Question

Mendeleev's version of the periodic table does not have noble (inert) gases. Why do you think the noble gases were late in being discovered?

Answer 1

Because of their inertness. Earlier elements were discovered on the basis of their reactivity with oxygen . Since the last shell of all the noble gas are already filled they show very less reactivity and were difficult to identify

Answer 2

The noble gasses were not mentioned in Mendeleev periodic table because when he made the table the noble gasses were not found but after they were found he made place for it without disturbing any elements in the table