An entire group of elements was unknown when the periodic table was first developed because they were unreactive and no known compounds were composed of them. What most likely led to the inclusion of these elements in the periodic table?

These elements are the noble gases, which are very unreactive and therefore were not recognized through their compounds. Improved technology made it possible to detect and identify them directly, leading scientists to add them as a group in the periodic table. Forming a compound would not be a likely discovery method for such unreactive elements.

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