Final answer:
The least reactive elements are helium and neon from the noble gases, as well as inert metals such as gold and platinum. Noble gases are unreactive due to their full valence electron shells, while inert metals have a low tendency to oxidize, making them minimally reactive.
Step-by-step explanation:
The elements that are considered the least reactive are from the group of noble gases, specifically helium (He) and neon (Ne) from group 18, alongside inert metals such as gold (Au) and platinum (Pt) found at the bottom of the activity series. The noble gases are extremely unreactive due to their filled valence electron shells, which make them stable and unlikely to gain or lose electrons. Inert metals, likewise, have a low tendency to be oxidized because of their minimal reactivity, which allows them to resist corrosion and remain unaltered in many environments.
Group 17 elements, known as halogens, are highly reactive nonmetals with iodine being the least reactive within the group. Though iodine does react with metals and can act as an oxidizing agent, these reactions typically require heating and are not as vigorous when compared to other halogens like fluorine or chlorine. Considering metals, alkali metals from group 1 are highly reactive, but inert metals such as gold and platinum exhibit the least reactivity among metals because they have the least tendency to lose electrons or engage in chemical reactions.