Five major differences between Mendeleev's periodic table and Modern periodic table are:-  S.No. Mendeleev's periodic table Modern Periodic table  (i) Elements have been arranged in increasing order of atomic masses. Elements have been arranged in increasing order of atomic number.  (ii) It consists of 8 groups  It consists of 18 groups  (iii) All the groups from I to VIII are divided into two sub groups No sub-groups. (iv) He left some gaps in the table for undiscovered elements. All elements are filled in increasing order of atomic number. No gaps left in the periodic table.  (v) Isotopes were placed in different groups.  Isotopes were placed in same groups.