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 recognized by the Society of Chemists until 1887. Another person to propose a periodic table was
Lothar Meyer, who published a paper in 1864 describing 28 elements classified by their valence, but with no predictions of new elements. After becoming a teacher in 1867, Mendeleev wrote
Principles of Chemistry (), which became the definitive textbook of its time. It was published in two volumes between 1868 and 1870, and Mendeleev wrote it as he was preparing a textbook for his course. This is when he made his most important discovery. As he attempted to classify the elements according to their
chemical properties, he noticed patterns that led him to postulate his periodic table; he claimed to have envisioned the complete arrangement of the elements in a
dream: Unaware of the earlier work on periodic tables going on in the 1860s, he made the following table: By adding additional elements following this pattern, Mendeleev developed his extended version of the periodic table. On 6 March 1869, he made a formal presentation to the Russian Chemical Society, titled
The Dependence between the Properties of the Atomic Weights of the Elements, which described elements according to both
atomic weight (now called
relative atomic mass) and
valence. This presentation stated that • The elements, if arranged according to their atomic weight, exhibit an apparent periodicity of properties. • Elements which are similar regarding their chemical properties either have similar atomic weights (e.g., Pt, Ir, Os) or have their atomic weights increasing regularly (e.g., K, Rb, Cs). • 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. • The elements which are the most widely diffused have small atomic weights. • 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. • 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. • 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. (Tellurium's atomic weight is 127.6, and Mendeleev was incorrect in his assumption that atomic weight must increase with position within a period.) • 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 in a Russian-language journal. Only a few months after, Meyer published a virtually identical table in a German-language journal. Mendeleev has the distinction of accurately predicting the properties of what he called
ekasilicon, ekaaluminium and ekaboron (
germanium,
gallium and
scandium, respectively). Mendeleev also proposed changes in the properties of some known elements. Prior to his work,
uranium was supposed to have valence 3 and atomic weight about 120. Mendeleev realized that these values did not fit in his periodic table, and doubled both to valence 6 and atomic weight 240 (close to the modern value of 238). For his predicted three 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 weight. 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 using Sanskrit prefixes to name "missing" elements, Mendeleev may have recorded his debt to the Sanskrit grammarians of ancient India, who had created theories of language based on their discovery of the two-dimensional patterns of speech sounds (exemplified by the
Śivasūtras in
Pāṇini's
Sanskrit grammar). Mendeleev was a friend and colleague of the Sanskritist
Otto von 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. The original draft made by Mendeleev would be found years later and published under the name
Tentative System of Elements. Dmitri Mendeleev is often referred to as the Father of the Periodic Table. He called his table or matrix, "the Periodic System". ==Later life==