
Element
atomic 
weight
Element
atomic 
weight
Element
atomic 
weight
l i
na
K
7
23
39
Ca
sr
Ba
40
88
137
Cl
Br
i
35.5
80
127
  t able 3.2  newlands’ o ctaves
Element l i Be B C n o f
At. wt. 7 9 11 12 14 16 19
Element na mg al si P s Cl
At. wt. 23 24 27 29 31 32 35.5
Element K Ca
At. wt. 39 40
relationship, referred to as the l aw of t riads, 
seemed to work only for a few elements, it was  
dismissed as coincidence. The next reported 
attempt to classify elements was made by a 
French 	 geologist,	 A.E.B. 	 de	 Chancourtois 	 in	
1862. He arranged the then known elements 
in order of increasing atomic weights and 
made a cylindrical table of elements to display 
Working independently, both the chemists in 
1869 proposed that on arranging elements in  
the increasing order of their atomic weights, 
similarities appear in physical and chemical 
properties at regular intervals. Lothar Meyer 
plotted the physical properties such as 
atomic volume, melting point and boiling 
point against atomic weight and obtained 
Unit 3.indd   75 9/9/2022   4:36:12 PM
Rationalised 2023-24
76 chemistry a periodically repeated pattern. Unlike 
Newlands, Lothar Meyer observed a change 
in length of that repeating pattern. By 1868, 
Lothar Meyer had developed a table of the 
elements that closely resembles the Modern 
Periodic Table. However, his work was not 
published until after the work of Dmitri 
Mendeleev, the scientist who is generally 
credited with the development of the Modern 
Periodic Table.
While Dobereiner initiated the study of 
periodic relationship, it was Mendeleev who 
was responsible for publishing the Periodic 
Law 	 for 	the	first 	time. 	It 	 states 	as 	follows	 :
The properties of the elements are 
a periodic function of their atomic 
weights.
Mendeleev arranged elements in horizontal 
rows and vertical columns of a table in order 
of their increasing atomic weights in such a 
way that the elements with similar properties 
occupied the same vertical column or group. 
Mendeleev’s system of classifying elements 
was more elaborate than that of Lothar 
Meyer’s. 	 He	 fully 	 recognized 	 the 	 significance 	
of periodicity and used broader range of 
physical and chemical properties to classify 
the elements. In particular, Mendeleev relied 
on the similarities in the empirical formulas 
and properties of the compounds formed by 
the elements. He realized that some of the 
el ements 	 di d 	 not 	 fit 	 i n 	 w i th 	 hi s 	 scheme 	 of	
classification 	 if 	 the 	 order 	 of 	 atomic 	 weight	
was strictly followed. He ignored the order 
of atomic weights, thinking that the atomic 
measurements might be incorrect, and placed 
the elements with similar properties together. 
For 	 example,	 iodine 	 with	 lower	 atomic	 weight	
than that of tellurium (Group VI) was placed 
in 	 Group 	 VII 	 along 	 with 	 fluorine, 	 chlorine,	
bromine because of similarities in properties 
(Fig. 	 3.1). 	 At 	 the 	 same 	 time, 	 keeping 	 his	
primary aim of arranging the elements of 
similar properties in the same group, he 
proposed that some of the elements were 
still undiscovered and, therefore, left several 
gaps 	 in	 the 	 table. 	 For	 example,	 both	 gallium	
and germanium were unknown at the time 
Mendeleev published his Periodic Table. 
He left the gap under aluminium and a gap 
under silicon, and called these elements 
Eka-aluminium and Eka-silicon. Mendeleev 
predicted not only the existence of gallium and 
germanium, but also described some of their 
general physical properties.  These elements 
were discovered later. Some of the properties 
predicted by Mendeleev for these elements 
and those found experimentally are listed in  
Table 3.3. 
The boldness of Mendeleev’s quantitative 
predictions and their eventual success 
made him and his Periodic Table famous. 
