chemistry

Noble gases are very stable due to having the maximum number of valence electrons their outer shell can hold.

For example,Helium has a completely filled shell (which is 2 electrons filled the first electron shell), it DOES NOT react with anything unless extreme conditions apllied.

As noble gases are very stable, they are difficult to be detected by reaction.

As noble gases are inert due to fully filled outermost shell electronic configuration, they are very stable and do not react with other chemicals normally.

Because no reaction can test noble gases, it cannot be discovered by experiment. So noble gases were the last gases in the air to be discovered0

Since the noble gases are inert and do not react readily, they were last to be discovered.