How does acid strength change from hydrogen fluoride to hydrogen iodide?

The acid strength of the hydrogen halides (HX) increases as you move down the group from hydrogen fluoride (HF) to hydrogen iodide (HI).12

While hydrogen fluoride is a weak acid, hydrogen chloride (HCl), hydrogen bromide (HBr), and hydrogen iodide (HI) are all strong acids that dissociate completely in aqueous solution. This trend in acidity is primarily attributed to the decreasing bond strength of the element-hydrogen bond as the atomic size of the halogen increases. Because the bond between hydrogen and the larger halogen atoms becomes weaker, the molecule is more capable of donating a proton in solution. Other contributing factors to this trend include electronegativity and ionic solvation energy.12