Mendeleev’s Periodic Table published in 1905 
is 	 shown	 in	 Fig.	3.1.
table 3.3 mendeleev’s Predictions for the Elements Eka-aluminium (Gallium) and  
Eka-silicon (Germanium)
Property
Eka-aluminium 
(predicted)
Gallium 
(found)
Eka-silicon 
(predicted)
Germanium 
(found)
atomic weight 68 70 72 72.6
density/(g/cm
3
) 5.9 5.94 5.5 5.36
melting point/K Low 302.93 High 1231
f ormula of oxide E
2
O
3
Ga
2
O
3
EO
2
GeO
2
f ormula of chloride E Cl
3
GaCl
3
ECl
4
GeCl
4
Unit 3.indd   76 9/9/2022   4:36:12 PM
Rationalised 2023-24
Page 4


The Periodic Table is arguably the most important concept 
in chemistry, both in principle and in practice. It is the 
everyday support for students, it suggests new avenues 
of research to professionals, and it provides a succinct 
organization of the whole of chemistry. It is a remarkable 
demonstration of the fact that the chemical elements are 
not a random cluster of entities but instead display trends 
and lie together in families. An awareness of the Periodic 
Table is essential to anyone who wishes to disentangle 
the world and see how it is built up from the fundamental 
building blocks of the chemistry, the chemical elements.
 Glenn T. Seaborg
Unit 3
Classifi Cation of El EmEnts and 
PEriodi City in Pro PErti Es
After studying this Unit, you will be 
able to 
• 	 appreciate how the concept of 
grouping elements in accordance 
to their properties led to the 
development of Periodic Table.
• 	 understand  the Periodic Law; 
•	 understand 	 the 	 	 significance 	 of	
atomic number and electronic 
configuration as the basis for 
periodic 	classification;
• 	 name the elements with  
Z >100 according to IUPAC 
nomenclature; 
• 	 classify elements into s, p, d, 
f blocks and learn their main 
characteristics;
• 	 recognise the periodic trends in 
physical and chemical properties 
of elements;
• 	 compare the reactivity of elements 
and correlate it with their 
occurrence in nature;
• 	 explain the relationship  between 
ionization enthalpy and metallic 
character;
• 	 use  scientific  vocabulary 
appropriately to communicate 
ideas related to certain important 
properties of atoms e.g., 
atomic/ionic radii, ionization 
enthalpy, electron gain enthalpy, 
electronegativity, valence of 
elements. 
In this Unit, we will study the historical development of the 
Periodic Table as it stands today and the Modern Periodic 
Law.	 We 	 will	 also	 learn	 how	 the	 periodic	 classification 	
follows as a logical consequence of the electronic 
configuration 	 of 	 atoms. 	 Finally, 	 we 	 shall 	 examine 	 some 	 of	
the periodic trends in the physical and chemical properties 
of the elements.
3.1 WHy do WE nEEd to Classify El EmEnts ?
We know by now that the elements are the basic units of 
all types of matter. In 1800, only 31 elements were known. 
By 	 	 1865, 	 the 	 number 	 of 	 identified 	 elements 	 had 	 more 	 than	
doubled to 63. At present 114 elements are known. Of 
them, the recently discovered elements are man-made. 
Efforts to synthesise new elements are continuing. With 
such 	 a	 large	 number	 of	 elements	 it	 is	 very	 difficult 	 to	
study individually the chemistry of all these elements and 
their innumerable compounds individually. To ease out 
this problem, scientists searched for a systematic way to 
organise their knowledge by classifying the elements. Not 
only that it would rationalize known chemical facts about 
elements, but even predict new ones for undertaking 
further study. 
