NCERT book Class 10 Science - Periodic Classification of Elements

 Page 1


Periodic Classification
of Elements
5 CHAPTER
I
n Class IX we have learnt that matter around us is present in the form
of elements, compounds and mixtures and the elements contain atoms
of only one type. Do you know how many elements are known till date?
At present, 118 elements are known to us. All these have different
properties. Out of these 118, only 94 are naturally occurring.
As different elements were being discovered, scientists gathered more
and more information about the properties of these elements. They found
it difficult to organise all that was known about the elements. They started
looking for some pattern in their properties, on the basis of which they
could study such a large number of elements with ease.
5.1 MAKING ORDER OUT OF CHA 5.1 MAKING ORDER OUT OF CHA 5.1 MAKING ORDER OUT OF CHA 5.1 MAKING ORDER OUT OF CHA 5.1 MAKING ORDER OUT OF CHAOS – EARL OS – EARL OS – EARL OS – EARL OS – EARLY Y Y Y Y
A A A A ATTEMPTS A TTEMPTS A TTEMPTS A TTEMPTS A TTEMPTS AT THE CL T THE CL T THE CL T THE CL T THE CLASSIFIC ASSIFIC ASSIFIC ASSIFIC ASSIFICA A A A ATION OF TION OF TION OF TION OF TION OF
ELEMENTS ELEMENTS ELEMENTS ELEMENTS ELEMENTS
We have been learning how various things or living beings can
be classified on the basis of their properties. Even in other
situations, we come across instances of organisation based
on some properties. For example, in a shop, soaps are kept
together at one place while biscuits are kept together elsewhere.
Even among soaps, bathing soaps are stacked separately from
washing soaps. Similarly, scientists made several attempts to
classify elements according to their properties and obtain an
orderly arrangement out of chaos.
The earliest attempt to classify the elements resulted in
grouping the then known elements as metals and non-metals.
Later further classifications were tried out as our knowledge
of elements and their properties increased.
5.1.1 Döbereiner’s Triads
In the year 1817, Johann Wolfgang Döbereiner, a German
chemist, tried to arrange the elements with similar properties into groups.
He identified some groups having three elements each. So he called these
groups ‘triads’. Döbereiner showed that when the three elements in a
Figure 5.1
Imagine you and your friends have
found pieces of an old map to reach
a treasure. Would it be easy or
chaotic to find the way to the
treasure? Similar chaos was there
in Chemistry as elements were
known but there was no clue as to
how to classify and study about them.
2022-23
Page 2


Periodic Classification
of Elements
5 CHAPTER
I
n Class IX we have learnt that matter around us is present in the form
of elements, compounds and mixtures and the elements contain atoms
of only one type. Do you know how many elements are known till date?
At present, 118 elements are known to us. All these have different
properties. Out of these 118, only 94 are naturally occurring.
As different elements were being discovered, scientists gathered more
and more information about the properties of these elements. They found
it difficult to organise all that was known about the elements. They started
looking for some pattern in their properties, on the basis of which they
could study such a large number of elements with ease.
5.1 MAKING ORDER OUT OF CHA 5.1 MAKING ORDER OUT OF CHA 5.1 MAKING ORDER OUT OF CHA 5.1 MAKING ORDER OUT OF CHA 5.1 MAKING ORDER OUT OF CHAOS – EARL OS – EARL OS – EARL OS – EARL OS – EARLY Y Y Y Y
A A A A ATTEMPTS A TTEMPTS A TTEMPTS A TTEMPTS A TTEMPTS AT THE CL T THE CL T THE CL T THE CL T THE CLASSIFIC ASSIFIC ASSIFIC ASSIFIC ASSIFICA A A A ATION OF TION OF TION OF TION OF TION OF
ELEMENTS ELEMENTS ELEMENTS ELEMENTS ELEMENTS
We have been learning how various things or living beings can
be classified on the basis of their properties. Even in other
situations, we come across instances of organisation based
on some properties. For example, in a shop, soaps are kept
together at one place while biscuits are kept together elsewhere.
Even among soaps, bathing soaps are stacked separately from
washing soaps. Similarly, scientists made several attempts to
classify elements according to their properties and obtain an
orderly arrangement out of chaos.
The earliest attempt to classify the elements resulted in
grouping the then known elements as metals and non-metals.
