Mendeleev's Periodic Table
Elements are arranged in the increasing order of their atomic masses.
> There are 8 Groups.
> Each groups are subdivided into sub group 'a' and 'b'
> Groups for Noble gas was not present as noble gases were not discovered by that time.
> There was no place for isotopes.
Modern periodic table
> Elements are arranged in the increasing order of their atomic numbers.
> There are 18 Groups.
> Groups are not subdivided into sub-groups.
> A separate group is meant for noble gases.
> This problem has been rectified as slots are determined according to atomic number.