MENDELEEV’S PERIODIC TABLE
Attempts to find regularities among the elements led the Russian Scientist, Dmitri I. Mendeleev to put forward a scheme of classification of elements in 1869. He gave a periodic law known after his name as Mendeleev’s periodic law. This law states that: The properties of elements are a periodic function of their atomic weights.
It means that when the elements are arranged in order of increasing atomic weights, the elements with similar properties recur after regular intervals. On the basis of this periodic law, Mendeleev constructed a periodic table in such a way that the elements were arranged horizontally in the order of their increasing atomic weights. However, he also kept in mind the similarities in the chemical properties of the elements. The main criterion of the judgment of similarities in the properties· was valency of the elements. Mendeleev observed that some of the elements did not fit in with his scheme of classification if the order of atomic weights was strictly followed. He ignored the order of atomic weights and placed the elements with similar chemical properties together. For example, iodine having atomic weight 127 was placed after tellurium (atomic weight 128), together with fluorine, chlorine and bromine due to similarities in properties.
Mendeleev left certain vacant places in his table which provided a clue for the discovery of new elements. Some of the properties could be predicted with a fair accuracy. For example, both gallium and germanium were not discovered at the time when Mendeleev proposed his periodic table. Mendeleev named these elements as Eka-Aluminium and Eka-Silicon respectively. Later on, when these elements were discovered, Mendeleev’s prediction proved remarkably correct. Some of the properties predicted by Mendeleev for these elements and those found experimentally.