Radioactivity is a nuclear phenomenon in which an unstable nucleus undergoes decay and is referred to as radioactive decay.
Three types of radioactive decay occur in nature : (i) α-decay (ii) β-decay (iii) γ-decay
Noble Gases
There are six elements in Group 18 of the Periodic Table - Helium (He), Neon (Ne), Argon (Ar), Krypton (Kr) , Xenon (Xe) and Radon(Rn) which were earlier termed as inert gases or rare gases.
Both of these terms are now no longer used as they are not so rare and not completely inactive. They do show some chemical reactivity in the form of some xenon and krypton compounds and hence are termed as noble gases.