# Chapter 2 - Atoms, Molecules, and Ions - Exercises - Page 75: 54

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.

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