Such a step-by-step filling of electron shells is a "standard of behaviour" for the inhabitants of the Periodic Table from the fourth period on. This is the basic strict regulation of the Big House of Chemical Elements. For this reason main and secondary subgroups are distinguished in it. The elements in which the outermost electron shells are being filled comprise the main subgroups. Those in which inner shells are being completed make up the secondary subgroups. But the fourth, N-shell, does not fill up in one step. Its completion is drawn out into three whole storeys of the Big House. The first electron of this shell appears in potassium which occupies flat 19. But the 32nd electron appears in it only in lutetium, a representative of the sixth period. Its atomic number is 71.

   So you see, the incongruity has a silver lining. In trying to account for it we and the physicists got a better insight into the structure of the Periodic System.

---