Later further classifications were tried out as our knowledge
of elements and their properties increased.
5.1.1 Döbereiner’s Triads
In the year 1817, Johann Wolfgang Döbereiner, a German
chemist, tried to arrange the elements with similar properties into groups.
He identified some groups having three elements each. So he called these
groups ‘triads’. Döbereiner showed that when the three elements in a
Figure 5.1
Imagine you and your friends have
found pieces of an old map to reach
a treasure. Would it be easy or
chaotic to find the way to the
treasure? Similar chaos was there
in Chemistry as elements were
known but there was no clue as to
how to classify and study about them.
2022-23
Science
80
triad were written in the order of increasing atomic masses; the atomic
mass of the middle element was roughly the average of the atomic masses
of the other two elements.
For example, take the triad consisting of lithium (Li), sodium (Na)
and potassium (K) with the respective atomic masses 6.9, 23.0 and 39.0.
What is the average of the atomic masses of Li and K? How does this
compare with the atomic mass of Na?
Given below (Table 5.1) are some groups of three elements. These
elements are arranged downwards in order of increasing atomic masses.
Can you find out which of these groups form Döbereiner triads?
Table 5.1
Group A Atomic Group B Atomic Group C Atomic
element mass element mass elements mass
N 14.0 Ca 40.1 Cl 35. 5
P 31.0 Sr 87.6 Br 79.9
As 74.9 Ba 137.3 I 126.9
You will find that groups B and C form Döbereiner triads. Döbereiner
could identify only three triads from the elements known at that time
(Table 5.2). Hence, this system of classification into triads was not found
to be useful.
Table 5.2
Döbereiner’s triads
Johann Wolfgang Döbereiner (1780-1849)
Johann Wolfgang Döbereiner studied as a
pharmacist at Münchberg in Germany, and then
studied chemistry at Strasbourg. Eventually he
became a professor of chemistry and pharmacy
at the University of Jena. Döbereiner made the
first observations on platinum as a catalyst and
discovered similar triads of elements which led to
the development of the Periodic Table of elements.
5.1.2 Newlands’ Law of Octaves
The attempts of Döbereiner encouraged other chemists to correlate the
properties of elements with their atomic masses. In 1866, John Newlands,
an English scientist, arranged the then known elements in the order of
increasing atomic masses. He started with the element having the lowest
atomic mass (hydrogen) and ended at thorium which was the 56
th
element. He found that every eighth element had properties similar to
that of the first. He compared this to the octaves found in music.
Therefore, he called it the ‘Law of Octaves’. It is known as ‘Newlands’
Law of Octaves’. In Newlands’ Octaves, the properties of lithium and
sodium were found to be the same. Sodium is the eighth element after
lithium. Similarly, beryllium and magnesium resemble each other. A
part of the original form of Newlands’ Octaves is given in Table 5.3.
Li Ca Cl
Na Sr Br
K Ba I
2022-23
Page 3


Periodic Classification
of Elements
5 CHAPTER
I
n Class IX we have learnt that matter around us is present in the form
of elements, compounds and mixtures and the elements contain atoms
of only one type. Do you know how many elements are known till date?
At present, 118 elements are known to us. All these have different
properties. Out of these 118, only 94 are naturally occurring.
As different elements were being discovered, scientists gathered more
and more information about the properties of these elements. They found
it difficult to organise all that was known about the elements. They started
looking for some pattern in their properties, on the basis of which they
could study such a large number of elements with ease.
5.1 MAKING ORDER OUT OF CHA 5.1 MAKING ORDER OUT OF CHA 5.1 MAKING ORDER OUT OF CHA 5.1 MAKING ORDER OUT OF CHA 5.1 MAKING ORDER OUT OF CHAOS – EARL OS – EARL OS – EARL OS – EARL OS – EARLY Y Y Y Y
A A A A ATTEMPTS A TTEMPTS A TTEMPTS A TTEMPTS A TTEMPTS AT THE CL T THE CL T THE CL T THE CL T THE CLASSIFIC ASSIFIC ASSIFIC ASSIFIC ASSIFICA A A A ATION OF TION OF TION OF TION OF TION OF
ELEMENTS ELEMENTS ELEMENTS ELEMENTS ELEMENTS
We have been learning how various things or living beings can
be classified on the basis of their properties. Even in other
situations, we come across instances of organisation based
on some properties. For example, in a shop, soaps are kept
together at one place while biscuits are kept together elsewhere.
