Evolution of Periodic Table and Classification of Elements

 
P
E
R
I
O
D
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C
L
A
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S
I
F
I
C
A
T
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O
F
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T
S
 
CONTENTS:
What is 
Periodic
 
table
Need 
for classification 
of
 
elements
Dobereiner’s
 
Triads
Newland’s Law of
 
Octaves
Mendeleev’s 
Periodic
 
Table
  Modern 
Periodic Table
  Trends 
in Modern 
Periodic
 
Table
 
WHAT IS A PERIODIC TABLE
 
It is the tabular display 
of elements organized
on the  basis of 
atomic 
number
, 
electronic
configurations
 
and 
 chemical
 properties
.
The 
table also 
shows 
four rectangular 
blocks
:
s-,  
p- 
d- 
and 
f-block. In 
general, 
within 
one
row  (period) the 
elements are metals on 
the
left
 
hand  side, and non-metals on 
the right
hand
 
side.
The 
rows 
of 
the table 
are called 
periods
; the
columns  are called
 
groups
 
NEED FOR CLASSIFICATION OF ELEMENTS
 
In the 
year 
1800
, 
only 
30 
elements
 
were
known. 
Now, at present, 
there 
are 
118
elements
 
known.
So, 
it became difficult for the 
scientist 
to
remember 
all 
the 
properties of
 
elements.
Therefore, there was 
a 
need to classify
elements.
Many scientist 
tried to classify 
elements
based on different criteria. Some
 
brought
success 
while 
some 
did not.
 
ORGANIZING THE ELEMENTS
 
• Chemists used the properties of
elements to sort them into groups.
• JW. Dobreiner grouped elements into
triads.
• A triad is a set of three elements with
similar properties
 
Dobereiner’s Triads
 
 
Dobereiner classified elements in the increasing
order of their atomic masses into groups of three
elements called triads. In each triad the atomic
mass of the middle element was approximately
equal to the average atomic mass of the other two
elements.
The defect in this classification was that all the then
known elements could not be correctly arranged into
triads.
 
 
Example 
of 
Dobereiner’s
 
Triad:
 
Element
 
Atomic
 
mass
 
Lithium:
 
7amu
Sod
i
um
:
 
 
2
3
a
mu
Potas
s
iu
m
:
 
39amu
 
So, this is 
one example of his 
triad. In this triad 
atomic mass
of  sodium 
is 
average of 
the other 
two 
atomic
 
mass.
i.e. 7+39/2 
= 
46/2 
=
 
23amu.
L
i
m
i
t
a
t
i
o
n
 
o
f
 
h
i
s
 
T
r
i
a
d
:
 
Dobereiner could only 
find three 
such 
triads from the elements
known at 
that 
time. 
Thus 
this system 
of 
classification 
was 
not
found  to be
 
useful
.
 
Newland 
arranged element
according
 
to  their 
increasing
atomic
 
mass.
H
e
 
f
o
u
n
d
 
o
u
t
 
t
h
a
t
 
e
v
e
r
y
 
e
i
g
h
t
e
l
e
m
e
n
t
 
 
h
a
d
 
p
r
o
p
e
r
t
i
e
s
 
s
i
m
i
l
a
r
 
t
o
f
i
r
s
t
.
 
T
h
i
s
 
 
c
a
m
e
 
t
o
 
b
e
 
k
n
o
w
n
 
a
s
N
e
w
l
a
n
d
s
 
l
a
w
 
o
f
 
 
o
c
t
a
v
e
.
Example:
Lithium and Sodium 
were found
 
to
 
be
same
.
 
Limi
t
a
tio
n
s
:
 
Law of octave 
was 
only applicable 
up to 
calcium
as after every eight element 
did not 
have same
properties 
to that 
of
 
first.
He assumed that 
only 
56 
element 
existed 
and
no 
more elements 
were to be
 
discovered.
In 
order 
to fit 
elements 
into the 
table,
Newlands’ 
adjusted two 
elements like
 
cobalt
and nickel 
in the 
same slot and also put some
unlike elements 
under the 
same
 
