The reactivities of the halogens decrease down the group ( At < I < Br < Cl < F). This is due to the fact that atomic radius increases in size with an increase of electronic energy levels. This lessens the attraction for valence electrons of other atoms, decreasing reactivity.
Fission is a term that means when a large atom breaks into smaller atoms. It’s counterpart, Fusion, is the opposite, where small atoms fuse into larger atoms.