Even among soaps, bathing soaps are stacked separately from
washing soaps. Similarly, scientists made several attempts to
classify elements according to their properties and obtain an
orderly arrangement out of chaos.
The earliest attempt to classify the elements resulted in
grouping the then known elements as metals and non-metals.
Later further classifications were tried out as our knowledge
of elements and their properties increased.
5.1.1 Döbereiner’s Triads
In the year 1817, Johann Wolfgang Döbereiner, a German
chemist, tried to arrange the elements with similar properties into groups.
He identified some groups having three elements each. So he called these
groups ‘triads’. Döbereiner showed that when the three elements in a
Figure 5.1
Imagine you and your friends have
found pieces of an old map to reach
a treasure. Would it be easy or
chaotic to find the way to the
treasure? Similar chaos was there
in Chemistry as elements were
known but there was no clue as to
how to classify and study about them.
2022-23
Science
80
triad were written in the order of increasing atomic masses; the atomic
mass of the middle element was roughly the average of the atomic masses
of the other two elements.
For example, take the triad consisting of lithium (Li), sodium (Na)
and potassium (K) with the respective atomic masses 6.9, 23.0 and 39.0.
What is the average of the atomic masses of Li and K? How does this
compare with the atomic mass of Na?
Given below (Table 5.1) are some groups of three elements. These
elements are arranged downwards in order of increasing atomic masses.
Can you find out which of these groups form Döbereiner triads?
Table 5.1
Group A Atomic Group B Atomic Group C Atomic
element mass element mass elements mass
N 14.0 Ca 40.1 Cl 35. 5
P 31.0 Sr 87.6 Br 79.9
As 74.9 Ba 137.3 I 126.9
You will find that groups B and C form Döbereiner triads. Döbereiner
could identify only three triads from the elements known at that time
(Table 5.2). Hence, this system of classification into triads was not found
to be useful.
Table 5.2
Döbereiner’s triads
Johann Wolfgang Döbereiner (1780-1849)
Johann Wolfgang Döbereiner studied as a
pharmacist at Münchberg in Germany, and then
studied chemistry at Strasbourg. Eventually he
became a professor of chemistry and pharmacy
at the University of Jena. Döbereiner made the
first observations on platinum as a catalyst and
discovered similar triads of elements which led to
the development of the Periodic Table of elements.
5.1.2 Newlands’ Law of Octaves
The attempts of Döbereiner encouraged other chemists to correlate the
properties of elements with their atomic masses. In 1866, John Newlands,
an English scientist, arranged the then known elements in the order of
increasing atomic masses. He started with the element having the lowest
atomic mass (hydrogen) and ended at thorium which was the 56
th
element. He found that every eighth element had properties similar to
that of the first. He compared this to the octaves found in music.
Therefore, he called it the ‘Law of Octaves’. It is known as ‘Newlands’
Law of Octaves’. In Newlands’ Octaves, the properties of lithium and
sodium were found to be the same. Sodium is the eighth element after
lithium. Similarly, beryllium and magnesium resemble each other. A
part of the original form of Newlands’ Octaves is given in Table 5.3.
Li Ca Cl
Na Sr Br
K Ba I
2022-23
Periodic Classification of Elements 81
Are you familiar with musical notes?
In the Indian system of music, there are seven musical notes in a scale – sa, re, ga, ma, pa,
da, ni. In the west, they use the notations – do, re, mi, fa, so, la, ti. The notes in a scale are
separated by whole and half-step frequency intervals of tones and semitones. A musician
uses these notes for composing the music of a song. Naturally, there must be some repetition
of notes. Every eighth note is similar to the first one and it is the first note of the next scale.
n It was found that the Law of Octaves was applicable only upto
calcium, as after calcium every eighth element did not possess
properties similar to that of the first.
n It was assumed by Newlands that only 56 elements existed in nature
and no more elements would be discovered in the future. But, later
on, several new elements were discovered, whose properties did not
fit into the Law of Octaves.
n In order to fit elements into his Table, Newlands adjusted two elements
in the same slot, but also put some unlike elements under the same
note. Can you find examples of these from Table 5.3? Note that cobalt
and nickel are in the same slot and these are placed in the same
column as fluorine, chlorine and bromine which have very different
properties than these elements. Iron, which resembles cobalt and
nickel in properties, has been placed far away from these elements.
