Is it among group 17 elements ? Halides ?
Florine, Chlorine, Bromine, Iodine
These are electronegative. They form anions. They accept electrons to become negatively charged. Hence Smaller size of Florine can attract the cation more easily due to higher Coulombic force. Florine stays in the compound more stably.
Attractive forces with Iodine are less, so it will react more readily.
F < Cl < Br < Iodine - reactivity