Mendeleev’s classification of elements in the form of periodic table was based on the fact that the physical and chemical properties of the element depend upon their atomic masses. In other words, Mcndeleev regarded the atomic mass as the fundamental property of elements. In the light of modem researches, this was no longer true. The beginning of the 20th century witnessed profound developments in theories about sub atomic particles. In 1913, the English physicist, Henry Mosley observed regularities in the characteristic x-ray spectra of the elements. A plot of √ v (where v is the frequency of x-rays emitted) against atomic number gave straight line and not the plot of √ v against atomic mass. He showed that the atomic number is more fundamental property of an element than its atomic mass. Mendeleev’s Periodic law was, therefore, accordingly modified. This is known as the Modern Periodic Law and can be stated as,

"The physical and chemical properties of the elements are periodic functions of their atomic numbers.”

This law implies that the physical and chemical properties of the elements depend on atomic numbers and this dependence shows periodicity. Therefore, if the elements are arranged in the order of increasing atomic numbers, the elements with similar properties should recur after regular intervals. The Periodic Law revealed important analogies among the 94 naturally occurring elements (neptunium and plutonium like actinium and protoactinium are also found in pitch blende an ore of uranium). It stimulated renewed interest in inorganic Chemistry and has carried into the present with the creation of artificially produced short-lived elements.

The modern periodic law added new dimensions to the periodic classification of elements. Based on the modern periodic law, several new periodic tables have been proposed in recent times. Among these new periodic tables the long form is extended form of the periodic table and is found to be the most useful.