With the discovery of noble gases, the Law of Octaves became irrelevant.
Thus, Newlands’ Law of Octaves worked well with lighter elements only.
QUESTIONS
1. Did Döbereiner’s triads also exist in the columns of Newlands’ Octaves?
Compare and find out.
2. What were the limitations of Döbereiner’s classification?
3. What were the limitations of Newlands’ Law of Octaves?
5.2 MAKING ORDER OUT OF CHA 5.2 MAKING ORDER OUT OF CHA 5.2 MAKING ORDER OUT OF CHA 5.2 MAKING ORDER OUT OF CHA 5.2 MAKING ORDER OUT OF CHAOS – MENDELÉEV OS – MENDELÉEV OS – MENDELÉEV OS – MENDELÉEV OS – MENDELÉEV’S ’S ’S ’S ’S
PERIODIC TABLE PERIODIC TABLE PERIODIC TABLE PERIODIC TABLE PERIODIC TABLE
Even after the rejection of Newlands’ Law of Octaves, many scientists
continued to search for a pattern that correlated the properties of elements
with their atomic masses.
?
Table 5.3 Newlands’ Octaves
Notes of music:
sa re  ga ma pa da ni
(do) (re) (mi) (fa) (so) (la) (ti)
H Li Be B C N O
F Na Mg Al Si P S
Cl K Ca Cr Ti Mn Fe
Co and Ni Cu Zn Y In As Se
Br Rb Sr Ce and La Zr — —
Do Y Do Y Do Y Do Y Do You Know? ou Know? ou Know? ou Know? ou Know?
2022-23
Page 4


Periodic Classification
of Elements
5 CHAPTER
I
n Class IX we have learnt that matter around us is present in the form
of elements, compounds and mixtures and the elements contain atoms
of only one type. Do you know how many elements are known till date?
At present, 118 elements are known to us. All these have different
properties. Out of these 118, only 94 are naturally occurring.
As different elements were being discovered, scientists gathered more
and more information about the properties of these elements. They found
it difficult to organise all that was known about the elements. They started
looking for some pattern in their properties, on the basis of which they
could study such a large number of elements with ease.
5.1 MAKING ORDER OUT OF CHA 5.1 MAKING ORDER OUT OF CHA 5.1 MAKING ORDER OUT OF CHA 5.1 MAKING ORDER OUT OF CHA 5.1 MAKING ORDER OUT OF CHAOS – EARL OS – EARL OS – EARL OS – EARL OS – EARLY Y Y Y Y
A A A A ATTEMPTS A TTEMPTS A TTEMPTS A TTEMPTS A TTEMPTS AT THE CL T THE CL T THE CL T THE CL T THE CLASSIFIC ASSIFIC ASSIFIC ASSIFIC ASSIFICA A A A ATION OF TION OF TION OF TION OF TION OF
ELEMENTS ELEMENTS ELEMENTS ELEMENTS ELEMENTS
We have been learning how various things or living beings can
be classified on the basis of their properties. Even in other
situations, we come across instances of organisation based
on some properties. For example, in a shop, soaps are kept
together at one place while biscuits are kept together elsewhere.
Even among soaps, bathing soaps are stacked separately from
washing soaps. Similarly, scientists made several attempts to
classify elements according to their properties and obtain an
orderly arrangement out of chaos.
The earliest attempt to classify the elements resulted in
grouping the then known elements as metals and non-metals.
Later further classifications were tried out as our knowledge
of elements and their properties increased.
5.1.1 Döbereiner’s Triads
In the year 1817, Johann Wolfgang Döbereiner, a German
chemist, tried to arrange the elements with similar properties into groups.
He identified some groups having three elements each. So he called these
groups ‘triads’. Döbereiner showed that when the three elements in a
Figure 5.1
Imagine you and your friends have
found pieces of an old map to reach
a treasure. Would it be easy or
chaotic to find the way to the
treasure? Similar chaos was there
in Chemistry as elements were
known but there was no clue as to
how to classify and study about them.
2022-23
Science
80
triad were written in the order of increasing atomic masses; the atomic
mass of the middle element was roughly the average of the atomic masses
of the other two elements.
For example, take the triad consisting of lithium (Li), sodium (Na)
and potassium (K) with the respective atomic masses 6.9, 23.0 and 39.0.
