Periodic table is an arrangement of elements according to the ascending order of atomic number of elements. In periodic table elements are classified into vertical and horizontal rows. Horizontal rows of the periodic table are known as periods. The periods of periodic table indicates the number of energy levels or shells, in which electrons are present, in an atom. The elements which are present in first period have only one shell which is filled with electrons. In second periodic table, electrons are present in two energy levels and so on. Moreover, the number of shells or orbits increases from up to down in a periodic table.