Final answer:
Fluoride (F-) is the weakest nucleophile in a polar protic solvent due to its small size and high electronegativity, which lead to a strong solvation shell that hinders its nucleophilic attack.
Step-by-step explanation:
In a polar protic solvent, the halide ion that is the weakest nucleophile is typically fluoride (F-). This is because fluoride is the smallest and most electronegative of the halogens listed, and thus it is most heavily solvated in a polar protic solvent like water or methanol. The solvation shell around fluoride ion makes it less available to attack an electrophilic center. On the other hand, I- is less solvated due to its larger size, making it a stronger nucleophile in polar protic solvents.
Therefore, among the options given (F-, Cl-, Br-, I-), fluoride (F-) is the weakest nucleophile when in a polar protic solvent.