Question

Why do ionisation energies decrease in going from F to I in group 17?

Answer 1

1. Atomic size increases down the group, so the outer electrons are farther from the nucleus.
2. Increased distance reduces the effective nuclear attraction on the valence electrons.
3. There is more inner electron shielding in heavier halogens, weakening nuclear pull on outer electrons.
4. As a result, less energy is needed to remove the outermost electron in iodine than in fluorine.

Thus, ionisation energy decreases from F to I in group 17.