Advanced Placement (AP) Chemistry Practice Exam

What is the periodic trend in atomic radius as you move down a group in the periodic table?

1. Atomic radius decreases due to increased nuclear charge

2. Atomic radius stays the same due to constant electron shielding

3. Atomic radius increases due to the addition of electron shells

4. Atomic radius varies irregularly without a clear trend

The correct answer is: Atomic radius increases due to the addition of electron shells

The atomic radius tends to increase as you move down a group in the periodic table primarily because of the addition of electron shells. Each subsequent element in a group has an additional principal energy level compared to the one above it, resulting in electrons being further from the nucleus. As new shells are added, the outermost electrons are contained in energy levels that are farther from the nucleus, which leads to an increase in the overall size of the atom. While there is an increase in nuclear charge as protons are added to the nucleus, the effect of increased distance due to additional electron shells outweighs the stronger attraction that the increased nuclear charge might provide. Additionally, these added shells result in electron shielding, where inner shell electrons repel outer shell electrons, diminishing the effective nuclear charge felt by outer electrons. This combination of factors leads to a clear trend of increasing atomic radius as you move down a group, reinforcing how fundamental atomic structure influences periodic trends.