Advanced Placement (AP) Chemistry Practice Exam

What is the trend of atomic radius as you move from left to right across a period?

1. It increases due to a larger number of neutrons

2. It decreases as protons are added to the nucleus

3. It remains constant

4. It increases due to added electron shells

The correct answer is: It decreases as protons are added to the nucleus

As you move from left to right across a period in the periodic table, atomic radius tends to decrease. This trend occurs because, as you traverse a period, protons are added to the nucleus, resulting in an increase in positive charge. This increased positive charge attracts the electrons more strongly, pulling them closer to the nucleus. Furthermore, the added electrons are being added to the same principal energy level, meaning they do not contribute significantly to electron shielding, which would counteract the increased nuclear charge. As a result, the effective nuclear charge experienced by the outermost electrons increases, pulling them in closer and reducing the atomic radius. This reduction in atomic radius highlights the trend of increased nuclear charge’s effect on size as you progress across a period. The other trends, such as the addition of electron shells, are more relevant when discussing vertical movements in the periodic table rather than horizontal movements across a period.