In Fig. 34 is shown Bohr’s arrangement of the periodic system in which the systematic correlation of the properties of the element appears somewhat clearer than in the usual plan (cf. p. 23). It shows great similarity with an arrangement proposed nearly thirty years ago by the Danish chemist, Julius Thomsen. The elements from scandium to nickel, where, in the neutral atom, the electron group of 3-quantum orbits is in a state of reconstruction, are placed in a frame; the neutral oblique lines connect elements which are “homologous,” _i.e._, similar in chemical and physical (spectral) respect. [Illustration: FIG. 34.—The periodic system of the elements. The elements where an inner group of orbits is in a stage of reconstruction are framed. The oblique lines connect elements which in physical and chemical respects have similar properties.] In _krypton_ (36) we again have a stable architecture with an outer group of eight electrons, four in 4₁ orbits and four in 4₂ orbits. Owing to the appearance of 4₃ orbits in the normal state of the atoms of elements with atomic number higher than 38, there follows in the fifth period of the natural system a reconstruction and provisional completion of the 4-quantum orbits to a group of eighteen electrons, which shows a great simplicity with the completion of the 3-quantum group in the fourth period. In Fig. 34 the elements where the 4-quantum group is in a state of reconstruction are framed. The 4-quantum group with eighteen electrons is of more stable construction than the group of eighteen 3-quantum orbits in the elements with an atomic number lower by eighteen. This is due to the fact that all the orbits in the first-mentioned group are oblong and therefore moored, so to say, in the inner groups, while in the complete group of 3-quantum orbits there are six circular orbits. This is the reason why silver, in contrast to copper, is monovalent.