In its elemental form, Iodine is a diatomic molecule in the gas phase. In the periodic table, Iodine is just right beside Xenon which is a stable element because it has no extra electrons. Thus, to conform to stability, Iodine must have to gain one more electron. That's why in its ionic form, its formula is I⁻. The reaction would be
I₂ → 2 I⁻
To achieve the Law of Definite Proportions from Dalton's Atomic Theory, we must balance the reaction. So in every neutral Iodine atom, 2 Iodide ions are formed.
