Iodine (or iodine) is a member of the halogen family, and halogens are the most reactive non-metals. Iodine is also a diatomic element, which means it is so reactive it is much more stable in molecular form. This gives it the formula I2. Aluminum exists in atomic form so the reaction between aluminum and iodine is
Al + I2 → AlI3
(if you need to balance it)
2Al + 3I2 → 2AlI3
