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.)

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