What is the average of the atomic masses of Li and K? How does this
compare with the atomic mass of Na?
Given below (Table 5.1) are some groups of three elements. These
elements are arranged downwards in order of increasing atomic masses.
Can you find out which of these groups form Döbereiner triads?
Table 5.1
Group A Atomic Group B Atomic Group C Atomic
element mass element mass elements mass
N 14.0 Ca 40.1 Cl 35. 5
P 31.0 Sr 87.6 Br 79.9
As 74.9 Ba 137.3 I 126.9
You will find that groups B and C form Döbereiner triads. Döbereiner
could identify only three triads from the elements known at that time
(Table 5.2). Hence, this system of classification into triads was not found
to be useful.
Table 5.2
Döbereiner’s triads
Johann Wolfgang Döbereiner (1780-1849)
Johann Wolfgang Döbereiner studied as a
pharmacist at Münchberg in Germany, and then
studied chemistry at Strasbourg. Eventually he
became a professor of chemistry and pharmacy
at the University of Jena. Döbereiner made the
first observations on platinum as a catalyst and
discovered similar triads of elements which led to
the development of the Periodic Table of elements.
5.1.2 Newlands’ Law of Octaves
The attempts of Döbereiner encouraged other chemists to correlate the
properties of elements with their atomic masses. In 1866, John Newlands,
an English scientist, arranged the then known elements in the order of
increasing atomic masses. He started with the element having the lowest
atomic mass (hydrogen) and ended at thorium which was the 56
th
element. He found that every eighth element had properties similar to
that of the first. He compared this to the octaves found in music.
Therefore, he called it the ‘Law of Octaves’. It is known as ‘Newlands’
Law of Octaves’. In Newlands’ Octaves, the properties of lithium and
sodium were found to be the same. Sodium is the eighth element after
lithium. Similarly, beryllium and magnesium resemble each other. A
part of the original form of Newlands’ Octaves is given in Table 5.3.
Li Ca Cl
Na Sr Br
K Ba I
2022-23
Periodic Classification of Elements 81
Are you familiar with musical notes?
In the Indian system of music, there are seven musical notes in a scale – sa, re, ga, ma, pa,
da, ni. In the west, they use the notations – do, re, mi, fa, so, la, ti. The notes in a scale are
separated by whole and half-step frequency intervals of tones and semitones. A musician
uses these notes for composing the music of a song. Naturally, there must be some repetition
of notes. Every eighth note is similar to the first one and it is the first note of the next scale.
n It was found that the Law of Octaves was applicable only upto
calcium, as after calcium every eighth element did not possess
properties similar to that of the first.
n It was assumed by Newlands that only 56 elements existed in nature
and no more elements would be discovered in the future. But, later
on, several new elements were discovered, whose properties did not
fit into the Law of Octaves.
n In order to fit elements into his Table, Newlands adjusted two elements
in the same slot, but also put some unlike elements under the same
note. Can you find examples of these from Table 5.3? Note that cobalt
and nickel are in the same slot and these are placed in the same
column as fluorine, chlorine and bromine which have very different
properties than these elements. Iron, which resembles cobalt and
nickel in properties, has been placed far away from these elements.
With the discovery of noble gases, the Law of Octaves became irrelevant.
Thus, Newlands’ Law of Octaves worked well with lighter elements only.
QUESTIONS
1. Did Döbereiner’s triads also exist in the columns of Newlands’ Octaves?
Compare and find out.
2. What were the limitations of Döbereiner’s classification?
3. What were the limitations of Newlands’ Law of Octaves?
5.2 MAKING ORDER OUT OF CHA 5.2 MAKING ORDER OUT OF CHA 5.2 MAKING ORDER OUT OF CHA 5.2 MAKING ORDER OUT OF CHA 5.2 MAKING ORDER OUT OF CHAOS – MENDELÉEV OS – MENDELÉEV OS – MENDELÉEV OS – MENDELÉEV OS – MENDELÉEV’S ’S ’S ’S ’S
PERIODIC TABLE PERIODIC TABLE PERIODIC TABLE PERIODIC TABLE PERIODIC TABLE
Even after the rejection of Newlands’ Law of Octaves, many scientists
continued to search for a pattern that correlated the properties of elements
with their atomic masses.
?
