Question

Which of the halide ions (f−, cl−, br−, and i−) is the most stable base? which is the least stable base?

Answer 1

Explanation:

It is known that on moving down a group there will occur a decrease in electronegativity of non-metals. Whereas stability of their conjugate bases increases on moving down the group.

This also means that their acid strength also increases.

For example, $HX + H_{2}O \rightarrow H_{3}O^{+} + X^{-}$

where, X = F, Cl, Br or I)

Therefore, acidity will increase in the following order.

                      HI > HBr > HCl > HF

Hence, the stability of their conjugate bases will be as follows.

                $I^{-} > Br^{-} > Cl^{-} > F^{-}$

Thus, we can conclude that the most stable base is $I^{-}$ and least stable base is $F^{-}$.

Answer 2

You should take note that the question is about stability. A compound is stable if it does not easily react with other elements. Hence, its reactivity must be low. As you move down the group, reactivity decreases. So, the halide at the very bottom is the least reactive. It would then be logical that the most stable conjugate base is I⁻ and the least stable conjugate base is the most reactive which is F⁻.