You must have visited a library. There are thousands of books in a large library. In spite of this if you ask for a particular book, the library staff can locate it easily. How is it possible? In library the books are classified into various categories and sub-categories. They are arranged on shelves accordingly. Therefore location of books becomes easy.
In the last two lessons you have studied about the structure of atoms and their electronic configurations. You have also studied that elements with similar electronic configurations show similar chemical properties. Electrons are filled in various shells and subshells in a fairly regular fashion. Therefore, properties of elements are repeated periodically. Such trends in their physical and chemical properties were noticed by chemists in the nineteenth century and attempts were made to classify elements on their basis long before structure of atom was known.
In this lesson we shall study about the earlier attempts for classification, the first successful classification which included all the known elements at that time namely Mendeleev’s periodic table, and about the long form of modern periodic table which is an improvement over Mendeleev’s work. Finally we shall learn about some properties of elements and their variations in the periodic table.