Write two reasons responsible for the late discovery of noble gases.

Explanation

(i) Most of the noble gas elements are not present in earth crust as minerals like other elements and are present in air to a very small extent.
(ii) Their atoms have stable electronic configuration of their outermost shells also called valence shells. (2 in case of He and 8 for other elements). They donot combine with atoms of other elements and donot exist independently. That is why noble gas elements were discovered at a later stage.