|MadSci Network: Science History|
Mendeleev left the noble gases out of his periodic table for a very good reason:
they were not known, and there were no known elements with similar properties
which would lead him to suspect that they existed.
Mendeleev's periodic table was highly empirical: he sorted the elements in order of atomic weight, then noticed that certain properties repeated periodically. If there was a set of properties that had a missing element in one period, he inserted an "undiscovered" element. But NO elements were known with noble gas properties, so there were no empty "noble gas" slots for him to fill.
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