Question

Consider four elements from Group 17: fluorine in the second period, chlorine in the third period, bromine in the fourth period, and iodine in the fifth period. Which element has the largest first ionization energy?

Answer 1

Fluorine
   For the group 17 elements (Halogens), the ionization energy decreases as the period increases (the elements become less reactive). And since of the 4 elements given, fluorine is from the lowest period, it has the highest ionization energy.

Answer 2

Answer is: fluorine has the largest first ionization energy.

The ionization energy (Ei) is the minimum amount of energy required to remove the valence electron, when element lose electrons, oxidation number of element grows (oxidation process).

Nonmetals are far right in the main group and they have highest ionization energy, because they have many valence electrons.

First ionization energy means that element lost one valence electron, because in fluorine (smallest atomic number) that electron is closest to nucleus, it requires most energy.