Final answer:
Ionization energy increases from left to right across a period due to stronger nuclear attraction and decreases down a group due to increased distance from the nucleus and electron shielding.
Step-by-step explanation:
The trend for ionization energy across a period and down a group in the periodic table describes how the energy needed to remove an electron from an atom changes depending on its position in the table. Ionization energy generally increases from left to right across a period. This is because, as you move from left to right, the atomic size decreases and the electrostatic interactions between the nucleus and valence electrons increase. As a result, more energy is required to remove an electron.
Conversely, the ionization energy decreases down a group. This decrease is observed because the valence electrons are further away from the nucleus and experience less electromagnetic pull, which makes them easier to remove. Essentially, the increasing distance from the nucleus and additional electron shells lead to a weaker attraction.