Table 5.3 Newlands’ Octaves
Notes of music:
sa re  ga ma pa da ni
(do) (re) (mi) (fa) (so) (la) (ti)
H Li Be B C N O
F Na Mg Al Si P S
Cl K Ca Cr Ti Mn Fe
Co and Ni Cu Zn Y In As Se
Br Rb Sr Ce and La Zr — —
Do Y Do Y Do Y Do Y Do You Know? ou Know? ou Know? ou Know? ou Know?
2022-23
Science
82
The main credit for classifying elements goes to Dmitri Ivanovich
Mendeléev, a Russian chemist. He was the most important contributor
to the early development of a Periodic Table of elements wherein the
elements were arranged on the basis of their fundamental property, the
atomic mass, and also on the similarity of chemical properties.
Dmitri lvanovich Mendeléev (1834-1907)
Dmitri lvanovich Mendeléev was
born in Tobolsk in Western Siberia,
Russia on 8 February 1834. After
his early education, Mendeléev
could join a university only due to
the efforts of his mother. Dedicating
his investigations to his mother he
wrote, “She instructed with
example, corrected with love and
travelled with me to places
spending her last resources and
strength. She knew that with the
aid of science without violence, with love but firmness, all
superstitions, untruth and errors can be removed.” The
arrangement of elements he proposed is called Mendeléev’s
Periodic Table. The Periodic Table proved to be the unifying
principle in chemistry. It was the motivation for the discovery of
some new elements.
When Mendeléev started his work, 63 elements were known. He
examined the relationship between the atomic masses of the elements
and their physical and chemical properties. Among chemical properties,
Mendeléev concentrated on the compounds formed by elements with
oxygen and hydrogen. He selected hydrogen and oxygen as they are
very reactive and formed compounds with most elements. The formulae
of the hydrides and oxides formed by an element were treated as one of
the basic properties of an element for its classification. He then took 63
cards and on each card he wrote down the properties of one element. He
sorted out the elements with similar properties and pinned the cards
together on a wall. He observed that most of the elements got a place in
a Periodic Table and were arranged in the order of their increasing atomic
masses. It was also observed that there occurs a periodic recurrence of
elements with similar physical and chemical properties. On this basis,
Mendeléev formulated a Periodic Law, which states that ‘the properties
of elements are the periodic function of their atomic masses’.
Mendeléev’s Periodic Table contains vertical columns called ‘groups’
and horizontal rows called ‘periods’ (Table 5.4).
2022-23
Page 5


Periodic Classification
of Elements
5 CHAPTER
I
n Class IX we have learnt that matter around us is present in the form
of elements, compounds and mixtures and the elements contain atoms
of only one type. Do you know how many elements are known till date?
At present, 118 elements are known to us. All these have different
properties. Out of these 118, only 94 are naturally occurring.
As different elements were being discovered, scientists gathered more
and more information about the properties of these elements. They found
it difficult to organise all that was known about the elements. They started
looking for some pattern in their properties, on the basis of which they
could study such a large number of elements with ease.
5.1 MAKING ORDER OUT OF CHA 5.1 MAKING ORDER OUT OF CHA 5.1 MAKING ORDER OUT OF CHA 5.1 MAKING ORDER OUT OF CHA 5.1 MAKING ORDER OUT OF CHAOS – EARL OS – EARL OS – EARL OS – EARL OS – EARLY Y Y Y Y
A A A A ATTEMPTS A TTEMPTS A TTEMPTS A TTEMPTS A TTEMPTS AT THE CL T THE CL T THE CL T THE CL T THE CLASSIFIC ASSIFIC ASSIFIC ASSIFIC ASSIFICA A A A ATION OF TION OF TION OF TION OF TION OF
ELEMENTS ELEMENTS ELEMENTS ELEMENTS ELEMENTS
We have been learning how various things or living beings can
be classified on the basis of their properties. Even in other
situations, we come across instances of organisation based
on some properties. For example, in a shop, soaps are kept
together at one place while biscuits are kept together elsewhere.
Even among soaps, bathing soaps are stacked separately from
washing soaps. Similarly, scientists made several attempts to
classify elements according to their properties and obtain an
orderly arrangement out of chaos.
The earliest attempt to classify the elements resulted in
grouping the then known elements as metals and non-metals.
Later further classifications were tried out as our knowledge
of elements and their properties increased.
