What is one of the reasons for the decrease in ionization energy down a group?

The correct answer is related to the concept of electron shielding, which plays a significant role in the trend of ionization energy as you move down a group in the periodic table. As you descend a group, additional electron shells are added, meaning that electrons are located further from the nucleus. This increase in the number of core electrons between the nucleus and the outermost electrons effectively shields the outermost electrons from the full attractive force of the nucleus.

As a result of this increased shielding, the effective nuclear charge experienced by the outermost electrons is reduced. Consequently, it requires less energy to remove an outer electron, leading to a decrease in ionization energy down the group. Understanding the interplay between shielding and ionization energy is crucial when analyzing trends in the periodic table.