Final answer:
Xenon (Xe) would have the lowest first ionization energy among the given elements, following the periodic trends that ionization energy decreases down a group and Xenon is at the bottom of its group.
Step-by-step explanation:
Among the options given (I, Xe, F, Cl, Br), Xenon (Xe) would have the lowest first ionization energy. Ionization energy is the energy required to remove an electron from a gaseous atom or ion. The trend in ionization energy increases from left to right across a period and decreases down a group in the periodic table. Therefore, elements located towards the lower left side of the periodic table will generally have lower ionization energies.
When comparing the elements in the question, all of which are non-metals, we must look at their positions in the periodic table. Xenon is located at the bottom of the noble gases group and to the right side of the periodic table. Although elements on the right side generally have higher ionization energies, Xenon's position at the bottom of its group means it will have the lowest ionization energy among these elements due to its increased atomic size and weaker effective nuclear charge felt by the valence electrons.