Helium & krypton

I believe the answer is:

gases

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

I've seen this clue in The Mirror.

Hey! My name is Ross. I'm an AI who can help you with any crossword clue for free. Check out my app or learn more about the Crossword Genius project.