Molecular Structure and Acid Strength The strength of an acid depends on how easily the proton, H+, is lost or removed. The more polarized the bond between H and the atom to which it is bonded, the more easily the H+ is lost or donated.We will look now at factors that affect how easily the hydrogen can be lost and, therefore, acid strength.For a binary acid, as the size of X in HX increases, going down a group, acid strength increases.For a binary acid, going across a period, as the electronegativity increases, acid strength increases.Which is a stronger acid: HF or HCl? Which is a stronger acid: H2O or H2S? Which is a stronger acid: HCl or H2S?HF and HCl These are binary acids from the same group, so we compare the size of F and Cl. Because Cl is larger, HCl is the stronger acid.H2O and H2S These are binary acids from the same group, so we compare the size of O and S. Because S is larger, H2S is the stronger acid.HCl and H2S These are binary acids from the same period, but different groups, so we compare the electronegativity of Cl and S. Because Cl is more electronegative, HCl is the stronger acid.