Final answer:
Ionization energy decreases from top to bottom in a group on the periodic table due to increased distance and reduced nuclear attraction for the valence electrons.
Step-by-step explanation:
Periodic Trend in Ionization Energy
Ionization energy refers to the amount of energy needed to remove an electron from an atom. In the periodic table, there is a clear trend in ionization energy across a period and down a group. As we move from top to bottom in a group, the ionization energy decreases. This decrease is because the valence electrons are further away from the nucleus due to the increasing number of electron shells, experiencing less electrostatic pull to the nucleus down the column.
On the other hand, ionization energy generally increases from left to right across a period. This is because valence electrons are in the same shell while more protons are being added to the nucleus, resulting in a stronger attraction between the valence electrons and the nucleus, thus requiring more energy to remove an electron.
Therefore, the correct answer to the original question is Option 2: Decreases. The ionization energy decreases from top to bottom within a group on the periodic table.