Which one of the following noble gases is not found in atmosphere?

All the noble gases except radon occur in the atmosphere. 

Xenon and radon are the rarest elements of the group. Radon is obtained as a decay product of ${ }^{226}\mathrm{Ra}$.

${ }_{88}^{226}\mathrm{Ra}{\to }_{86}^{222}\mathrm{Rn}{+}_2^4\mathrm{He}$