Predict which atom has a higher ionization energy. cl or i

The answer is chlorine (Cl)

Explanation:

Ionization energy is the minimum amount of energy required to remove a valence electron from an atom in its gaseous state. The closer the valence electron (negatively charged) is to the nucleus (positively charged), the higher the ionization energy required to remove it, hence ionization energy generally decreases down the group.

Between the two, chlorine comes first in the group before iodine and hence has a higher ionization energy (as explained earlier) than Iodine