How does electronegativity affect the strength of an oxyacid?

The strength of an oxyacid—an oxygen-containing acid—is determined by its ability to dissociate in water to form hydronium ions.

The electronegativity of the central nonmetal atom is a primary factor in predicting this strength: as the electronegativity of the central atom increases, the acid strength of the compound increases.1

For example, because chlorine is more electronegative than sulfur, which is in turn more electronegative than phosphorus, perchloric acid (HClO₄) is a stronger acid than sulfuric acid (H₂SO₄), which is stronger than phosphoric acid (H₃PO₄).1

Additionally, for a specific central atom, acid strength increases as the oxidation number of that atom increases. For instance, nitric acid (HNO₃), where nitrogen has an oxidation state of +5, is a stronger acid than nitrous acid (HNO₂), where nitrogen is in a +3 state.1