5.1.1 Döbereiner’s Triads
In the year 1817, Johann Wolfgang Döbereiner, a German
chemist, tried to arrange the elements with similar properties into groups.
He identified some groups having three elements each. So he called these
groups ‘triads’. Döbereiner showed that when the three elements in a
Figure 5.1
Imagine you and your friends have
found pieces of an old map to reach
a treasure. Would it be easy or
chaotic to find the way to the
treasure? Similar chaos was there
in Chemistry as elements were
known but there was no clue as to
how to classify and study about them.
2022-23
Science
80
triad were written in the order of increasing atomic masses; the atomic
mass of the middle element was roughly the average of the atomic masses
of the other two elements.
For example, take the triad consisting of lithium (Li), sodium (Na)
and potassium (K) with the respective atomic masses 6.9, 23.0 and 39.0.
What is the average of the atomic masses of Li and K? How does this
compare with the atomic mass of Na?
Given below (Table 5.1) are some groups of three elements. These
elements are arranged downwards in order of increasing atomic masses.
Can you find out which of these groups form Döbereiner triads?
Table 5.1
Group A Atomic Group B Atomic Group C Atomic
element mass element mass elements mass
N 14.0 Ca 40.1 Cl 35. 5
P 31.0 Sr 87.6 Br 79.9
As 74.9 Ba 137.3 I 126.9
You will find that groups B and C form Döbereiner triads. Döbereiner
could identify only three triads from the elements known at that time
(Table 5.2). Hence, this system of classification into triads was not found
to be useful.
Table 5.2
Döbereiner’s triads
Johann Wolfgang Döbereiner (1780-1849)
Johann Wolfgang Döbereiner studied as a
pharmacist at Münchberg in Germany, and then
studied chemistry at Strasbourg. Eventually he
became a professor of chemistry and pharmacy
at the University of Jena. Döbereiner made the
first observations on platinum as a catalyst and
discovered similar triads of elements which led to
the development of the Periodic Table of elements.
5.1.2 Newlands’ Law of Octaves
The attempts of Döbereiner encouraged other chemists to correlate the
properties of elements with their atomic masses. In 1866, John Newlands,
an English scientist, arranged the then known elements in the order of
increasing atomic masses. He started with the element having the lowest
atomic mass (hydrogen) and ended at thorium which was the 56
th
element. He found that every eighth element had properties similar to
that of the first. He compared this to the octaves found in music.
Therefore, he called it the ‘Law of Octaves’. It is known as ‘Newlands’
Law of Octaves’. In Newlands’ Octaves, the properties of lithium and
sodium were found to be the same. Sodium is the eighth element after
lithium. Similarly, beryllium and magnesium resemble each other. A
part of the original form of Newlands’ Octaves is given in Table 5.3.
Li Ca Cl
Na Sr Br
K Ba I
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Periodic Classification of Elements 81
Are you familiar with musical notes?
In the Indian system of music, there are seven musical notes in a scale – sa, re, ga, ma, pa,
da, ni. In the west, they use the notations – do, re, mi, fa, so, la, ti. The notes in a scale are
separated by whole and half-step frequency intervals of tones and semitones. A musician
uses these notes for composing the music of a song. Naturally, there must be some repetition
of notes. Every eighth note is similar to the first one and it is the first note of the next scale.
n It was found that the Law of Octaves was applicable only upto
calcium, as after calcium every eighth element did not possess
properties similar to that of the first.
n It was assumed by Newlands that only 56 elements existed in nature
and no more elements would be discovered in the future. But, later
on, several new elements were discovered, whose properties did not
fit into the Law of Octaves.
n In order to fit elements into his Table, Newlands adjusted two elements
in the same slot, but also put some unlike elements under the same
note. Can you find examples of these from Table 5.3? Note that cobalt
and nickel are in the same slot and these are placed in the same
column as fluorine, chlorine and bromine which have very different
properties than these elements. Iron, which resembles cobalt and
nickel in properties, has been placed far away from these elements.
With the discovery of noble gases, the Law of Octaves became irrelevant.
Thus, Newlands’ Law of Octaves worked well with lighter elements only.
QUESTIONS
1. Did Döbereiner’s triads also exist in the columns of Newlands’ Octaves?
Compare and find out.
