In 1869, the Russian chemist Mendeleyev (1834 – 1907) proposes to classify the chemical elements in a periodic table of elements. The classification of elements according to their atomic number shows a clear periodicity of their properties. In 1913, Bohr’s and Rutherford’s works of on the atom and the nucleus explained the periodicity of properties of elements and gave a physical meaning to the atomic number of the periodic table, identifying it both to the number of electrons orbiting around the nucleus and to the number of protons in the nucleus. We now know that a row of the table corresponds to the filling with electrons of the outer shell of atoms. At the end of the row, rare gases are atoms with a shell saturated with electrons, the transition to the next row corresponding to the filling of a new shell with electrons.