Oxides and oxoacids

Oxygen forms compounds, oxides, with almost every other element, and how an oxide behaves in water follows a clear trend tied to the periodic table. Oxides of strongly electropositive metals, such as sodium oxide (Na2O), dissolve in water to give basic (alkaline) solutions — the metal's low electronegativity means it readily gives up electron density to oxygen, leaving the oxide ion free to accept a proton from water and generate hydroxide ions. Oxides of strongly electronegative nonmetals, such as sulfur trioxide (SO3), instead dissolve to give acidic solutions — the nonmetal holds electron density tightly, and the resulting molecule reacts with water by accepting a hydroxide-like fragment and releasing a proton. Between these extremes, elements near the metal-nonmetal boundary of the periodic table, such as aluminium and zinc, form amphoteric oxides that react with both strong acids and strong bases, a direct chemical signature of an element sitting on that boundary.

Oxoacids extend this same logic to acids built around a central atom bonded to one or more oxygen atoms, at least one of which also bears an ionisable hydrogen (such as sulfuric acid, H2SO4, or nitric acid, HNO3). Two factors control an oxoacid's strength. First, for a fixed number of oxygen atoms, acid strength increases with the central atom's electronegativity, since a more electronegative central atom pulls electron density away from the O-H bond, weakening it and making the proton easier to release. Second, and often more dramatically, for the same central element in different oxidation states, acid strength increases sharply with the number of additional (non-hydrogen-bearing) oxygen atoms attached — compare hypochlorous acid (HClO, chlorine oxidation state +1) with perchloric acid (HClO4, chlorine oxidation state +7): each additional highly electronegative oxygen atom withdraws further electron density from the O-H bond and also stabilises the resulting anion by spreading its negative charge across more oxygen atoms, both effects favouring proton loss and making perchloric acid one of the strongest common acids while hypochlorous acid is comparatively weak.