Have you ever seen a house with all its bays or sections planned identically according to a type design, except for one which differs entirely from the rest, as if a different architect had made it? It is not very likely that you have.

Well, the Big House is just such a curious structure. Mendeleyev fashioned one of its sections quite uniquely. It may be added that he had to.

The section in question is the eighth group of the Periodic System. The elements in it are arranged in threes. Furthermore, they are not on each floor, but only in the long periods of the table. Iron, cobalt, and nickel are in one of them and the platinum metals, in the other two.

Mendeleyev tried hard to find more suitable places for them. But he was finally obliged to add an eighth group to the Periodic Table.

Why an eighth? Simply because the last group before that was the seventh, the one with the halogens.

But that makes the group number purely formal.

A valence of plus eight in the eighth group is a rare exception rather than the rule. Only ruthenium and osmium try to conform, though they find it far from easy; their oxides, RuO₄ and OsO₄, are unstable.

None of the other metals have ever reached such �heights,� despite all the scientists� efforts to help them.

Let us try to solve this riddle together.

Note that the platinum metals participate in chemical reactions very reluctantly. That is why chemists now often use platinum laboratory ware for their experiments. Platinum and its companions are the �noble gases,� as it were, among the metals. It is therefore not without reason that they have been called �noble� for ages. Note also that they occur in nature in the native, uncombined state.

Now take iron, for instance. Ordinary iron behaves chemically like a moderately active element. Pure iron is very stable.

(By the way, here is something to think about. Maybe many of the elements, not only metals, are highly resistant to chemical influences when extra pure.)

It is not the outermost, but the second-last electron shell in the atoms of the platinum metals that is responsible for their �nobility.�

This shell lacks but a very few electrons to make a complete set of eighteen, an eighteen-electron shell being also a fairly stable structure. That is why the platinum metals are not inclined to give away electrons from this shell. Nor can they accept electrons, because they are metals, after all. (The element palladium actually has a complete set of eighteen electrons in its second-last (N) shell, and no electrons in its outer (O) shell.)

This �irresoluteness� of the platinum metals accounts for their peculiar behaviour.

Still, the eighth group does not fit very well into the logic of the Mendeleyev Table. To eliminate this inconsistency chemists have suggested combining the eighth and zero groups into one.

The future will show whether this is the right thing to do.

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