 Lothar Meyer presented his table several months after Mendeleev, but opposed his Periodic law.Initially, no theoretical explanation for the Periodic Law was available and it was used only as an empirical principle, but, with the development of the quantum mechanics, it became possible to understand the theoretical basis for the Periodic Law.The periodic recurrence of elements with similar physical and chemical properties, when the elements are listed in order of increasing atomic number, results directly from the periodic recurrence of similar electronic configurations in the outer shells of respective atoms.