2. What were the limitations of Döbereiner’s classification?
3. What were the limitations of Newlands’ Law of Octaves?
5.2 MAKING ORDER OUT OF CHA 5.2 MAKING ORDER OUT OF CHA 5.2 MAKING ORDER OUT OF CHA 5.2 MAKING ORDER OUT OF CHA 5.2 MAKING ORDER OUT OF CHAOS – MENDELÉEV OS – MENDELÉEV OS – MENDELÉEV OS – MENDELÉEV OS – MENDELÉEV’S ’S ’S ’S ’S
PERIODIC TABLE PERIODIC TABLE PERIODIC TABLE PERIODIC TABLE PERIODIC TABLE
Even after the rejection of Newlands’ Law of Octaves, many scientists
continued to search for a pattern that correlated the properties of elements
with their atomic masses.
?
Table 5.3 Newlands’ Octaves
Notes of music:
sa re  ga ma pa da ni
(do) (re) (mi) (fa) (so) (la) (ti)
H Li Be B C N O
F Na Mg Al Si P S
Cl K Ca Cr Ti Mn Fe
Co and Ni Cu Zn Y In As Se
Br Rb Sr Ce and La Zr — —
Do Y Do Y Do Y Do Y Do You Know? ou Know? ou Know? ou Know? ou Know?
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Science
82
The main credit for classifying elements goes to Dmitri Ivanovich
Mendeléev, a Russian chemist. He was the most important contributor
to the early development of a Periodic Table of elements wherein the
elements were arranged on the basis of their fundamental property, the
atomic mass, and also on the similarity of chemical properties.
Dmitri lvanovich Mendeléev (1834-1907)
Dmitri lvanovich Mendeléev was
born in Tobolsk in Western Siberia,
Russia on 8 February 1834. After
his early education, Mendeléev
could join a university only due to
the efforts of his mother. Dedicating
his investigations to his mother he
wrote, “She instructed with
example, corrected with love and
travelled with me to places
spending her last resources and
strength. She knew that with the
aid of science without violence, with love but firmness, all
superstitions, untruth and errors can be removed.” The
arrangement of elements he proposed is called Mendeléev’s
Periodic Table. The Periodic Table proved to be the unifying
principle in chemistry. It was the motivation for the discovery of
some new elements.
When Mendeléev started his work, 63 elements were known. He
examined the relationship between the atomic masses of the elements
and their physical and chemical properties. Among chemical properties,
Mendeléev concentrated on the compounds formed by elements with
oxygen and hydrogen. He selected hydrogen and oxygen as they are
very reactive and formed compounds with most elements. The formulae
of the hydrides and oxides formed by an element were treated as one of
the basic properties of an element for its classification. He then took 63
cards and on each card he wrote down the properties of one element. He
sorted out the elements with similar properties and pinned the cards
together on a wall. He observed that most of the elements got a place in
a Periodic Table and were arranged in the order of their increasing atomic
masses. It was also observed that there occurs a periodic recurrence of
elements with similar physical and chemical properties. On this basis,
Mendeléev formulated a Periodic Law, which states that ‘the properties
of elements are the periodic function of their atomic masses’.
Mendeléev’s Periodic Table contains vertical columns called ‘groups’
and horizontal rows called ‘periods’ (Table 5.4).
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Periodic Classification of Elements 83
Mendeléev’s Periodic Table was published in a German journal in 1872. In the formula
for oxides and hydrides at the top of the columns, the letter ‘R’ is used to represent any
of the elements in the group. Note the way formulae are written. For example, the hydride
of carbon, CH
4
, is written as RH
4
 and the oxide CO
2
, as RO
2
.
Table 5.4 Mendeléev’s Periodic Table
5.2.1 Achievements of Mendeléev’s Periodic Table
While developing the Periodic Table, there were a few instances where
Mendeléev had to place an element with a slightly greater atomic mass
before an element with a slightly lower atomic mass. The sequence was
inverted so that elements with similar properties could be grouped
together. For example, cobalt (atomic mass 58.9) appeared before nickel
(atomic mass 58.7). Looking at Table 5.4, can you find out one more
such anomaly?
Further, Mendeléev left some gaps in his Periodic Table. Instead of
looking upon these gaps as defects, Mendeléev boldly predicted the
existence of some elements that had not been discovered at that time.
Mendeléev named them by prefixing a Sanskrit numeral, Eka (one) to
the name of preceding element in the same group. For instance,
scandium, gallium and germanium, discovered later, have properties
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