On March 6, 1869, Dmitry Mendeleev presented to the Russian Chemical Society a method for organizing chemical elements. The most interesting thing is that the Russian chemist could not know why his table looked exactly like this, but nevertheless ordered the chemical elements according to the correct principle and even predicted the discovery of three elements that were not yet known, writes "Dagens Nycheter".
On March 6, 1869, Dmitry Mendeleev presented to the Russian Chemical Society a method for organizing chemical elements. To celebrate the 150th anniversary of this event, the UN General Assembly and UNESCO have declared 2019 the International Year of the Periodic System.
When French chemist Paul-Émile Lecoq de Boisbaudran investigated the minerals of the Pyrenees mountains in 1875, he discovered a new element that he named gallium after the Roman name for France. He reported his discovery, but soon received a letter in which he was asked to investigate the density of the element again, because he probably measured it incorrectly. De Boisbaudran did so and discovered that the sender of the letter, the Russian chemist Dmitry Mendeleev, was right. The density of the element was not 4.7 grams per square centimeter, as he originally thought, but 5.9 - almost the same as Mendeleev predicted.
"It was after this that the periodic table became famous all over the world," says Michael Gordin, professor of modern history at Princeton University and author of A Well-Ordered Thing: Dmitry Mendeleev and the Shadow of the Periodic Table: Dmitrii Mendeleev and the Shadow of the Periodic Table).
It all started in St. Petersburg in February 1869. Dmitry Mendeleev wrote a textbook on chemistry in two volumes. The first part was already ready: he devoted the first 500 pages of the book to the four elements - carbon, oxygen, hydrogen and nitrogen. In the last chapter, Mendeleev describes the so-called halogens: fluorine, chlorine, bromine and iodine - a group of elements whose properties are very similar. For example, they readily react with metals and form salts such as sodium chloride, which is common food grade salt.
The second volume was supposed to be no less thick. But out of 63 known elements, the chemist told in the first part only about eight.
“He had 55 elements left to present in the second volume. So he tried to find a way to organize them so that he could describe them as economically as possible,”says Michael Gordin.
To begin with, Dmitry Mendeleev thought about alkali metals - lithium, sodium, potassium and rubidium. All these are soft and light metals that actively interact with water, forming an alkaline base, in other words, a solution with a high pH. Comparing their atomic weight, or atomic mass (an indicator of how much one atom of a substance weighs), he found that the difference between two alkali metals is very similar to the difference between two halogens.
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“He began to think that there might be some natural system to explain this phenomenon,” says Michael Gordin.
There are many common myths about how Dmitry Mendeleev came up with the idea of the periodic system. Some say that he had a card game with the properties of chemical elements on the cards, while others say that the final decision came to him in a dream. But according to Michael Gordin, this is not true. Mendeleev's documents indicate that he built the table step by step, in the process of work he started from halogens and alkali metals, and then tried to find suitable places for other elements.
Finally he realized that everything was ready, rewrote the manuscript and sent it to the press. The work was published on February 17, 1869, which actually means March 1, 1869, because then Russia had not yet switched to the Gregorian calendar. Dmitri Mendeleev wrote the title in Russian and French: "An experience of classifying elements based on their atomic weight and chemical similarity" (the tendency to react with other elements). Then he changed his mind and deleted the word "classification", writing "system" instead, but forgot to change the article in the French version: left une from classification ("classification", feminine) instead of un for système ("system", masculine) …
"This is where the typo in the very first printed versions of the periodic table came from," explains Michael Gordin.
The first table of Dmitri Mendeleev was not like the periodic systems that now hang in every class of chemistry around the world. But his idea was correct, and the breakthrough in science was so significant that the UN General Assembly and UNESCO declared 2019 the International Year of the Periodic System to celebrate its 150th anniversary.
Most surprising of all, Mendeleev could not have known why his table looks like this. The elements in it are ordered by atomic number, that is, by the number of protons in the nucleus of the atom. But the British physicist Henry Moseley found it out only in 1913 - six years after Mendeleev's death. Which column an element falls into is determined by the organization of the electrons in the valence shell of the atom, but electrons were not discovered until 1897 by Joseph John Thomson, another British physicist.
“Mendeleev treated electrons with great suspicion, he did not decide whether to believe in them or not. And he certainly didn't know anything about quantum mechanics. It wasn't until 1923 that Niels Bohr formulated the periodic table in terms of quantum theory,”says Michael Gordin.
From the very beginning, Dmitry Mendeleev realized that his table was missing at least three elements. When he published an improved version of the table in 1871, he named them ekabor, ekaaluminium, and ekasilicon, where eka means one in Sanskrit. He also made very detailed predictions about their properties and how they might be detected. That is why he was so sure that Boisbaudran had incorrectly measured the density of gallium, because all its other properties were the same as eka-aluminum. Then in 1879 scandium was discovered, and in 1886 - germanium.
“First and foremost, germanium was incredibly similar to ekosilicon. When it was discovered, it was a brilliant success of the periodic system, which truly glorified Mendeleev all over the world,”says Michael Gordin.
Maria Gunther