revision preference src: 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 electronic theory of atomic structure, it became possible to understand the theoretical basis for the Periodic Law. <SEP> tgt: Initially, no theoretical explanation for the Periodic Law was available and it was used only as an empirical principle. With the development of the electronic theory of atomic structure, it became possible to understand the theoretical basis for the Periodic Law.