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
2024-25
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
2024-25
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
2024-25
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
2024-25
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
2024-25
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
2024-25
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
2024-25
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
2024-25
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
2024-25
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
2024-25
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
2024-25
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
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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
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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
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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
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