This is because the iodide ion has a larger electron cloud than the bromide ion. This means that the electrons in the iodide ion are more spread out and therefore less tightly bound to the nucleus. As a result, it is easier to remove an electron from an iodide ion than a bromide ion.
The larger the size of the anion, the easier it is to remove an electron.
This is because the valence electrons are farther from the nucleus and are therefore less attracted to it.
The trend in ionization energy for the halogens is:
F > Cl > Br > I
This means that it is easiest to remove an electron from an iodide ion and most difficult to remove an electron from a fluoride ion.
