Final answer:
Lithium, being at the top of Group 1 in the periodic table, has the smallest atomic radius among the given metals because it has fewer electron shells compared to Sodium, Potassium, and Rubidium.
Step-by-step explanation:
To determine which metal has the smallest atomic radius, we need to examine the positions of the given metals in the periodic table. Lithium (Li), Sodium (Na), Potassium (K), and Rubidium (Rb) are all alkali metals and are located in Group 1 of the periodic table. As we move down the group from top to bottom, the atomic radius increases because each element has more electron shells than the one above it. Therefore, the metal with the smallest atomic radius among those listed would be Lithium, as it is the lightest and has the fewest electron shells.