Final answer:
The element in question is likely Krypton (Kr), based on the clues provided about its position on the periodic table, state at room temperature, and atomic properties such as ionization energy and atomic radius.
Step-by-step explanation:
The student is looking for an element that satisfies the following clues: It is to the right of the stair-step line, it is a gas at room temperature, not diatomic, not the smallest atom in its group, not radioactive, it's harder to remove an electron from it than from the element in period 4 of its group, and it has a larger atomic radius of the elements remaining. Considering these clues, the element is a noble gas, which belongs to Group 18. Clue 6 suggests that we are comparing elements in the same group but in different periods, specifically looking at the ionization energy, which indicates we need an element in a higher period than 4. Clue 7 suggests that within the remaining elements, we want the one with the larger atomic radius. Comparing noble gases above period 4 and taking into account that atomic size increases down a group, we can deduce that the element is likely Krypton (Kr), which is in period 4 and lighter than Xenon (Xe), which would be its nearest heavier noble gas neighbor.