NCERT Textbook - Periodic Classification of Elements Class 10 Notes | EduRev

General Science(Prelims) by IRS Divey Sethi

UPSC : NCERT Textbook - Periodic Classification of Elements Class 10 Notes | EduRev

 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, 114 elements are known to us. Around the year 1800, only
30 elements were known. All these had seemingly different properties.
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.
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, 114 elements are known to us. Around the year 1800, only
30 elements were known. All these had seemingly different properties.
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.
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
KBa I
Read More
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