What is the trend of first ionization energy down a group?

The trend of first ionization energy down a group in the periodic table is characterized by a decrease due to increased atomic radius. As you move down a group, the number of electron shells increases. This added distance between the nucleus and the outermost electrons results in a larger atomic radius.

With more electron shells, the positive charge from the nucleus is less effective at attracting the outermost electrons because of increased electron shielding. The inner shell electrons repel the outer shell electrons, making it easier to remove an electron from the atom. This means that the energy required to remove an electron (the ionization energy) decreases as you go down a group.

Therefore, option C is correct as it accurately describes the trend observed in ionization energy when moving down a group in the periodic table.