The main points of distinction between Mendeleev’s Periodic Table and Modern Periodic Table are:

Mendeleev’s Periodic Table:

1. It regards the atomic masses of the elements as the basis of classification.

2. No separate positions or slots have been allotted to the isotopes of an element since they have different atomic masses.

3. No justification is made for placing hydrogen in group IA along with alkali metals.

4. Except for the elements, in group VIII, the remaining groups have been divided with sub-groups A and B.

5. Position of certain elements based on their atomic masses are misfits. For example, the element cobalt (atomic mass = 58.9) has been placed ahead of nickel (atomic mass = 58.7)

6. Electronic configurations and properties of the elements cannot be predicted from their positions in the table.

7. It is not very systematic and is difficult to remember.

Modern Periodic Table:

1. It regards the atomic number of the elements as the basis of classification.

2. There is no need for separate slots for the isotopes since they have the same atomic numbers.

3. Justification has been made for placing hydrogen along with alkali metals in group 1 since both hydrogen and alkali metals have one valence electron.

4. There are no subgroups and all groups are independent in nature.

5. Modern Periodic Table is free from such anomalies. The element cobalt is placed before nickel since its atomic number (27) is more than that of nickel (28).

6. Both electronic configurations and certain properties of the elements can be predicted from their positions in the periodic table.

7. It is quite systematic and is easy to remember.