The periodic system of chemical elements (periodic table) is a classification of chemical elements that establishes the dependence of various properties of elements on the charge of the atomic nucleus. The system is a graphic expression of the periodic law established by the Russian chemist D.I. Mendeleev in 1869. Its initial version was developed by D. I. Mendeleev in 1869-1871 and established the dependence of the properties of elements on their atomic weight (in modern terms, on atomic mass). In the modern version of the system, it is assumed that the elements are reduced to a two-dimensional table in which each column (group) determines the main physicochemical properties, and the rows are periods that are somewhat similar to each other.