Advanced Placement (AP) Chemistry Practice Exam

What is the significance of the steep increase in ionization energy in the ionization energy trend?

1. It indicates the transition between electron shells

2. It shows a decrease in atomic radius

3. It allows for easier ion formation

4. It reflects changes in molecular mass

The correct answer is: It indicates the transition between electron shells

The steep increase in ionization energy in the trend is significant because it indicates the transition between electron shells. When an electron is removed from an atom, the energy required to remove that electron can vary significantly based on its proximity to the nucleus and the stability of the electron configuration. As electrons are removed progressively, especially when transitioning from a completed shell to a new outer shell—representing a significant increase in distance from the nucleus or a change in energy level—the ionization energy can show a marked increase. This reflects the stability associated with completely filled or half-filled subshells, which require much more energy to disrupt. For elements where this sharp increase is observed, it denotes that an electron is being removed from a much more stable core set of electrons (closer to the nucleus) rather than from a higher energy level where electrons are more loosely held. Thus, this part of the trend is pivotal in understanding both the electron configurations and the general reactivity patterns of elements in the periodic table.