The results of the discovery of the Periodic law by D. I. Mendeleev are considered, and the actual formulation of this law is given. Some examples of the use of symmetry groups in modern science are given. It is shown that the SO(4,2) group allows presenting the contents of the Periodic system of elements in full coincidence with the experimentally established structure of electronic shells of corresponding atoms without involving any additional quantum numbers characterizing the properties of atoms. adynamic substantiation of the use of representations of the dynamic symmetry group of the quantum system, isovalent to hydrogen, for a mathematical description of the properties of the symmetry of the Periodic system of elements is proposed. Using it, the splitting of the infinite-dimensional unitary representations of the group SO(4,2) into the finite-dimensional multiplets, determined by the quantum numbers describing the states of electrons, was implemented. A problem of inclusion of isotopes of elements in the Periodic system of elements is discussed.