Final answer:
The acidity of protons bonded to oxygen versus sulfur depends on electronegativity and the size of the atom. Sulfur's larger size typically makes its protons more acidic, despite oxygen's higher electronegativity, due to better charge stabilization in the conjugate base.
Step-by-step explanation:
When comparing the acidity of protons attached to oxygen and sulfur in compounds that can form similar resonance structures, we need to consider both the electronegativity and the size of the atom to which the proton is bonded.
Electronegativity is important because more electronegative atoms attract more electron density, weakening the bond to the proton, making the proton more acidic. On the other hand, the size of the atom is critical because larger atoms can better accommodate the negative charge in the conjugate base following deprotonation, providing additional stability.
Generally, oxygen is more electronegative than sulfur, which would lead us to believe that protons attached to oxygen would be more acidic. However, the larger size of sulfur means that the conjugate bases formed after deprotonation (like thiolates compared to alkoxides) are better stabilized.
This size effect often makes the hydrogen atoms bonded to sulfur more acidic than those bonded to oxygen, as seen in the stronger acidity of thiols compared to alcohols.
In polyoxy acids and their salts, the overall stability and therefore the acidity can also be influenced by the oxidation number of the central atom and the number of highly electronegative atoms (like oxygen or fluorine) bonded to it. Such as H₂SO₄ being a stronger acid than H₂SO₃ due to its higher oxidation state of sulfur and the presence of a larger number of oxygen atoms.