1. Isotopes were discovered long time after Mendeleev developed the periodic table. All isotopes of the same element have different atomic masses but same atomic number. They have similar chemical properties.
2. As isotopes have the same chemical properties but different atomic masses, a challenge was posed in placing them in Mendeleev’s periodic table.
3. In Mendeleev’s periodic table, elements are arranged in increasing order of their atomic masses such that chemically similar elements are placed together in a group.
4. So, it became difficult to place isotopes in Mendeleev’s periodic table. Therefore, isotopes were not given separate place in Mendeleev’s periodic table.
5. In Modern periodic table, elements are arranged in increasing order of their atomic number. Hence, all the isotopes of an elements occupy the same position in the modern periodic table.