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917 @Be 6 (O) 0 ee ane eT, , € Periodic Classification of, Elements, , , , Modern Periodic Law, , In the year 1913, Henry Moseley, a young physicist from England studied the, frequencies of the X-rays which were emitted when certain metals were, bombarded with high speed electrons. He found that in all the cases, the square, root of the frequency was directly proportional to the atomic number of the, atom of the metal. He further stated that there was no co-relation between the, frequency and the atomic mass., , When Moseley plotted a graph between square root of frequency and atomic, numbers of different metals, a straight line was obtained. But it was not the, case when a graph was plotted between square root of frequency and atomic, masses of the metals. These studies led Moseley to believe that atomic number, and not the atomic mass is, the fundamental property of an element., , According to him, atomic numbers must form the basis of the classification of, the elements in the periodic table., , Moseley gave the Modern Periodic Law which states that: Physical and chemical, properties of the elements are the periodic function of their atomic, numbers., , Atomic mass of an element is due to the mass of protons and neutrons present, in the nucleus of its atom., , As the number of electrons in an atom are given by the atomic number and not, by the mass number therefore, atomic number should form the basis of the, classification of the elements in the periodic table and not atomic mass as, predicted by Mendeleev., , Cause of Periodicity: Periodicity may be defined as the repetition of the, similar properties of the elements placed ina group and separated by certain, definite gaps of atomic numbers., , Electronic configuration of Alkali metals, , , , , , , , , , Element Atomic Number Electronic Configuration, Lithium 3 21, Sodium 1 28,1, Potassium 19 28,81, Rubidium 37 2,8,18,8,1, Cesium 55 2,8,18,18,8,1, Francium 87 2,8,18,18,32,8,1, It is clear that all of them have one electron each in the valence shell of their, atoms., Electronic configuration of Halogens, |, Element Atomic Number Electronic Configuration, , = O gq