Answer
a) The noble gas elements are:
$\Longrightarrow$ Helium - He
$\Longrightarrow$ Neon - Ne
$\Longrightarrow$ Argon - Ar
$\Longrightarrow$ Krypton - Kr
$\Longrightarrow$ Xenon - Xe
$\Longrightarrow$ Radon - Ra
Radon only has radioactive isotopes.
b) Promethium - Pm - has only radioactive isotopes
Work Step by Step
a) All elements of Group 18 of he Periodic Table are noble gases, namely helium [He], neon [Ne], argon [Ar], krypton [Kr], xenon [Xe]), and one that only has radioactive isotopes radon [Ra].
b) Promethium is a man-made element, as all its isotopes are radioactive with very short half-lives of less than 20 years.
