Final answer:
The ion with the smallest diameter is E) Lithium (Li+).
Step-by-step explanation:
The ion with the smallest diameter is Lithium (Li+).
The size of an ion is determined by the effective nuclear charge and the number of electrons. In this case, the lithium cation (Li+) has the smallest radius among the listed options.
When a neutral atom becomes a cation (loses electrons), it loses electron-electron repulsion and the remaining electrons are pulled closer to the nucleus. This results in a smaller ion.
Among the options given, Li+ has the smallest radius and therefore the smallest diameter.