Dmitri Mendeleev - Periodic Table

Periodic Table

In 1863 there were 56 known elements with a new element being discovered at a rate of approximately one per year.

Other scientists had previously identified periodicity of elements. John Newlands described a Law of Octaves, noting their periodicity according to relative atomic weight in 1864, publishing it in 1865. His proposal identified the potential for new elements such as germanium. The concept was criticized and his innovation was not recognised by the Society of Chemists until 1887. Another person to prose a periodic table was Lothar Meyer, who published a paper in 1864 describing 28 elements classified by their valence, but with no prediction of new elements.

After becoming a teacher, Mendeleev wrote the definitive textbook of his time: Principles of Chemistry (two volumes, 1868–1870). As he attempted to classify the elements according to their chemical properties, he too noticed patterns that led him to postulate his periodic table. Mendeleev was unaware of the earlier work on periodic tables going on in the 1860s. He made the following table, and by adding additional elements following this pattern, developed his extended version of the periodic table.

On 6 March 1869, Mendeleev made a formal presentation to the Russian Chemical Society, entitled The Dependence between the Properties of the Atomic Weights of the Elements, which described elements according to both atomic weight and valence. This presentation stated that

1. The elements, if arranged according to their atomic weight, exhibit an apparent periodicity of properties.
2. Elements which are similar regarding their chemical properties have atomic weights which are either of nearly the same value (e.g., Pt, Ir, Os) or which increase regularly (e.g., K, Rb, Cs).
3. The arrangement of the elements in groups of elements in the order of their atomic weights corresponds to their so-called valencies, as well as, to some extent, to their distinctive chemical properties; as is apparent among other series in that of Li, Be, B, C, N, O, and F.
4. The elements which are the most widely diffused have small atomic weights.
5. The magnitude of the atomic weight determines the character of the element, just as the magnitude of the molecule determines the character of a compound body.
6. We must expect the discovery of many yet unknown elements–for example, two elements, analogous to aluminium and silicon, whose atomic weights would be between 65 and 75.
7. The atomic weight of an element may sometimes be amended by a knowledge of those of its contiguous elements. Thus the atomic weight of tellurium must lie between 123 and 126, and cannot be 128. Here Mendeleev seems to be wrong as the "atomic mass" of tellurium (127.6) remains higher than that of iodine (126.9) as displayed on modern periodic tables, but this is due to the way atomic masses are calculated, based on a weighted average of all of an element's common isotopes, not just the one-to-one proton/neutron-ratio version of the element to which Mendeleev was referring.
8. Certain characteristic properties of elements can be foretold from their atomic weights.

Mendeleev published his periodic table of all known elements and predicted several new elements to complete the table. Only a few months after, Meyer published a virtually identical table. Some consider Meyer and Mendeleev the co-creators of the periodic table, but virtually everybody agrees that Mendeleev's accurate prediction of the qualities of what he called ekasilicon, ekaaluminium and ekaboron (germanium, gallium and scandium, respectively) qualifies him for the majority of the credit for the table.

For his predicted eight elements, he used the prefixes of eka, dvi, and tri (Sanskrit one, two, three) in their naming. Mendeleev questioned some of the currently accepted atomic weights (they could be measured only with a relatively low accuracy at that time), pointing out that they did not correspond to those suggested by his Periodic Law. He noted that tellurium has a higher atomic weight than iodine, but he placed them in the right order, incorrectly predicting that the accepted atomic weights at the time were at fault. He was puzzled about where to put the known lanthanides, and predicted the existence of another row to the table which were the actinides which were some of the heaviest in atomic mass. Some people dismissed Mendeleev for predicting that there would be more elements, but he was proven to be correct when Ga (gallium) and Ge (germanium) were found in 1875 and 1886 respectively, fitting perfectly into the two missing spaces.

By giving Sanskrit names to his "missing" elements, Mendeleev showed his appreciation and debt to the Sanskrit grammarians of ancient India, who had created sophisticated theories of language based on their discovery of the two-dimensional patterns in basic sounds. According to Professor Paul Kiparsky of Stanford University, Mendeleev was a friend and colleague of the Sanskritist Böhtlingk, who was preparing the second edition of his book on Pāṇini at about this time, and Mendeleev wished to honor Pāṇini with his nomenclature. Noting that there are striking similarities between the periodic table and the introductory Śiva Sūtras in Pāṇini's grammar, Prof. Kiparsky says:

he analogies between the two systems are striking. Just as Panini found that the phonological patterning of sounds in the language is a function of their articulatory properties, so Mendeleev found that the chemical properties of elements are a function of their atomic weights. Like Panini, Mendeleev arrived at his discovery through a search for the "grammar" of the elements...