Classification of Elements and Periodicity in properties – Notes
Need and Genesis of Classification
A periodic table helps classify all the known elements in rows and columns such that the periodicity in their properties becomes apparent. The periodic law defines this periodicity.
The classification of elements in a periodic table helps predict the properties of unknown elements.
Johann Dobereiner was the first scientist to observe trends in the properties of elements in 1817; he stated the law of triads based on his observations.
The first periodic table that applied to all the known elements was formulated by A.E.B de Chancourtois in 1862.
Chancourtois’s cylindrical periodic table was based on atomic weights of elements and displayed the periodic recurrence in their properties.
John Alexander Newlands gave the law of Octaves which states that when the elements are arranged in the increasing order of their atomic weights, the properties of every eighth element are similar to that of the first element.
Lothar Meyer and Dmitri Mendeleev independently proposed the Periodic Law, as we know it today, in 1869.
Modern Periodic Table
The English physicist, Henry Moseley observed regularities in the characteristic X-ray spectra of the elements. He proved that the atomic number is a more fundamental property of an element than its atomic mass.