Question

ionisation energy of fluorine is more than that of iodine why

Answer 1

Ionization energy is known as energy required to remove valence electron.
Fluorine is smaller than Iodine. The force of attraction of nucleus on electrons is more in case of Fluorine and the valence electrons are tightly held as compared iodine. So it is easy to remove an electron from valence shell of iodine than Fluorine. Fluorine need more energy to remove the valence electron hence it has high ionization energy as compared to iodine.