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Describe Mendeleev's Merits and Demerits in the periodic table?
Merits of  Mendeleev's periodic table

(i)  Systematic study of the elements. Mendeleev's classification condensed the study of  about 90 elements (only 65 were known at that time ,but he left a provision for many
more ) to the study of only 8 groups of elements .

(ii) Prediction of new elements and their properties . While arranging the elements of increasing atomic masses, Mendeleev laid more stress on the similarities  in the properties of the elements .He did not follow the order of increasing atomic mass rigidly .So whenever certain elements did not seem fit into the expected positions ,he left gaps in the periodic table for the undiscovered elements .From his classification ,Mendeleev could even predict the properties of those elements which were yet to be discovered .their properties could be predicted from the properties of their neighbours .

(iii) Correction of certain atomic masses .By placing elements according to the similarity in their properties ,he was also able to correct certain atomic masses .For example he corrected the atomic masses of beryllium (Be),gold (Au) and platinum (Pt)


Defects in the Mendeleev's periodic table

(i) Position of hydrogen.Since hydrogen forms both the positive ion like alkali metals and negative ions like halogens ,hence it resembles both therefore it can be placed in either of the groups .

(ii) Separation of chemically similar elements .Certain elements which appear to be chemically similar ,like Cu ,Ag and Au and Pt etc have been placed in separate groups .

(iii) Grouping chemically disimilar elements .Certain chemically dissimilar elements have been grouped together in the Mendeleev's table .

(iv) Inversion in the periodic table .Certain pairs of elements had to be placed in the reverse order of atomic masses in order to to conform to the periodic law .

(v) Position of isotopes .Isotopes have similar atomic properties but different atomic masses .According to the periodic law ,therefore isotopes of an element must have been placed at separate places in the periodic table .But isotopes were not given separate place in the periodic table .

(vi) Position of lanthanides and actinides . Lanthanide and actinide group consist of 14 elements each .All these 14 elements have been assigned a single place in the periodic table despite having different atomic masses .


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