, 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.