Final answer:
The atomic radii increase as you go down a column of the periodic table. Rubidium has a bigger atomic radius than potassium because rubidium is below potassium on the periodic table and has an additional electron shell.
Step-by-step explanation:
As you go down a column of the periodic table, the atomic radii increase. This is because the valence electron shell is getting larger and there is a larger principal quantum number, so the valence shell lies physically farther away from the nucleus. This trend can be summarized as follows:
Rubidium (Rb) is below potassium (K) on the periodic table. Since rubidium is below potassium, it has an additional electron shell, making its atomic radius larger than that of potassium.