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Explain MENDELEEV'S PERIODIC TABLE ANDITS DRAWBACKS,LIMITATIONS AND ACHEIVEMENTS .

 Mendeleev’s Periodic Law    

  According to the law:
"The properties of element are the periodic function of their atomic masses".
 
  Mendeleev arranged the known elements according to increasing order of their atomic masses because, according to him fundamental property of an element was atomic mass. He arranged these eight  groups were further divided into subgroups.
  Mendeleev’s periodic table was very precise and provided the basis of modern periodic classification.

MERITS OF MENDELEEV’S PERIODIC TABLE
    
  Mendeleev’s periodic table offered the following advantages in understanding the properties of   elements.
  1. There was a regular gradation in physical and chemical properties of elements.
  2. The group number of an element indicates highest oxidation state that it can attain.
  3. There were many vacant spaces in Mendeleev's periodic table for the elements to be discovered. He named them Eka-Boron, Eka-Aluminium and Eka-silicone He also predicted the properties of these  undiscovered elements including atomic masses. These elements were discovered as Sc ,Ga and Ge  with same features as he predicted.
  4. Mendeleev’s arrangement helped to correct atomic masses of a number of elements.

Original statement of Mendeleev's periodic law
        Published: March 1869
  1. The elements, if arranged according to their atomic weights show an evident periodicity of properties.
  2. Elements which are similar as regards their chemical properties have atomic weights which are either nearly the same value or which increase regularly.
  3. The arrangement of the elements or of groups of elements in the order of their atomic weights, corresponds with their so called valencies.
  4. The elements which are most widely distributed in nature have small atomic weights, and sharply defined properties. They are therefore typical elements.
  5. The magnitude of the atomic weight determines the character of an element.
  6. The discovery of many as yet unknown elements may be expected.
  7. The atomic weight of an element may sometimes be corrected by the aid of a knowledge of those of adjacent elements.
  8. Certain characteristic properties of the elements can be foretold from their atomic weights.

DEMERITS IN MENDELEEV’S PERIODIC TABLE
    
  1. For placing the elements in proper groups, the order of the elements according to atomic mass was reversed in certain cases. He placed Iodine (127) after Tellurium (128) Potassium (39) and Ni (58) after Co (59). Which is against his periodic law but correct according to properties.
  2. Mendeleev’s periodic table does not provide a clear idea about the structure of atom.
  3. Lanthanide and Actinide have been assigned places in the periodic table which is against the periodic law.
  4. Alkali metal and coinage metals (Cu, Ag and Au) which differ widely in properties are placed into the same group.
  5. There is no separate position for isotopes in his periodic table.
  6. The change in atomic mass of two successive elements is not constant. Hence it is not possible to predict the number of missing elements by knowing the atomic masses of two known elements.


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