Which of the following noble gas is not present in air?

Correct answer: (d) Radon

Explanation

Of the noble gases, argon (about 0.93%), neon, helium, krypton and xenon are regular parts of air.

Radon is not counted among air's constituents: it is a radioactive gas from the decay of radium in rocks and soil, and it occurs only in traces near the ground, in mines and in basements.