Hydrogen Halide Acid Strength

The acid strength of hydrogen halides in water is governed mainly by the strength of the hydrogen-halogen bond, which decreases down the group as the halogen atom grows larger. Hydroiodic acid, HI, has the weakest and longest H-I bond because of poor overlap between hydrogen's small orbital and iodine's large orbital, so it ionises most readily in water to release protons, making it the strongest acid among the hydrogen halides. HF, despite fluorine's high electronegativity, is a weak acid because its short strong H-F bond and extensive hydrogen bonding resist ionisation. The option for HF based on hydrogen bonding actually explains its weakness, not strength. The option for HCl based on it being common has no chemical basis. The option for HBr being smallest is factually wrong, since iodine is larger. This bond-strength basis for acidity is a key NCERT trend. Plausibility check: acid strength increases in the order HF < HCl < HBr < HI as the bond weakens, confirming HI as the strongest.