Final answer:
Oxygen (O) will have the smallest ionic radius among the listed elements when it becomes an oxide ion. Lithium (Li) will have the smallest radius among the metals due to fewer electron shells, while sulfur (S) and potassium (K) will have larger radii.
Step-by-step explanation:
The element with the smallest ionic radius among Li, K, O, and S will be Oxygen (O) when it gains electrons to form an oxide ion (O2-). In the ionic form, nonmetals generally have smaller radii than metals because they gain electrons and their electron-electron repulsion leads to a smaller ionic size. Considering the other elements presented (Li, K), lithium (Li) will have the smallest radius among the metals due to having fewer electron shells. Potassium (K) will have a larger ionic radius compared to Li due to the additional electron shells. Sulfur (S) will have a slightly larger radius than oxygen because it is in the same group but has an additional electron shell.