Question

Why were noble gases discovered late?

Answer 1

The noble gasses are inert or unreactive due to the fact that they all follow the rule of octet and have complete `s^2p^6` orbital arrangements in the highest energy level.

Because most elements were discovered through their reactivity with other elements, typically with oxygen, it was difficult for scientists to work with a substance that seemed to have little or no chemical properties based upon the lack of reactivity.

I hope this was helpful.
SMARTERTAECHER

Very good historical presentation on this website: http://chemwiki.ucdavis.edu/Inorganic_Chemistry/Descriptive_Chemistry/Elements_Organized_by_Block/2_p-Block_Elements/Group_18%3A_The_Noble_Gases/1Group_18%3A_Properties_of_Nobel_Gases