Unit 3.indd   74 9/9/2022   4:36:12 PM
Rationalised 2023-24
75 Classifi Cation of El Em Ents and P Eriodi City in Pro PErti Es 3.2 GEnEsis of PEriodi C 
Classifi Cation
Classification 	 of 	 elements 	 into 	 groups 	 and	
development of Periodic Law and Periodic 
Table are the consequences of systematising 
the knowledge gained by a number of 
scientists through their observations and 
experiments. The German chemist, Johann 
Dobereiner 	 in 	 early 	 1800’s 	 was 	 the 	 first 	 to 	
consider the idea of trends among properties 
of elements. By 1829 he noted a similarity 
among the physical and chemical properties 
of several groups of three elements (t riads). In 
each case, he noticed that the middle  element  
of each of the t riads had an atomic weight 
about half way between the atomic weights of 
the other two (Table 3.1). Also the properties 
of the middle element were in between those 
of the other two members. Since Dobereiner’s 
the periodic recurrence of properties. This 
also did not attract much attention. The 
English chemist, John Alexander Newlands 
in 1865 profounded the l aw of o ctaves. He 
arranged the elements in increasing order 
of their atomic weights and noted that every 
eighth element had properties similar to the 
first 	 element 	 (Table 	 3.2). 	 The 	 relationship 	 was	
just like every eighth note that resembles the 
first 	 in	 octaves 	 of	 music.	 Newlands’s	 Law	 of	
Octaves seemed to be true only for elements 
up to calcium. Although his idea was not 
widely accepted at that time, he, for his work, 
was later awarded Davy Medal in 1887 by the 
Royal Society, London.
The Periodic Law, as we know it today 
owes its development to the Russian chemist, 
Dmitri Mendeleev (1834-1907) and the 
German chemist, Lothar Meyer (1830-1895). 
t able 3.1  dobereiner’s t riads
Element
atomic 
weight
Element
atomic 
weight
Element
atomic 
weight
l i
na
K
7
23
39
Ca
sr
Ba
40
88
137
Cl
Br
i
35.5
80
127
  t able 3.2  newlands’ o ctaves
Element l i Be B C n o f
At. wt. 7 9 11 12 14 16 19
Element na mg al si P s Cl
At. wt. 23 24 27 29 31 32 35.5
Element K Ca
At. wt. 39 40
relationship, referred to as the l aw of t riads, 
seemed to work only for a few elements, it was  
dismissed as coincidence. The next reported 
attempt to classify elements was made by a 
French 	 geologist,	 A.E.B. 	 de	 Chancourtois 	 in	
1862. He arranged the then known elements 
in order of increasing atomic weights and 
made a cylindrical table of elements to display 
Working independently, both the chemists in 
1869 proposed that on arranging elements in  
the increasing order of their atomic weights, 
similarities appear in physical and chemical 
properties at regular intervals. Lothar Meyer 
plotted the physical properties such as 
atomic volume, melting point and boiling 
point against atomic weight and obtained 
Unit 3.indd   75 9/9/2022   4:36:12 PM
Rationalised 2023-24
76 chemistry a periodically repeated pattern. Unlike 
Newlands, Lothar Meyer observed a change 
in length of that repeating pattern. By 1868, 
Lothar Meyer had developed a table of the 
elements that closely resembles the Modern 
Periodic Table. However, his work was not 
published until after the work of Dmitri 
Mendeleev, the scientist who is generally 
credited with the development of the Modern 
Periodic Table.
While Dobereiner initiated the study of 
periodic relationship, it was Mendeleev who 
was responsible for publishing the Periodic 
Law 	 for 	the	first 	time. 	It 	 states 	as 	follows	 :
The properties of the elements are 
a periodic function of their atomic 
weights.
Mendeleev arranged elements in horizontal 
rows and vertical columns of a table in order 
of their increasing atomic weights in such a 
way that the elements with similar properties 
occupied the same vertical column or group. 
Mendeleev’s system of classifying elements 
was more elaborate than that of Lothar 
Meyer’s. 	 He	 fully 	 recognized 	 the 	 significance 	
of periodicity and used broader range of 
physical and chemical properties to classify 
the elements. In particular, Mendeleev relied 
on the similarities in the empirical formulas 
and properties of the compounds formed by 
the elements. He realized that some of the 
el ements 	 di d 	 not 	 fit 	 i n 	 w i th 	 hi s 	 scheme 	 of	
classification 	 if 	 the 	 order 	 of 	 atomic 	 weight	
was strictly followed. He ignored the order 
of atomic weights, thinking that the atomic 
measurements might be incorrect, and placed 
the elements with similar properties together. 
