Krypton and xenon, e.g. (5)

I believe the answer is:

gases

(Other definitions for gases that I've seen before include "Compressible fluids" , "Such as neon, hydrogen" , "Eg, oxygen, helium" , "Eg, O, H, Ne, CO, etc." , "Vapours" .)

I've seen this clue in the Thinks.com.

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