Mendeleev's Periodic table and it's defects

November 03, 2018 0 Comments

A) Mendeleev's Periodic Law and Significance

The first meaningful and remarkable contribution in the field of classification of elements was made by a Russian chemist, Dmitri I. Mendeleev in 1869. Mendeleev used a broader range of physical and chemical properties of elements as compared to those used by Lother Meyer and studied the formulae and properties of several compounds of an element. On the basis of these studies, he developed a law known as Mendeleev's Periodic Law. This law can be stated as follows :-

The physical and chemical properties of the elements (as well as the formulae and properties of their compounds) are the periodic function of their atomic weight (atomic masses).

Following two inferences can be drawn from this periodic law.

  • The physical and chemical properties of the elements depend upon their atomic masses.

  • The dependence of physical and chemical properties of elements upon their atomic masses is periodic.

B) Mendeleev's Periodic Table

On the basis of this periodic law, Mendeleev arranged all the elements known at that time in the form of a table which is now known as Mendeleev's Periodic Table. In this table, the elements were arranged horizontally in ten series in the order of their increasing atomic weights. These series were divided into seven horizontal and eight vertical columns. The horizontal columns were called periods while the vertical columns were named as groups. Mendeleev tried to place the elements having similar properties in the same group. At several places he ignored the order of atomic masses in order to group together the elements having similar properties.

The important features of the modified form of periodic table are as follows :- 

i) Series : All the known elements have been arranged in ten horizontal columns called series.

ii) Groups : There are nine vertical columns in the periodic table, these are called groups.

iii) Periods : The ten horizontal series have been arranged in seven horizontal columns called periods.

Original view of Mendeleev's Periodic Table

C) Defects Of Mendeleev's Periodic Table

In spite of the remarkable contributions made to chemistry, Mendeleev's periodic table had certain defects. These defects are as follows :-

  • Position of Hydrogen
  • Position of Isotopes
  • Position of lanthanoids and actinoids
  • Variable valency of elements
  • Anomalous pair of elements
  • Similar elements placed in different groups
  • Dissimilar elements placed in same group
  • Eight group

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