note.
S
o
 
t
h
i
s
 
l
a
w
 
o
n
l
y
 
w
o
r
k
e
d
 
w
e
l
l
 
w
i
t
h
 
l
i
g
h
t
e
r
e
l
e
m
e
n
t
s
h
e
n
c
e
 
i
t
 
w
a
s
 
n
o
t
 
a
p
p
l
i
c
a
b
l
e
 
w
i
t
h
e
l
e
m
e
n
t
s
h
a
v
e
 
a
t
o
m
i
c
 
m
a
s
s
 
a
b
o
v
e
 
4
0
u
 
He 
arranged elements on the bases of 
Atomic
 
mass
and 
chemical
 
properties
.
He 
selected hydrogen and oxygen and reacted
 
every
elements 
with it 
and formed 
its
 
compounds.
Compounds of hydrides and oxides 
were treated 
as
one of the basic properties of an element for 
its
classification.
H
e
 
f
o
r
m
u
l
a
t
e
d
 
P
e
r
i
o
d
i
c
 
L
a
w
:
P
r
o
p
e
r
t
i
e
s
 
o
f
 
e
l
e
m
e
n
t
s
 
a
r
e
 
t
h
e
 
p
e
r
i
o
d
i
c
 
f
u
n
c
t
i
o
n
 
o
f
t
h
e
i
r
 
a
t
o
m
i
c
 
m
a
s
s
.
 
Predicted 
existence of some 
elements that
were not discovered 
at 
that time. 
Eg:  Gallium,
Scandium 
and
 
Germanium.
Predict properties 
of elements on 
basis 
of  
its
position.
Nobel gases 
which were discovered 
later
could fit 
in his
 
table.
 
Achievements 
of
 
Mendeleev
 
LIMITATIONS 
OF MENDELEEV’S
 
TABLE
 
Position of the 
Isotopes 
could 
not be
explained.
Wrong order of atomic mass could not  be
explained in 
some 
case. Eg:  Cobalt(58.9)
came 
before
 
Nickle(58.7).
Hydrogen was not 
assigned correct  position. It
had 
properties of 
both 
alkali  
and
 
Halogen
s.
 
In 
1913 Henry 
Moseley showed 
that
atomic number of an element 
was 
more
fundamental property than atomic
 
mass.
M
o
d
e
r
n
 
p
e
r
i
o
d
i
c
 
l
a
w
:
P
r
o
p
e
r
t
i
e
s
 
o
f
 
e
l
e
m
e
n
t
s
 
a
r
e
t
h
e
 
p
e
r
i
o
d
i
c
f
u
n
c
t
i
o
n
 
o
f
 
t
h
e
i
r
 
a
t
o
m
i
c
 
n
u
m
b
e
r
.
 
Modern periodic 
table 
consists of
 
18
groups
 and
 7
 
periods.
 
Through the modern periodic
 
table:
1.
Position of isotope is
 
clear.
2.
Position of Nickle and cobalt 
is
 
clear.
3.
Position of hydrogen 
is well
 
explained.
 
Trends 
in
 
Periods
 
P
e
r
i
o
d
 
 
H
o
r
i
z
o
n
t
a
l
 
r
o
w
s
 
o
f
 
e
l
e
m
e
n
t
 
i
n
 
a
 
p
e
r
i
o
d
i
c
t
a
b
l
e
 
a
r
e
 
c
a
l
l
e
d
p
e
r
i
o
d
s
.
Trends
 
:-
1.
V
a
l
e
n
c
e
 
e
l
e
c
t
r
o
n
 
:
 
F
r
o
m
 
l
e
f
t
 
t
o
 
r
i
g
h
t
 
n
u
m
b
e
r
 
o
f
v
a
l
e
n
c
e
 
e
l
e
c
t
r
o
n
 
i
n
c
r
e
a
s
e
s
.
2.
A
t
o
m
i
c
 
N
u
m
b
e
r
 
:
 
F
r
o
m
 
l
e
f
t
 
t
o
 
r
i
g
h
t
 
t
h
e
r
e
 
i
s
c
o
n
s
e
c
u
t
i
v
e
 
a
t
o
m
i
c
 
n
u
m
b
e
r
.
3.
V
a
l
e
n
c
y
 
:
 
F
r
o
m
 
l
e
f
t
 
t
o
 
r
i
g
h
t
 
V
a
l
e
n
c
y
 
i
n
c
r
e
a
s
e
s
 
f
r
o
m
1 
to 
4 and 
then decrease to
 
0.
4.
A
t
o
m
i
c
 
s
i
z
e
 
:
 
