Now, were we not too hasty in concluding that in undertaking to explain the structure of the Periodic Table the physicists found no defects in its construction? It would be a good thing if a definite electron shell were filled regularly in the inhabitants of each floor of the Big House, and if each floor started with an alkali metal and ended in an inert gas The capacity of each period would then be equal to the capacity of the electron shell.

   Alas, we are obliged to speak of this in the subjunctive mood: if this, if that. Actually the balance does not tally. The third period of Mendeleyev's Table accommodates less inhabitants than there are electrons in the third shell, the M-shell. And so on.
A sad incongruity. But this incongruity holds the clue to the essence of the Periodic System. Though the third period ends in argon, the third M-shell of the latter's atom is not completed. The completed shell should contain 18 electrons, but so far there are only 8 in it. Argon is followed by potassium which belongs to the fourth period, being the first inhabitant of the fourth floor. But instead of placing its latest electron in its third shell, the potassium atom prefers having it in its fourth, N-shell. This is no accident, but again a strict regularity established by the physicists. It is simply that no atom can have more than 8 electrons in its outside shell. The combination of 8 outer electrons is a very stable arrangement.

   In calcium, potassium's next-door neighbour, the newest electron also finds it "more advantageous" to occupy the outermost shell, because then the energy supply of the calcium atom is smaller than with any other electron distribution. But in scandium, which follows calcium, the tendency to continue filling the outer shell of the atom vanishes. Its new electron "dives" into the incomplete second-last M-shell. And since this shell has ten vacancies (we already know that the maximum capacity of the M -shell is 18 electrons), the atoms of the next ten elements, from scandium to zinc, gradually fill up their M-shells. Finally, in zinc, all the electrons of the M-shell are in place. After this the N-shell again begins to accept electrons. As soon as it contains a total of 8 electrons we get the inert gas krypton. In rubidium the old story repeats itself: the fifth shell appears before the fourth is complete.

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