Advanced Placement (AP) Chemistry Practice Exam

As you move left to right on the periodic table, what happens to ionization energy?

1. It decreases

2. It remains unchanged

3. It increases

4. It fluctuates

The correct answer is: It increases

As you move from left to right across a period in the periodic table, the ionization energy generally increases. This trend occurs due to several factors related to atomic structure. Firstly, as you move across a period, the number of protons in the nucleus increases while electrons are added to the same energy level. This increase in positive charge in the nucleus enhances the nuclear attraction exerted on the electrons. As a result, more energy is required to remove the outermost electron, leading to a higher ionization energy. In addition to the increase in nuclear charge, the increase in effective nuclear charge (which is the net positive charge experienced by electrons) also contributes to this trend. Even though electrons are added, they do not significantly shield each other from the increasing nuclear charge because they occupy the same principal energy level. Therefore, the overall attraction between the nucleus and the outermost electron becomes stronger, necessitating more energy to remove that electron. These trends make it clear that ionization energy increases as you progress from left to right across a period.