Final Answer:
When looking at the trend for ionization energy within a group, you should consider that ionization energy generally decreases down a group.
Step-by-step explanation:
In a group (vertical column) of the periodic table, elements have the same number of valence electrons, which are located in the outermost energy level. As you move down a group, the principal quantum number (n) of the outermost energy level increases, meaning that the valence electrons are farther from the nucleus. The increased distance reduces the attractive force between the positively charged nucleus and the negatively charged electrons, making it easier to remove an electron. As a result, ionization energy generally decreases down a group.
This trend is observed because the outer electrons are shielded by inner electron shells, and the increased distance outweighs the increased nuclear charge. Therefore, it becomes easier to remove an electron as you move down a group.