Genesis of periodic classification-
Classification of elements into groups and development of periodic law and table are the results of systematising the knowledge gained by sort of scientists through their observations and experiments or Genesis of periodic classification. The German chemist, Johann Dobereiner in early 1800’s was the first to believe the thought of trends among properties of elements. By 1829 he noted a similarity among the physical and chemical properties of several groups of three elements (Triads). In each case, he noticed that the middle element of each of the Triads had an relative mass about half way between the atomic weights of the other two (Table 3.1). Also the properties of the middle element were in between those of the other two members. Since Dobereiner’s relationship, mentioned because the Law of Triads, seemed to work only for a few of elements, it had been dismissed as coincidence. subsequent reported decide to classify elements was made by a French geologist, A.E.B. de Chancourtois in 1862. He arranged the then known elements so as of accelerating atomic weights and made a cylindrical table of elements to display the periodic recurrence of properties. This also didn’t attract much attention. English chemist, John Alexander Newlands in 1865 profounded the Law of Octaves. He arranged the weather in increasing order of their atomic weights and noted that every eighth element had properties almost just like the primary element (Table 3.2). the connection was a bit like every quaver that resembles the primary in octaves of music. Newlands’s Law of Octaves seemed to be true only for elements up to calcium. Although his idea wasn’t widely accepted at that time , he, for his work, was later awarded Davy Medal in 1887 by the Royal Society , London.
The periodic law , as we all realize it today owes its development to the Russian chemist, Mendeleyev (1834-1907) and thus the German chemist, Lothar Meyer (1830-1895). Working independently, both the chemists in 1869 proposed that on arranging elements within the increasing order of their atomic weights, similarities appear in physical and chemical properties at regular intervals. Lothar Meyer plotted the physical properties like atomic volume, melting point and boiling point against relative mass and obtained a periodically repeated pattern. Unlike Newlands, Lothar Meyer observed a change long of that repeating pattern. By 1868, Lothar Meyer had developed a table of the weather that closely resembles the fashionable table . However, his work wasn’t published until after the work of Mendeleyev , the scientist who is typically credited with the event of the fashionable table .
While Dobereiner initiated the study of periodic relationship, it had been Mendeleev who was responsible for publishing the periodic law for the first time. It states as follows :
The properties of the weather are a periodic function of their atomic weights.
Mendeleev arranged elements in horizontal rows and vertical columns of a table so as of their increasing atomic weights in such how that the weather with similar properties occupied an equivalent vertical column or group. Mendeleev’s system of classifying elements was more elaborate than that of Lothar Meyer’s. He fully recognized the importance of periodicity and used broader range of physical and chemical properties to classify the weather . especially , Mendeleev relied on the similarities within the empirical formulas and properties of the compounds formed by the weather . He realized that variety of the weather didn’t squeeze along side his scheme of classification if the order of relative mass was strictly followed. He ignored the order of atomic weights, thinking that the atomic measurements might be incorrect, and placed the weather with similar properties together. for instance , iodine with lower relative mass than that of tellurium (Group VI) was placed in Group VII in conjunction with fluorine, chlorine, bromine thanks to similarities in properties (Fig. 3.1). At the same time, keeping his primary aim of arranging the weather of comparable properties within an equivalent group, he proposed that variety of the weather were still undiscovered and, therefore, left several gaps within the table. for instance , both gallium and germanium were unknown at the time Mendeleev published his table . He left the gap under aluminium and a distinct segment under silicon, and called these elements EkaAluminium and Eka-Silicon. Mendeleev predicted not only the existence of gallium and germanium, but also described variety of their general physical properties. These elements were discovered later. a number of the properties predicted by Mendeleev for these elements and other people found experimentally are listed in Table 3.3.
The boldness of Mendeleev’s quantitative predictions and their eventual success made him and his table famous. Mendeleev’s table published in 1905 is shown in Fig. 3.1. Genesis of periodic classification.