Compare and contrast the arrangement of elements in Mendeleev’s periodic table and the Modern periodic table.

Mendeleev’s periodic table
Elements are arranged in the increasing order of their atomic masses.
There are 8 groups.
Each groups are subdivided into subgroup ‘a’ and ‘b’
There were no place of isotopes

Modern periodic table
Elements are arranged in the increasing order of their atomic numbers.
There are 18 groups
Groups are not subdivided into subgroups.
This problem has been rectified as slots are determined according to atomic number.