D
e
c
r
e
a
s
e
s
 
f
r
o
m
 
l
e
f
t
 
t
o
 
r
i
g
h
t
.
5.
M
e
t
a
l
l
i
c
C
h
a
r
a
c
t
e
r
 
:
 
M
e
t
a
l
l
i
c
 
c
h
a
r
a
c
t
e
r
 
d
e
c
r
e
a
s
e
s
f
r
o
m
 
l
e
f
t
 
t
o
 
r
i
g
h
t
.
6.
C
h
e
m
i
c
a
l
 
r
e
a
c
t
i
v
i
t
y
 
:
 
F
r
o
m
 
l
e
f
t
 
t
o
 
r
i
g
h
t
,
 
d
e
c
r
e
a
s
e
s
t
h
e
n
 
i
n
c
r
e
a
s
e
s
.
7.
N
a
t
u
r
e
 
o
f
 
o
x
i
d
e
s
 
:
 
F
r
o
m
 
l
e
f
t
 
t
o
 
r
i
g
h
t
 
b
a
s
i
c
c
h
a
r
a
c
t
e
r
 
d
e
c
r
e
a
s
e
s
 
a
n
d
 
t
h
e
n
 
a
c
i
d
i
c
 
c
h
a
r
a
c
t
e
r
i
n
c
r
e
a
s
e
s
.
 
Trends 
in
 
Groups
 
G
r
o
u
p
 
 
V
e
r
t
i
c
a
l
 
c
o
l
u
m
n
s
 
i
n
 
p
e
r
i
o
d
i
c
 
t
a
b
l
e
 
i
s
 
k
n
o
w
n
 
a
s
groups.
Trends
 
:-
1.
V
a
l
e
n
c
e
 
E
l
e
c
t
r
o
n
 
:
 
H
a
v
e
 
s
a
m
e
 
n
u
m
b
e
r
 
o
f
 
v
a
l
e
n
c
e
e
l
e
c
t
r
o
n
.
2.
V
a
l
e
n
c
y
 
:
 
E
l
e
m
e
n
t
s
 
i
n
 
a
 
g
r
o
u
p
 
h
a
v
e
 
a
 
s
a
m
e
 
V
a
l
e
n
c
y
.
3.
A
t
o
m
i
c
 
s
i
z
e
 
:
 
I
n
c
r
e
a
s
e
s
 
f
r
o
m
 
u
p
 
t
o
 
d
o
w
n
.
4.
M
e
t
a
l
l
i
c
 
C
h
a
r
a
c
t
e
r
 
:
 
O
n
 
g
o
i
n
g
 
d
o
w
n
 
m
e
t
a
l
l
i
c
 
c
h
a
r
a
c
t
e
r
i
n
c
r
e
a
s
e
s
.
5.
C
h
e
m
i
c
a
l
 
R
e
a
c
t
i
v
i
t
y
 
:
 
O
n
 
g
o
i
n
g
 
d
o
w
n
 
c
h
e
m
i
c
a
l
 
r
e
a
c
t
i
v
i
t
y
of metal 
increases but 
chemical 
reactivity 
of 
non
 
metal
decreases.
6.
N
a
t
u
r
e
 
o
f
 
o
x
i
d
e
s
 
:
 
A
l
l
 
e
l
e
m
e
n
t
s
 
i
n
 
g
r
o
u
p
 
h
a
s
 
s
a
m
e
n
a
t
u
r
e
 
o
f
 
o
x
i
d
e
s
.
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The Periodic Table is a systematic arrangement of elements based on atomic number and properties. Over time, chemists developed various classification methods such as Dobereiner's Triads, Newland's Law of Octaves, Mendeleev's Periodic Table, and the Modern Periodic Table to organize the increasing number of known elements. The need for classification arose as the number of elements grew, making it challenging to remember properties. Chemists like JW Dobereiner attempted to group elements into triads based on similar properties. However, limitations were encountered, leading to the development of more advanced periodic tables.


