Question

what is the basic difference in the approach between Mendeleev periodic table and Modern Periodic Table

Answer 1

Mendeleev’s Periodic law is based upon atomic weights of elements, whereas Modem Periodic Law is based upon atomic number of elements. Mendeleev stated : "The properties of the elements are a periodic function of their atomic weights."
Modern Periodic Law states : “The physical and chemical properties of the elements are periodic functions of their atomic numbers.”

Answer 2

mendleev arranged the elements in increasing order of atomic mass.
modern periodic table is arranged in increasing order of atomic number.
this is the basic difference.