question_answer 127)

    Give an account of the process adopted by Mendeleev for the classification of elements. How did he arrive at "periodic law"?

Answer:

                When Mendeleev selected his work, 63elements were known. He examined the relationship between the atomic masses of the elements and their physical and chemical properties. Among the chemical properties, he concentrated on the compounds formed by elements with hydrogen and oxygen, He selected hydrogen and oxygen as they are very reactive and formed compounds with most elements. The formulae of the hydrides and oxides formed by elements were treated as one of the basic properties of the elements for their classification. He then took 63 cards and on each card he wrote down the properties of one element. He sorted out the elements with similar properties and paired the cards together on a wall. He observed that most of the elements get a place in a periodic table and were arranged in order of their increasing atomic masses. It was also observed that there occurs aperiodic recurrence of elements with similar physical and chemical properties. On this basis, Mendeleev formulated a periodic law, which states that "the properties of elements are the periodic function of their atomic masses".