11th Class Chemistry Classification of Elements and Periodicity in Properties

  • question_answer 103)   Write the drawbacks in Mendeleev's periodic table that led to its modifications.


      Mendeleev is presented the periodic table containing 63 known elements. This table was arranged on the basis of atomic mass of the elements. The original table consisted eight vertical columns called groups. This table consisted a number of discrepancies such as anomalous pairs; position of isotopes and position of lanthanides and actinides etc. Some of these discrepancies are described below: (i) Anomalous pairs: The original periodic law based on atomic masses is violated in case of four pairs of elements in order to give them positions on the basis of properties. The elements having higher atomic masses have been assigned position before the elements having lower atomic masses at four places as shown below:

      (a) Ar k (b) Co Ni (c) Te I (d) Th Pa
    At. mass 40   39 59. 58.6 127.5 127 232 2 31
    At. no. 18 19 27  28 52     53 90 91
      The discrepancy disappears, if the elements are arranged in order of increasing atomic numbers. (ii) Position of isotopes: Isotopes are atoms of the same element having different atomic masses but same atomic number. The isotopes of the same element will be given different positions, if atomic mass is taken as a basis. This will disturb the symmetry of the table. In modem table, one position is<fixed for one atomic number and since all the isotopes of an element have the same atomic number, these are assigned only one position.

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