By the time Morozov was released, the inert gases had already been discovered and had found their place in the table of elements.

   It is said that Morozov visited Mendeleyev not long before the latter's death, and that the two great compatriots had a long talk about the Periodic Law. Unfortunately, the contents of this talk are unknown. Mendeleyev died not long before the mystery of the inertness of the noble gases was solved. The secret was as follows.

   The physicists who had so often, and still do; come to the aid of the chemists, established that an outer shell containing 8 electrons is very stable. It is a sort of ideal of stability of an electron shell. And hence, there is no reason for it to donate or accept electrons. Thus the reason for the "nobility" of the inert gases is the 8 electrons in their outermost shell (or 2-in the case of the helium atom). The two electron shell of helium is no less stable than the octet ones of the other chemical sloths.

   And another thing became clear to the chemists: the addition of the zero group to the Periodic Table was not just a forced measure; without it the Periodic System would have looked like an unfinished edifice, becauuse each of its periods ends in an inert gas after which the next electron shell begins to fill up and thus the next storey of the Big House begins to form.

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