The difference between Mendeleev’s and Long-form of periodic table are:

Mendeleev’s periodic table

  • Elements have been arranged in order of increasing atomic mass.
  • There are only nine vertical columns called groups.
  • All the transitional elements are arbitrarily placed together in single group VIII.
  • The inert gases were not known at the time of Mendeleev.
  • No proper places are assigned to isotopes of elements.
  • Dissimilar metals are placed together.
  • There are misfit anomalous pairs on the basis of atomic mass.
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Long-form of periodic table

  • Elements have been arranged in the order of increasing atomic number.
  • There are eighteen vertical columns, called groups.
  • The inert gases have been placed at the periods 1 to 6 i.e. O (group)
  • Isotopes of elements are assigned the same place as their respective elements as they have same atomic number.
  • Dissimilar metals are placed in separated sub groups.
  • The misfit anomalous pairs have been removed, when atomic number is taken as the basis of classification.