The physicist has thus piled Ossa upon Pelion in his quantitative proof of the existence of electronic orbits within atoms. About the shapes of these orbits he has some little information (Fig. 27) but about their orientations he is as yet pretty largely in the dark. The diagrams[162] on the accompanying pages, Figs. 28, 29, and 31, represent hypothetical conceptions, due primarily to Bohr, of the electronic orbits in a group of atoms. Since, however, these orbits are some sort of space configurations, the accompanying plane diagrams are merely schematic. They may be studied in connection with Fig. 27, Table XV, and Bohr’s diagram[163] of the periodic system of the elements shown in Fig. 30. These contain the most essential additions which Bohr made in 1922 and 1923 to the simple theory developed in 1913. The most characteristic feature of these additions is the conception of the penetration, in the case of the less simple atoms, of electrons in highly elliptical orbits into the region inside the shells of lower quantum number. [Pg 222] Fig. 28—Hypothetical atomic structures [Pg 223] This gives, so Bohr believes, these penetrating electron-orbits in some cases a smaller mean potential energy, and therefore a higher stability, than some of the orbits corresponding to the smaller quantum numbers. A glance at the group of elements beginning with argon, the last element in shell 3, in both Table XV and Fig. 30, will make clear the meaning of this statement. The fourth column of Table XV shows that Bohr assigns to argon four very elliptical orbits of shape and four of shape . Glancing down the same column to copper, or lower, one sees that there are eighteen possible third-shell orbits, namely, six of shape six of shape , and six of shape , i.e., there are in the third shell in argon ten unfilled orbits. But when a new electron is added, as we pass from argon to potassium, it goes, according to Bohr, into the orbit, thus giving potassium univalent properties like lithium and sodium (see Fig. 28). Similarly, calcium is shown in Table XV as taking its two extra electrons into its orbits. But as now the nuclear charge gets stronger and stronger with increasing atomic number, the empty third-shell orbits gain in stability over the fourth-shell ones, and a stage of reconstruction sets in with scandium (Fig. 30) and continues down to copper, all the added electrons now going inside to fill the ten empty orbits in the third shell, with the result that the chemical properties, which depend on the outer or valence electrons, do not change much while this is going on. With copper (see Table XV) the eighteen third-shell orbits are completely filled and one electron is in the orbit (see also Fig. 29), and from there down to krypton the chemical properties progress normally much as they do from Mg to Ar. [Pg 224] Fig. 29—Hypothetical atomic structures [Pg 225]