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Mendeleev’s Periodic Law

The first really successful arrangement of elements was that of Dmitri Ivanocich Mendeleev a Russian chemist, gave a scheme for the classification of elements in 1869.
 

Dmitri Ivanocich Mendeleev


Mendeleev stated the famous periodic law according to which "Properties of elements are a periodic function of their atomic weight’’.
 

Mendeleev arranged elements in the order of their increasing atomic weights in the form of a table, which is known as Mendeleev's Periodic Table. Some gaps were left as the elements to fill up these gaps had not been discovered at that time.
 

Example : he left a gap below silicon and called the yet undiscovered element as eka silicon. Similarly he named scandium as eka-boron and gallium as eka-aluminium. Later, when these elements were discovered, it was amazing to know that they fitted so well in the periodic table.

 

 

Period

1

Group

I

II

III

IV

V

VI

VII

VIII

A

B

A

B

A

B

A

B

A

B

A

B

A

B

H
1.0

 

 

 

 

 

 

 

2

Li
6.9

Be
9.0

B
10.8

C
12.0

N
14.0

O
16.0

F
19.0

 

3

Na
23.0

Mg
24.3

Al
27.0

Si
28.1

P
31.0

S
32.1

Cl
35.5

 

4

K
39.1

Ca
40.1

 

Ti
47.9

V
50.9

Cr
52.0

Mn
54.9

  Fe           Co             Ni
55.8         58.9          58.7

Cu
63.5

Zn
65.4

 

 

As
74.9

Se
79.0

Br
79.9

 

5

Rb
85.5

Sr
87.6

Y
88.9

Zr
91.2

Nb
92.9

Mo
95.9

 

  Ru          Rh             Pd
101.1      102.9        106.4

Ag
107.9

Cd
112.4

In
114.8

Sn
118.7

Sb
121.8

Te
127.6

I
126.9

 

6

Cs
132.9

Ba
137.3

La
138.9

 

Ta
180.9

W
183.9

 

  Os          Ir             Pt
190.2      192.2        195.1

Au
197.0

Hg
200.6

Ti
204.4

Pb
202.2

Bi
207.2

 

 

 

 Mendeleev's periodic table (based on the original table of 1871)


In Mendeleev 's periodic table there were six horizontal rows of elements which are called periods of elements. The eight vertical rows of elements are called groups. Elements with similar chemical properties were placed in the same group and within a group various elements showed a regular gradation in their properties.

 

Defects of Mendeleev’s Periodic Table

  • Hydrogen resembles elements of Group IA as well as element of Group VIII A. Like alkali metals it reacts with halogens and forms HCl and HI. It reacts with alkali metals also. So he could not assign a correct position to it in his periodic table.
  • There was no provision for placing isotopes in his periodic table.
     
  • All the 14 rare earth metals have been placed together at one place inspite of their different atomic weights.
     
  • All these defects led to the modification in Mendeleev’s periodic table. In 1913, Henry Moseley showed that the atomic number of an element is a more fundamental property than its atomic mass. So Mendeleev’s periodic table was modified and atomic number was adopted as the basis of Modern periodic table.




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