On this day 118 years ago, on February 2, 1907 (December 20, 1906 lunar calendar), the Russian chemist Mendeleev died. On February 2, 1907, the Russian chemist Dmitri Ivanovich Mendeleev (1834-1907) died. His most important contribution to chemistry was the establishment of the periodic classification of elements. Mendeleev wrote "Principles of Chemistry" between 1869 and 1871. At the same time, he deeply explored the relationship between the properties of the elements, and arranged all known elements in the order of increasing atomic weight. This periodic table gradually became the skeleton of most chemical theories. In 1887, he proposed the theory of solution hydration, which was a pioneer of the modern solution theory. Mendeleev also studied the relationship between the volume of gases and liquids and temperature and pressure, and discovered the critical temperature of gases. Further reading: Why was Mendeleev able to decipher the element "code"? Further reading: Why was Mendeleev able to decipher the element "code"? Mendeleev's greatest contribution was the discovery of the famous periodic law of chemical elements. It is said that he deciphered the element code. What happened? In ancient times, both the East and the West believed that matter was composed of the most basic "elements". In China, these "elements" were gold, wood, water, fire, and earth. In ancient Greece, it was earth, air, water, and fire. But in modern times, these "elements" could not resist the test of science. For example, the French chemist Lavoisier proved that water can be produced by burning hydrogen in oxygen. Later, it was also found that electricity can split water into hydrogen and oxygen, so water is not an element. Another so-called "element", air, was found to be about 1/5 oxygen, and the rest are other gases that do not support combustion (mainly nitrogen), so air is not an element either. Therefore, scientists define an element as a substance that cannot be decomposed by chemical methods. Hydrogen, oxygen, nitrogen, chlorine, carbon, sulfur, phosphorus, iron, copper, gold, silver, etc. are the real elements that make up matter. In 1789, the French chemist Lavoisier published a list of 33 chemical elements, and then there was a frenzy in Europe to search for new elements, and the number of elements discovered soon reached more than 60. These elements have different properties and appear to be disorganized. This situation makes people feel confused: how many elements are there in the world? What are the relationships between elements? How should new elements be found? Scientists made various efforts to find patterns, but the results were not satisfactory until the Russian chemist Mendeleev found the "code" to crack the elements. The biggest difficulty Mendeleev faced at that time was that some of the elements that had been discovered had inaccurate atomic weights. This is like when people are ranked according to their height, a tall person of 1.8 meters is treated as a short person of 1.4 meters. There are many elements that have not yet been discovered, just as there are still absent elements in the queue. It is really difficult to find a pattern in such a line. But Mendeleev was not discouraged. He wrote down the atomic weights and main properties of the 63 elements he already knew at that time on cards, and then arranged the cards repeatedly to find patterns in them. He found that if the elements were arranged in atomic weights from small to large, an element with similar properties to the previous element would appear at every distance. This is like when people are lined up in order from low to high, a person in red will appear every few people. Not only that, but behind the first person wearing red clothes, there are people wearing orange, yellow, green, blue, blue, and purple clothes in turn. When the second person wearing red clothes appears, there are people wearing orange, yellow, green, blue, blue, and purple clothes in turn. This color seven group will repeat several times, but starting from the third row, the color seven group will be divided into two sections by 10 people wearing variegated clothes, the first two (red, orange) and the last five (yellow, green, blue, blue, purple). Of course, there are still some positions in this repeated queue that are still vacant. If you divide the row into sections, each containing a group of seven people of color, and arrange them in parallel so that each color is aligned, a column in red and a column in green, with people in variegated clothes on either side, you form an array. The people in each row are arranged by height, and the people in each column are the same color. This is basically Mendeleev's first periodic table of elements published in 1869, just by rotating it 90 degrees clockwise. Each row is called a period, and the properties of the elements in it gradually change from metal to non-metal (from red to purple). Each column is called a family, and the properties of the elements in it are similar to each other (the clothes are the same color). With this list, the laws of periodic changes in the properties of elements as their atomic weights rise are clearly revealed. How did Mendeleev overcome the difficulty of inaccurate atomic weights? In addition to personally re-measuring the atomic weights of some elements, he also judged according to the properties of the elements. This is like the color of the clothes mentioned above. If the person in orange went to the green column by atomic weight, the atomic weight may be in doubt. A specific example is the element beryllium. According to the atomic weight of beryllium at the time, which was 13.5, it should be ranked after carbon with atomic weight 12. But according to the chemical properties of beryllium, it should be between lithium (atomic weight 7) and boron (atomic weight 11), and the atomic weight should be around 9. The atomic weight of beryllium, which was later re-determined, was indeed 9. Sometimes, in order to align the colors, it was necessary to leave a space in the row. Mendeleev did not think this was a violation of the periodic law, but rather that it was the position that the undiscovered elements should occupy. For example, there are two spaces between zinc and arsenic, which correspond to aluminum and silicon. He called these two unknown elements "aluminum-like" and "silicon-like", and based on their positions in the periodic table and the properties of the "neighbors" up, down, left and right, he inferred the properties of these two elements. These two elements were indeed discovered later, and they were called gallium and germanium, respectively. Their properties are also almost exactly the same as Mendeleev predicted! These facts show that Mendeleev has cracked the "code" of the elements! This is of great theoretical significance and practical value. Each element finds its place in the periodic table. No new elements will be found in the rows and columns that have been filled, and the spaces not only predict the undiscovered elements, but also predict their properties based on their positions in the periodic table, which greatly accelerates the process of finding these unknown elements. However, in Mendeleev's time, people did not know the structure of the atom, so he did not know the real reason behind this law, nor did he know how to deal with those "wearing variegated clothes", which were later called transitional elements. It was only through the work of the British scientist Mosley that it was discovered that the number of positive charges in the nucleus (that is, the atomic number) determines the chemical properties of the elements. What the periodic table actually reflects is the periodic change in the number of outermost electrons of an element as its atomic number rises. Before that, Mendeleev had discovered the periodic law of elements based on inexact atomic weights and sequences with vacancies, reflecting his keen insight and intelligent imagination.