For 	 example,	 iodine 	 with	 lower	 atomic	 weight	
than that of tellurium (Group VI) was placed 
in 	 Group 	 VII 	 along 	 with 	 fluorine, 	 chlorine,	
bromine because of similarities in properties 
(Fig. 	 3.1). 	 At 	 the 	 same 	 time, 	 keeping 	 his	
primary aim of arranging the elements of 
similar properties in the same group, he 
proposed that some of the elements were 
still undiscovered and, therefore, left several 
gaps 	 in	 the 	 table. 	 For	 example,	 both	 gallium	
and germanium were unknown at the time 
Mendeleev published his Periodic Table. 
He left the gap under aluminium and a gap 
under silicon, and called these elements 
Eka-aluminium and Eka-silicon. Mendeleev 
predicted not only the existence of gallium and 
germanium, but also described some of their 
general physical properties.  These elements 
were discovered later. Some of the properties 
predicted by Mendeleev for these elements 
and those found experimentally are listed in  
Table 3.3. 
The boldness of Mendeleev’s quantitative 
predictions and their eventual success 
made him and his Periodic Table famous. 
Mendeleev’s Periodic Table published in 1905 
is 	 shown	 in	 Fig.	3.1.
table 3.3 mendeleev’s Predictions for the Elements Eka-aluminium (Gallium) and  
Eka-silicon (Germanium)
Property
Eka-aluminium 
(predicted)
Gallium 
(found)
Eka-silicon 
(predicted)
Germanium 
(found)
atomic weight 68 70 72 72.6
density/(g/cm
3
) 5.9 5.94 5.5 5.36
melting point/K Low 302.93 High 1231
f ormula of oxide E
2
O
3
Ga
2
O
3
EO
2
GeO
2
f ormula of chloride E Cl
3
GaCl
3
ECl
4
GeCl
4
Unit 3.indd   76 9/9/2022   4:36:12 PM
Rationalised 2023-24
77 Classifi Cation of El Em Ents and P Eriodi City in Pro PErti Es PERIODIC	SYSTEM	 OF	 THE 	 ELEMENTS	IN	 GROUPS	AND	 SERIES
Fig. 3.1  Mendeleev’s Periodic Table published earlier
Unit 3.indd   77 9/9/2022   4:36:12 PM
Rationalised 2023-24
Page 5


The Periodic Table is arguably the most important concept 
in chemistry, both in principle and in practice. It is the 
everyday support for students, it suggests new avenues 
of research to professionals, and it provides a succinct 
organization of the whole of chemistry. It is a remarkable 
demonstration of the fact that the chemical elements are 
not a random cluster of entities but instead display trends 
and lie together in families. An awareness of the Periodic 
Table is essential to anyone who wishes to disentangle 
the world and see how it is built up from the fundamental 
building blocks of the chemistry, the chemical elements.
 Glenn T. Seaborg
Unit 3
Classifi Cation of El EmEnts and 
PEriodi City in Pro PErti Es
After studying this Unit, you will be 
able to 
• 	 appreciate how the concept of 
grouping elements in accordance 
to their properties led to the 
development of Periodic Table.
• 	 understand  the Periodic Law; 
•	 understand 	 the 	 	 significance 	 of	
atomic number and electronic 
configuration as the basis for 
periodic 	classification;
• 	 name the elements with  
Z >100 according to IUPAC 
nomenclature; 
• 	 classify elements into s, p, d, 
f blocks and learn their main 
characteristics;
• 	 recognise the periodic trends in 
physical and chemical properties 
of elements;
• 	 compare the reactivity of elements 
and correlate it with their 
occurrence in nature;
• 	 explain the relationship  between 
ionization enthalpy and metallic 
character;
• 	 use  scientific  vocabulary 
appropriately to communicate 
ideas related to certain important 
properties of atoms e.g., 
atomic/ionic radii, ionization 
enthalpy, electron gain enthalpy, 
electronegativity, valence of 
elements. 
