The basis of classification of elements adopted was atomic masses of the elements. The elements were arranged in order of increasing atomic masses.
Elements with similar properties were kept in a particular grwoup in order of increasing aromic masses.
Elements placed in a particular group (or sub group) were having same valency.
Note : Please note that when Mendeleev arranged the elements in the Periodic Tabel only 63 elements were known. Even noble gas or inert gas elements were adjust at a later stage. Moreover, many groups were left in the table.
