Here we have to determine the formula of the ionic compound formed between a and iodine.
The ionic compounds are those which remains in ionic state in its solution phase. The formula of the compound aF₃ which reveals the element a is trivalent i.e. a³⁺. It will remain in ionic state as follows: aF₃ → a³⁺ + 3F⁻.
As the element a is trivalent thus with iodine also it will combine like fluorine (as both belongs to the same halogen group). The formula of the compound will be aI₃ and it will be ionic in nature. The ions will be aI₃ → a³⁺ + 3I⁻.
Thus the formula of the ionic compound between a and iodine is determined.