What is the basic difference in approach between Mendeleev Periodic Law and the Modem Periodic Law

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.”