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  1. PERIODIC CLASSIFICATION OF ELEMENTS CONTENTS: What is Periodic table Need for classification of elements Dobereiner s Triads Newland s Law of Octaves Mendeleev s Periodic Table Modern Periodic Table Trends in Modern Periodic Table

  2. WHAT IS A PERIODIC TABLE It is the tabular display of elements organized on the basis of atomic number, electronic configurations and chemical properties. The table also shows four rectangular blocks: s-, p- d- and f-block. In general, within one row (period) the elements are metals on the left hand side, and non-metals on the right hand side. The rows of the table are called periods; the columns are called groups

  3. NEED FOR CLASSIFICATION OF ELEMENTS In the year 1800, only 30 elements were known. Now, at present, there are 118 elements known. So, it became difficult for the scientist to remember all the properties of elements. Therefore, there was a need to classify elements. Many scientist tried to classify elements based on different criteria. Some brought success while some did not.

  4. ORGANIZING THE ELEMENTS Chemists used the properties of elements to sort them into groups. JW. Dobreiner grouped elements into triads. A triad is a set of three elements with similar properties

  5. Dobereiners Triads Dobereiner classified elements in the increasing order of their atomic masses into groups of three elements called triads. In each triad the atomic mass of the middle element was approximately equal to the average atomic mass of the other two elements. The defect in this classification was that all the then known elements could not be correctly arranged into triads.

  6. Example of Dobereiners Triad: Element Atomic mass Lithium: Sodium: Potassium: 7amu 23amu 39amu So, this is one example of his triad. In this triad atomic mass of sodium is average of the other two atomic mass. i.e. 7+39/2 = 46/2 = 23amu. Limitation of his Triad: Dobereiner could only find three such triads from the elements known at that time. Thus this system of classification was not found to be useful.

  7. Newland arranged element according to their increasing atomic mass. He found out that every eight element had properties similar to first. This came to be known as Newlands law of octave. Example: Lithium and Sodium were found to be same.

  8. Limitations: Law of octave was only applicable up to calcium as after every eight element did not have same properties to that of first. He assumed that only 56 element existed and no more elements were to be discovered. In order to fit elements into the table, Newlands adjusted two elements like cobalt and nickel in the same slot and also put some unlike elements under the same note. So this law only worked well with lighter elements hence it was not applicable with elements have atomic mass above 40u

  9. He arranged elements on the bases of Atomic mass and chemical properties. He selected hydrogen and oxygen and reacted every elements with it and formed its compounds. Compounds of hydrides and oxides were treated as one of the basic properties of an element for its classification. He formulated Periodic Law: Properties of elements are the periodic function of their atomic mass.

  10. Achievements of Mendeleev Predicted existence of some elements that were not discovered at that time. Eg: Gallium, Scandium and Germanium. Predict properties of elements on basis of its position. Nobel gases which were discovered later could fit in his table.

  11. LIMITATIONS OF MENDELEEVS TABLE Position of the Isotopes could not be explained. Wrong order of atomic mass could not be explained in some case. Eg: Cobalt(58.9) came before Nickle(58.7). Hydrogen was not assigned correct position. It had properties of both alkali and Halogens.

  12. In 1913 Henry Moseley showed that atomic number of an element was more fundamental property than atomic mass. Modern periodiclaw: Properties of elements are function of their atomic number. the periodic

  13. Modern periodic table consists of 18 groups and 7 periods. Through the modern periodic table: 1. Position of isotope is clear. 2. Position of Nickle and cobalt is clear. 3. Position of hydrogen is well explained.

  14. Trends in Periods Period Horizontal rows of element in aperiodic table are called periods. Trends :- 1. Valence electron : From left to right number of valence electron increases. 2. Atomic Number : From left to right there is consecutive atomic number. 3. Valency : From left to right Valency increasesfrom 1 to 4 and then decrease to 0. 4. Atomic size : Decreases from left to right. 5. Metallic Character : Metallic character decreases from left to right. 6. Chemical reactivity : From left to right, decreases then increases. 7. Nature of oxides : From left to right basic character decreases and then acidic character increases.

  15. Trends in Groups Group Vertical columns in periodic table is known as groups. Trends :- 1. Valence Electron : Have same number of valence electron. 2. Valency : Elements in a group have a sameValency. 3. Atomic size : Increases from up to down. 4. Metallic Character : On going down metallic character increases. 5. Chemical Reactivity : On going down chemical reactivity of metal increases but chemical reactivity of non metal decreases. 6. Nature of oxides : All elements in group has same nature of oxides.

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