Modern Periodic Law

Moseley in year 1912 studied the frequencies of X-rays, which were emitted by certain metals when bombarded with speed electrons. He found that, in all the cases, the square root of the frequency was directly proportional to the atomic number of the metal and not to its atomic mass. This observation lead to the development of Modern Periodic Law. The Modern Periodic Law states that, “The physical and chemical properties of the elements are the periodic function of their atomic numbers.”

It means that if the elements are arranged in order of increasing atomic numbers, the elements with similar properties appear after regular intervals.

Moseley’s view appears to be correct, because atomic mass of an element is due to the mass of protons and neutrons present in nucleus. Since nucleus is located well within the atom and is shielded by electrons, it cannot interact with the reagent and hence cannot effect the chemical properties. Actually it must be the electrons which interact with the reagent because electrons are exposed to environment. Thus the properties of an atom depends the atomic number not on atomic mass.