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3. CLASSIFICATION OF ELEMENTS AND PERIODICITY IN PROPERTIE:, , Earlier classifications, , 1) Dobereiner’s classification:, , Johann Dobereiner classified elements into small groups each containing three elements. These small, groups were called triads., , E.g. for triads are: i) 7Li na K, ii) ca sr Ba, In triads, the atomic mass of the middle element is approximately the average of the other two, elements. This is known as Law of Triads. This classification was applicable to very few elements and so it wa, rejected., , 2) Newlands classification:, , Newland arranged elements in the increasing order of their atomic masses. He noted that the properties, every eighth element, starting from a given element, are similar to that of the first element. The relationship, just like the resemblance of first and eighth musical notes. He named this as law of octaves., , But his classification was rejected since the law of octaves was applicable to elements upto calcium. Also, the discovery of noble gases, the properties of eighth element become not similar to that of the first elemen, , 3) Mendeleev’s classification:, , Dimitri Mendeleev classified the elements in the increasing order of their atomic weights. He founded thi, the properties of elements repeat after a regular interval. Based on this observation, he proposed a periodic, law which states that “The properties of elements are the periodic functions of their atomic weights.” That, when elements are arranged in the increasing order of their atomic weights, their properties repeat after a, regular interval., , Mendeleev arranged elements in horizontal rows (periods) and vertical columns (groups) in such a way tl, the elements with similar properties occupied in the same group. He mainly depends on the similarities in th, empirical formulae and the properties of the compounds formed by the elements. He realized that some of t, elements did not fit in with his scheme of classification if the order of increasing atomic weight was strictly, followed. So he ignored the order of atomic weights and placed the elements with similar properties togethe, , When Mendeleev proposed his periodic table, some of the elements were not discovered. He left some, vacant places (gaps) for them in the periodic table and predicted some of their properties. For e.g. both Galli, and Germanium were not discovered at that time. He named these elements as Eka-Aluminium and Eka-Silic, respectively and predicted their properties. These elements were discovered later and found that Mendelee\, predictions were correct., , Merits of Mendeleev’s periodic table, , 1) It was the first comprehensive classification of elements., , 2) He corrected the wrong atomic weights of some elements and placed them in correct position in the, , periodic table., , 3) He left vacant places for undiscovered elements and predicted some of their properties., , 4) Elements with similar properties are placed in the same group., , Demerits of Mendeleev’s periodic table, 1) Elements with dissimilar properties are found in same group., 2) He could not give an exact position for hydrogen., 3) He could not give exact position for Lanthanoids and Actinoids and also for isotopes., 4) Mendeleev’s periodic table did not strictly obey the increasing order of atomic weights., , 39