In this Unit, we will study the historical development of the 
Periodic Table as it stands today and the Modern Periodic 
Law.	 We 	 will	 also	 learn	 how	 the	 periodic	 classification 	
follows as a logical consequence of the electronic 
configuration 	 of 	 atoms. 	 Finally, 	 we 	 shall 	 examine 	 some 	 of	
the periodic trends in the physical and chemical properties 
of the elements.
3.1 WHy do WE nEEd to Classify El EmEnts ?
We know by now that the elements are the basic units of 
all types of matter. In 1800, only 31 elements were known. 
By 	 	 1865, 	 the 	 number 	 of 	 identified 	 elements 	 had 	 more 	 than	
doubled to 63. At present 114 elements are known. Of 
them, the recently discovered elements are man-made. 
Efforts to synthesise new elements are continuing. With 
such 	 a	 large	 number	 of	 elements	 it	 is	 very	 difficult 	 to	
study individually the chemistry of all these elements and 
their innumerable compounds individually. To ease out 
this problem, scientists searched for a systematic way to 
organise their knowledge by classifying the elements. Not 
only that it would rationalize known chemical facts about 
elements, but even predict new ones for undertaking 
further study. 
Unit 3.indd   74 9/9/2022   4:36:12 PM
Rationalised 2023-24
75 Classifi Cation of El Em Ents and P Eriodi City in Pro PErti Es 3.2 GEnEsis of PEriodi C 
Classifi Cation
Classification 	 of 	 elements 	 into 	 groups 	 and	
development of Periodic Law and Periodic 
Table are the consequences of systematising 
the knowledge gained by a number of 
scientists through their observations and 
experiments. The German chemist, Johann 
Dobereiner 	 in 	 early 	 1800’s 	 was 	 the 	 first 	 to 	
consider the idea of trends among properties 
of elements. By 1829 he noted a similarity 
among the physical and chemical properties 
of several groups of three elements (t riads). In 
each case, he noticed that the middle  element  
of each of the t riads had an atomic weight 
about half way between the atomic weights of 
the other two (Table 3.1). Also the properties 
of the middle element were in between those 
of the other two members. Since Dobereiner’s 
the periodic recurrence of properties. This 
also did not attract much attention. The 
English chemist, John Alexander Newlands 
in 1865 profounded the l aw of o ctaves. He 
arranged the elements in increasing order 
of their atomic weights and noted that every 
eighth element had properties similar to the 
first 	 element 	 (Table 	 3.2). 	 The 	 relationship 	 was	
just like every eighth note that resembles the 
first 	 in	 octaves 	 of	 music.	 Newlands’s	 Law	 of	
Octaves seemed to be true only for elements 
up to calcium. Although his idea was not 
widely accepted at that time, he, for his work, 
was later awarded Davy Medal in 1887 by the 
Royal Society, London.
The Periodic Law, as we know it today 
owes its development to the Russian chemist, 
Dmitri Mendeleev (1834-1907) and the 
German chemist, Lothar Meyer (1830-1895). 
t able 3.1  dobereiner’s t riads
Element
atomic 
weight
Element
atomic 
weight
Element
atomic 
weight
l i
na
K
7
23
39
Ca
sr
Ba
40
88
137
Cl
Br
i
35.5
80
127
  t able 3.2  newlands’ o ctaves
Element l i Be B C n o f
At. wt. 7 9 11 12 14 16 19
Element na mg al si P s Cl
At. wt. 23 24 27 29 31 32 35.5
Element K Ca
At. wt. 39 40
relationship, referred to as the l aw of t riads, 
seemed to work only for a few elements, it was  
dismissed as coincidence. The next reported 
attempt to classify elements was made by a 
French 	 geologist,	 A.E.B. 	 de	 Chancourtois 	 in	
1862. He arranged the then known elements 
in order of increasing atomic weights and 
made a cylindrical table of elements to display 
Working independently, both the chemists in 
1869 proposed that on arranging elements in  
the increasing order of their atomic weights, 
similarities appear in physical and chemical 
properties at regular intervals. Lothar Meyer 
plotted the physical properties such as 
atomic volume, melting point and boiling 
point against atomic weight and obtained 
Unit 3.indd   75 9/9/2022   4:36:12 PM
Rationalised 2023-24
76 chemistry a periodically repeated pattern. Unlike 
Newlands, Lothar Meyer observed a change 
in length of that repeating pattern. By 1868, 
Lothar Meyer had developed a table of the 
elements that closely resembles the Modern 
Periodic Table. However, his work was not 
published until after the work of Dmitri 
Mendeleev, the scientist who is generally 
credited with the development of the Modern 
Periodic Table.
