In 1869, the celebrated Russian scientist, Mendeleev put forward his famous periodic law-a memorable event in the history of the systematic study of chemistry. The law states that the physical and chemical properties of the elements are periodic functions of their atomic weights. That is, if the elements are arranged in the order of increasing atomic weights, the properties of the elements change in a regular manner from one member to another but their properties are repeated after a definite interval. This phenomenon of periodic change in properties of elements is called periodicity of elements. The classification of elements based on Mendeleev’s periodic law is called periodic classification and the arrangement of elements, he made, in the form of a table showing periodic variation of their properties with atomic weight is known as Mendeleev’s periodic table.
Each of the 4th, 5thand 6th periods is further divided into even and odd series. Taking the hydrogen-series as the 1st series, the elements occupying places in the series of even serial numbers are said to be the elements of even series and the elements placed in the series of odd serial numbers are called elements of the odd series.