When it comes to the trend of reactivity in halogens, the reactivity decreases down the table. This is because in the nucleus of the atoms (mix of protons and neutrons), there is less of an attraction of the protons to the electrons on the outer shell, which gets further away going down. Since halogens are non metals, they need to gain electrons to become stable, and so this lesser attraction to the electrons may not allow this at least immediately. Fluorine is the most reactive out of all, have less of its outer shells and so being able to react much quickly.
The atoms of elements can gain or lose electrons and become ions. Ions are charged particles that have gained or lost electrons. The atoms of elements can gain or lose electrons to form monatomic ions (made from a single atom of an element).
