To determine which of hydrochloric acid (HCl) or hydroiodic acid (HI) is more polar, we need to analyze their Lewis structures and the nature of the bond between hydrogen and the halogen.
In the Lewis structure of HCl, the chlorine atom has a higher electronegativity (approximately 3.16 on the Pauling scale) compared to hydrogen (2.20). This difference in electronegativity results in a polar covalent bond with a significant dipole moment, where the chlorine atom carries a partial negative charge (δ-) and hydrogen carries a partial positive charge (δ+).
On the other hand, in the Lewis structure of HI, iodine has a lower electronegativity (approximately 2.66) compared to hydrogen. The bond between hydrogen and iodine is still polar, but the dipole moment is less pronounced than that in HCl, because the difference in electronegativity is smaller.
Given this analysis, HCl is the more polar molecule compared to HI. The higher electronegativity of chlorine compared to iodine leads to a stronger dipole moment in HCl. Therefore, when considering polarity based on Lewis structures, hydrochloric acid (HCl) is more polar than hydroiodic acid (HI).