Stories About Chemistry
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.