Chlorine has two isotopes, viz, C1-35 and C1-37. Their atomic masses are 35 and 37 respectively. Their chemical properties are same. Where should these be placed in Mendeleev’s periodic table? In different places or in the same place?
  Mendeleev’s Periodic Table is based on atomic mass numbers. As the atomic mass numbers of these isotopes are different, they will have to be given different places in the Mendeleev’s Periodic Table.
  However, the properties of these isotopes are same. Thus, the elements with same properties will land in different groups.
  This is a demerit of Mendeleev’s Periodic Table.
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