While Dobereiner initiated the study of 
periodic relationship, it was Mendeleev who 
was responsible for publishing the Periodic 
Law 	 for 	the	first 	time. 	It 	 states 	as 	follows	 :
The properties of the elements are 
a periodic function of their atomic 
weights.
Mendeleev arranged elements in horizontal 
rows and vertical columns of a table in order 
of their increasing atomic weights in such a 
way that the elements with similar properties 
occupied the same vertical column or group. 
Mendeleev’s system of classifying elements 
was more elaborate than that of Lothar 
Meyer’s. 	 He	 fully 	 recognized 	 the 	 significance 	
of periodicity and used broader range of 
physical and chemical properties to classify 
the elements. In particular, Mendeleev relied 
on the similarities in the empirical formulas 
and properties of the compounds formed by 
the elements. He realized that some of the 
el ements 	 di d 	 not 	 fit 	 i n 	 w i th 	 hi s 	 scheme 	 of	
classification 	 if 	 the 	 order 	 of 	 atomic 	 weight	
was strictly followed. He ignored the order 
of atomic weights, thinking that the atomic 
measurements might be incorrect, and placed 
the elements with similar properties together. 
For 	 example,	 iodine 	 with	 lower	 atomic	 weight	
than that of tellurium (Group VI) was placed 
in 	 Group 	 VII 	 along 	 with 	 fluorine, 	 chlorine,	
bromine because of similarities in properties 
(Fig. 	 3.1). 	 At 	 the 	 same 	 time, 	 keeping 	 his	
primary aim of arranging the elements of 
similar properties in the same group, he 
proposed that some of the elements were 
still undiscovered and, therefore, left several 
gaps 	 in	 the 	 table. 	 For	 example,	 both	 gallium	
and germanium were unknown at the time 
Mendeleev published his Periodic Table. 
He left the gap under aluminium and a gap 
under silicon, and called these elements 
Eka-aluminium and Eka-silicon. Mendeleev 
predicted not only the existence of gallium and 
germanium, but also described some of their 
general physical properties.  These elements 
were discovered later. Some of the properties 
predicted by Mendeleev for these elements 
and those found experimentally are listed in  
Table 3.3. 
The boldness of Mendeleev’s quantitative 
predictions and their eventual success 
made him and his Periodic Table famous. 
Mendeleev’s Periodic Table published in 1905 
is 	 shown	 in	 Fig.	3.1.
table 3.3 mendeleev’s Predictions for the Elements Eka-aluminium (Gallium) and  
Eka-silicon (Germanium)
Property
Eka-aluminium 
(predicted)
Gallium 
(found)
Eka-silicon 
(predicted)
Germanium 
(found)
atomic weight 68 70 72 72.6
density/(g/cm
3
) 5.9 5.94 5.5 5.36
melting point/K Low 302.93 High 1231
f ormula of oxide E
2
O
3
Ga
2
O
3
EO
2
GeO
2
f ormula of chloride E Cl
3
GaCl
3
ECl
4
GeCl
4
Unit 3.indd   76 9/9/2022   4:36:12 PM
Rationalised 2023-24
77 Classifi Cation of El Em Ents and P Eriodi City in Pro PErti Es PERIODIC	SYSTEM	 OF	 THE 	 ELEMENTS	IN	 GROUPS	AND	 SERIES
Fig. 3.1  Mendeleev’s Periodic Table published earlier
Unit 3.indd   77 9/9/2022   4:36:12 PM
Rationalised 2023-24
78 chemistry 3.3 mod Ern PEriodi C la W and tHE 
Pr EsEnt form of t HE PEriodi C 
ta Bl E
We must bear in mind that when Mendeleev 
developed his Periodic t able, chemists 
knew nothing about the internal structure 
of atom. However, the beginning of the 20
th
 
century witnessed profound developments 
in theories about sub-atomic particles. In 
1913, the English physicist, Henry Moseley 
observed regularities in the characteristic 
X-ray spectra of the elements. A plot of  
(where 	is frequency of X-rays emitted) 
against atomic number (Z) gave a straight 
line and not the plot of  vs atomic mass. 
