PERIODIC CLASSIFICATION OF ELEMENTS.pdf

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PERIODIC CLASSIFICATION OF ELEMENTS




Classification of Elements


The ne
ed to simplify and o
rganize the study of elements and their large number of
compounds led to the development of the periodic table.



Dobereiner’s Triad

The initial attempt towards classification of elements was made by J.W.
Dobereiner
, a German
chemist. He tried to arrange t
he elements with similar properties into groups. He identified
certain groups of three elements with similar properties. He called these groups of three
elements as
Dobereiner’s Triads
.

He showed that when the three elements of a triad were
arranged in ord
er of their increasing atomic masses, the atomic mass of the middle element
was roughly the average of the atomic masses of the other two elements.


Element

Atomic Mass

Li

6.9

Na

23

K

39

95
.
22
2
39
9
.
6
=
+


For example, Li, Na, K ; Ca, Sr, Ba ; S,

Se, Te and Cl, Br, I constitute
Dobereiner’s

triads. The idea of
Dobereiner’s

triad was rejected because it could not be applied to all the
elements known at that time.



Newlands Law of Octaves


Next attempt towards classification of elements was made by

John Alexander Newland
who gave
Newlands’s Law of Octaves.

According to this law,
when elements were arranged
in increasing order of their atomic masses, the properties of every eighth element was similar
to the first one
.



Limitations of Newlands Law of

Octaves

1.

The Newlands’ law of octaves was rejected because it could be applied only to
elements having atomic masses upto 40
u
, i.e., calcium


2.

When noble gases were discovered, the properties of the
ninth

element were now
similar to that of the firs
t.



Mendeleev’s Classification



Mendeleev classified the elements on the basis of similarity in physical properties and
similarity in the formulae of their hydrides and oxides
.


Mendeleev’s Periodic Law


Mendeleev’s Periodic Law

states that
the physical
and chemical properties of the
elements are a periodic function of their atomic masses.



Salient Features of Mendeleev’s Classification


(a)

In Mendeleev’s periodic table, elements are arranged in order of their increasing
atomic masses in such a way that

elements with similar properties are placed in the same
vertical column called the
group.

To be sure that elements with similar properties fell in the
same group, Mendeleev had to place an element with slightly greater atomic mass (i.e., Te)
before an ele
ment of slightly lower atomic mass (i.e., I). Similarly, cobalt was placed ahead of
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nickel.


(b)

Mendeleev even left some
gaps
in the periodic table for those elements which
were yet to be discovered. For example, gallium and germanium.



Mendeleev’s Perio
dic Table consists of seven horizontal rows called
periods
and eight
vertical columns called
groups.

The periods are numbered from 1 to 7 while the groups are
designated as I, II, III, IV, V, VI, VII and VIII. Except group VIII, each group is divided into
two
subgroups designated as A and B.




Limitations of Mendeleev’s Periodic Table

1.

Anomalous position of hydrogen

2.

Position of isotopes

3.

Wrong order of atomic masses of some elements

4.

Uncertainty in prediction of new elements.



Modern Periodic Tab
le


Henry Moseley in 1913 showed that atomic number was a more fundamental property
of an element than its atomic mass. Therefore,
atomic number
or

electron number

was
adopted as the basis of classification of elements.



Modern Periodic Law


Mendeleev’s
periodic law was thus modified to Modern periodic law which states that
the properties of elements are a periodic function of their atomic number.



Salient Features of Modern Periodic Table

1.

When the elements are arranged in increasing order of their at
omic numbers, the
anomalies of Mendeleev’s periodic table are removed. However,
the position of
hydrogen still remains anomalous
.

It can be placed either along with alkali metals of
group 1 or along with halogens of group 17 of the Modern periodic table.


2.

In the
Modern or Long form of the periodic table,

elements are arranged in
increasing order of their atomic numbers.


3.

The Modern periodic table is
based upon electronic configuration of elements.


4.

The periodicity in properties of elements is due t
o periodicity in their outer electronic
configurations.


5.

The numbers 2, 8, 8, 18, 18 and 32 after which the properties of elements get repeated
are the
magic numbers

on

which this classification is based.



6.

The Modern periodic table consists of 18 ve
rtical columns called
groups

and 7
horizontal rows called
periods.


7.

Each period starts with the filling of electrons in a new electronic shell and the elements
in a period have consecutive atomic numbers.



Valence Electron


All the elements in a group
have identical outer
-
shell electronic configuration.
However, the number of inner filled energy shells increases as we go down the group. Further,
the elements in a group do not have consecutive atomic numbers.

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Group No.

1

2

13

14

15

16

17

18

Atomic No.

11

12

13

14

15

16

17

18

Symbol

Na

Mg

Al

Si

P

S

Cl

Ar

Electronic Configuration

2, 8, 1

2, 8, 2

2, 8, 3

2, 8, 4

2, 8, 5

2, 8, 6

2, 8, 7

2, 8, 8

No. of valence electron

1

2

3

4

5

6

7

8

Valency

1

2

3

4

3

2

1

0

Atomic Radii

1
86

1
36

125

117

116

104

99

1
9
0



Element

Atomic No.

E.C.

Atomic Radii

Period Two

Li

3

2, 1

152

Period Three

Na

11

2, 8, 1

186

Period Four

K

19

2, 8, 8, 1

231



Valency


The valency of elements in a group is fixed but in a period first it increases from 1 to 4
and then decreases to z
ero.


Valency

In
creases
and then decreases in a period

Remains the
same down the
group




A
tomic size/Atomic Radii


The atomic size decreases across a period from left to right but increases down a
group.



Atomic Size / Radii

D
ecreases from left to right in
a
period

Increases from
top to bottom in
a group




Metallic and Non
-
Metallic Character


Across a period, the metallic charact
er decreases while the non
-
metallic character
increases. Conversely, on moving down a group, the metallic character increases while the
non
-
metallic character decreases.



Metallic Character

Decreases along a period

Increases down
the group




Nature of Oxides

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The oxides of metals are
basic

while
those of non
-
metals are
acidic

in nature.



The periodicity in the properties of elements such as valency, atomic size and
metallic/non
-
metallic character can be explained on the basis of outer
-
shell electronic
configuration of the elements.






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