The Noble Gases
Uniquely stable, they seemed to participate in no chemical reactions. [blank] by understanding the stability of the noble gases, physicists discovered the key to chemical [blank] itself. Dmitri Mendeleev added the noble gases to his periodic table in 1902, where he arranged the elements in rows and columns according to their [blank] weight.Mendeleev was able to see repeating (or periodic) patterns in their properties. The noble gases appeared regularly in the [blank] table, occurring in every eighth position, at least amongst the lighter elements. Physicists struggled to find a model that would explain this curious observation.
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