He thereby showed that the atomic number 
is a more fundamental property of an element 
than its atomic mass. mendeleev’s Periodic 
l aw 	 w as , 	 t he r e f o r e , 	 ac c or di ngl y 	 m o di fie d. 	 Thi s	
is known as the modern Periodic l aw and 
can 	 be	stated 	as 	:
The physical and chemical properties 
of the elements are periodic functions 
of their atomic numbers.
The Periodic l aw revealed important 
analogies among the 94 naturally occurring 
elements (neptunium and plutonium like 
actinium and protoactinium are also found 
in pitch blende – an ore of uranium). It 
stimulated renewed interest in Inorganic 
Chemistry and has carried into the present 
with the creation of artificially produced 
short-lived elements.
You may recall that the atomic number 
is equal to the nuclear charge (i.e., number 
of protons) or the number of electrons in 
a neutral atom. It is then easy to visualize 
t he 	 si gni ficance 	 of 	 quant um 	 number s 	 and	
electronic 	 configurations 	 in 	 periodicity 	 of	
elements. In fact, it is now recognized that the 
Periodic Law is essentially the consequence 
of the periodic variation in electronic 
configurations, 	 which 	 indeed	 determine	 the	
physical and chemical properties of elements 
and their compounds.
Numerous forms of Periodic Table have 
been devised from time to time. Some 
forms emphasise chemical reactions and 
valence, whereas others stress the electronic 
configuration 	 of 	 elements. 	 A 	 modern	 version,	
the so-called “long form” of the Periodic 
t able	 of	 the	 elements	 (Fig. 	 3.2),	 is	 the	 most	
convenient and widely used. The horizontal 
rows (which Mendeleev called series) are 
called periods and the vertical columns, 
groups. Elements having similar outer 
electronic configurations in their atoms 
are arranged in vertical columns, referred 
to as groups or families. According to the 
recommendation of International Union of 
Pure and Applied Chemistry (IUPAC), the 
groups are numbered from 1 to 18 replacing 
the older notation of groups IA …  VIIA, VIII, 
IB …  VIIB and 0.
There are altogether seven periods. The 
period number corresponds to the highest 
principal quantum number (n) of the elements 
i n 	 t he 	 pe r i o d. 	 The 	 fir s t 	 pe r i o d 	 co nt a i ns 	 2	
elements. The subsequent periods consists of 
8, 8, 18, 18 and 32 elements, respectively. The 
seventh period is incomplete and like the sixth 
period would have a theoretical maximum 
(on the basis of quantum numbers) of 32 
elements. In this form of the Periodic Table, 
14 elements of both sixth and seventh periods 
(lanthanoids and actinoids, respectively) are 
placed in separate panels at the bottom*.
3.4 nomEnClat Ur E of El EmEnts 
Wit H atomi C nUmBErs > 100
The naming of the new elements had been 
traditionally the privilege of the discoverer 
(or discoverers) and the suggested name was 
ratified 	 by 	 the 	 IUPAC. 	 In 	 recent 	 years 	 this 	 has	
led to some controversy. The new elements 
with very high atomic numbers are so unstable 
that only minute quantities, sometimes only 
*
 
Glenn T. Seaborg’s work in the middle of the 20
th
 century starting with the discovery of plutonium in 1940, followed by 
those of all the transuranium elements from 94 to 102 led to reconfiguration of the periodic table placing the actinoids below 
the lanthanoids. In 1951, Seaborg was awarded the Nobel Prize in chemistry for his work. Element 106 has been named 
Seaborgium (Sg) in his honour.
Unit 3.indd   78 9/9/2022   4:36:13 PM
Rationalised 2023-24