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Periodic Table History and Noble Gases


The Russian chemist Dmitri Mendeleev is generally credited with organizing the chemical elements into a comprehensive periodic table. This was, in large part, due to the fact that he was able to use his table to predict the existence of elements, like gallium, which were not known at the time (1872) but were discovered later​.

Which of the following is the BEST explanation for why Mendeleev was unable to predict the existence of the noble gases (group 18)?


Noble gases are not found on Earth.


Noble gases are stable and very unreactive and therefore have few observable properties.


Noble gases are synthetic elements and do not occur naturally.


Noble gases are very reactive and do not exist as stable elements.