By the middle of the 19th century, scientists had finally realized that they would not be able to artificially transform one element into another. On the other hand, they were constantly discovering new chemical elements that had to be systematized somehow. For this purpose, scientists came up with different theories about hydrogen, about primacy, etc., but all of them were unsuccessful and had no success in scientific circles.
By the time you remember, the atomic weights of the elements had already been calculated. Therefore, before we move on, I advise you to take into your hands the Mendeleev table. For a clearer, so to speak, the idea of the subject of the conversation.
So, in 1817 the German chemist Johann Döbereiner discovered that similar elements can be grouped into so-called triads - groups of three elements. The atomic weight of the middle term of this triad was equal to the arithmetic mean of the two outermost elements. For example, in the triad of potassium (39.1), rubidium (85.4) and cesium (132.9), the average value between potassium and cesium is 86, which is approximately equal to the weight of rubidium.
Since then, the idea of a natural relationship between the chemical and physical properties of the elements and their atomic weights has firmly settled in the minds of scientists. Many of them tried to discover the "general" law of atomic scales, but only in the 70s of the XIX century, this was done by chemists D. Newlands and L. Meyer.
In 1864, Newlands outlined the idea of the periodicity of atomic scales in the London Royal Society. He said: "If we put all the elements in rows, eight in each, by their atomic weights, we will make sure that each of the eight elements are followed by similar elements. This rule of Newlands called "octave law".
Alas, the thought of the dependence of the atomic scales on other properties of the elements seemed so improbable at the time that the chairman of the Society asked the author an ironic question: "Has the referent tried to place the elements in alphabetical order?
Another German chemist, L. Meyer, had for several years applied a system of elements based on the periodicity of their atomic scales in his lectures, but he did not dare to publish it in scientific journals.
Only Dmitry Ivanovich Mendeleev (1834-1907), a Russian encyclopedic scientist, distinguished himself in this matter with unprecedented courage. He brought his periodic table almost to perfection and, more interestingly, predicted several facts, which were destined to soon be realized. Of course, at first there were some defects in the table, but Mendeleev called them the consequence of incompleteness of knowledge at that time.
Dmitry Mendeleev was born in Tobolsk to a large family (he was the sixteenth child). His father was the director of the Tobolsk high school, and this circumstance helped Mendeleev, who had a strong aversion to Latin, to graduate from this school. According to biographer Mendeleev, Sister Dmitry married a Latin teacher from the same gymnasium, and it was said that due to this circumstance the future scientist had safely bypassed all the obstacles associated with studying Latin.
Soon, however, Mendeleev encountered more serious obstacles on his way to further education. His father was blind, and his pension was insignificant, even for the most essential products. Dmitry and his mother moved to Moscow with her brother. There he was to enter the governor's office and become an ordinary official, but despite the lack of funds, he began to think about entering the St. Petersburg Pedagogical Institute. Thanks to the recommendation received at the school, without which there was nothing to worry about at that time, Dmitry entered the Institute. The director of the institute, a former comrade of Mendeleev's father, even gave him a place in the dormitory.
After graduating from the institute, Mendeleev was appointed a teacher at the Simferopol gymnasium. From there he moved to Odessa, but in 1856 he returned to St. Petersburg, where he became a private associate professor at the University. Three years Dmitry devoted to the development of several monographs, printing, and defense of master's work "On specific volumes".
Soon he was sent abroad. For two years he worked in Heidelberg under the guidance of scientists from Bunsen, Kirchhoff, and Kopp, which had a significant impact on the nature of his future work. A particularly important role in Mendeleev's life was played by the congress of German naturalists held in Karlsruhe in 1860. It was at this congress that molecular theory first gained general recognition and was officially recognized as a scientific theory.
In 1861, after returning from a trip abroad, Mendeleev returned to his duties as a private associate professor at the University, but in 1863 he became a professor at the Institute of Technology. At the same time, he published his extensive doctoral work on alcohol-water compounds and the first edition of the well-known "Fundamentals of Chemistry". You can learn about what this work was about and what experiments were conducted for it on the YouTube channel "Chemistry is simple" by watching the video "How Mendeleev did not invent vodka". By the way, maybe it was this work that he discovered his periodic law.