* Chlorine is a highly reactive nonmetal. It readily gains electrons to achieve a stable electron configuration. This makes it a strong oxidizing agent, meaning it readily reacts with other elements.
* Nickel and Iron are both transition metals, which are generally less reactive than halogens like chlorine. They can form compounds, but they are not as reactive as chlorine.
* Helium is a noble gas. Noble gases have a full outer shell of electrons, making them extremely stable and unreactive.
In summary: Reactivity generally increases moving from left to right across a period on the periodic table and decreases moving down a group. Chlorine is a halogen and sits near the top right of the periodic table, making it highly reactive.
