Dmitry Ivanovich Mendeleyev liked to tell that the idea of the periodic system came to him in a dream. Like many chemists of the middle of the 19th century, he tried to systematize a huge number of chemical elements that were discovered. Mendeleyev then worked on the book “Fundamentals of Chemistry”, and it always seemed to him that for the substances that he described, there must certainly be some sort of order that would make them more than just an occasional set of elements. It was such a way of ordering, he saw such a law in a dream.
In his table (today we call it a periodic table or a system of elements), Mendeleev arranged the chemical elements along rows in order of increasing mass, selecting the length of the rows so that the chemical elements in one column had similar chemical properties. For example, the rightmost column of the table contains helium, neon, argon, krypton, xenon, and radon. These are noble gases * – substances that are reluctant to react with other elements and exhibit low chemical activity. In contrast, the elements of the leftmost column – lithium, sodium, potassium, etc. – react violently with other substances, the process is explosive. Analogous statements can be made about the chemical properties of elements in other columns of the table – inside the column these properties are similar, but they vary from one column to the next.
It is impossible not to pay tribute to the boldness of Mendeleyev’s thoughts, who decided to publish their results. On the one hand, the table in its original form contained many empty cells. Elements, the existence of which we now know, was still only to be discovered. (Indeed, the discovery of these elements, including scandium and germanium, became one of the greatest triumphs of the periodic system.) On the other hand, Mendeleev had to admit that the atomic weights of some elements were not measured correctly, otherwise they would not fit into the system. And again it turned out that he was right.
The periodic system in its first version simply reflected the state of affairs existing in nature. As in the case of the Keplerian laws of planetary motion, the table did not explain why this should be so. And only with the advent of quantum mechanics and, in particular, the Pauli prohibition principle, did the true meaning of the arrangement of elements in the periodic table become clear.
Today we look at the periodic table in terms of how electrons fill the electron layers in the atom (see Aufbau Principle). The chemical properties of the atom (that is, what kind of bonds will be formed with other atoms) are determined by the number of electrons in the outer layer. So, hydrogen and lithium have only one external electron, so in chemical reactions they behave similarly. In turn, helium and neon both have filled outer shells, and also behave similarly, but not at all like hydrogen and lithium.
Chemical elements up to uranium (contains 92 protons and 92 electrons) are found in nature. Starting with the number 93 there are artificial elements created in the laboratory. So far, the biggest number announced by scientists is 118.