Oxides and oxoacids

P₄O₆ (4) and P₄O₁₀ (5) can be obtained through direct reaction of the elements, the PV compound 'phosphorus pentoxide' being the normal result when phosphorus burns in air. Under carefully controlled circumstances intermediate oxides P₄O_n (n=7, 8, 9) can be complete. The oxides of As and Sb have polymeric structures, and involve a mixed valency compound Sb₂O₄ with Sb^III within pyramidal coordination and octahedral Sb^V.

P⁴O¹⁰ is a very powerful dehydrating agent, that is reacting with water to form phosphoric acid H₃PO₄. This is a weak tribasic acid with consecutive acidity constants exemplifying Pauling's rules: pK₁=2.15, pK₂=7.20 and pK₃= 12.37. Neutral solutions include about equal concentrations of H₂PO₄^-and HPO₄^2-and are extensively used as buffers. A wide range of metal orthophosphates, consisting of ions with each feasible stage of deprotonation, are known. More addition of P₄O₁₀ to concentrated phosphoric acid results in the formation polyphosphates with P-OP linkages like in silicates. These linkages are kinetically stable in aqueous solution and are significant in biology. Metaphosphates like KPO₃ have infinite chains of corner-sharing octahedra like within the isoelectronic metasilicates like CaSiO₃.

The P^III oxoacid phosphorous acid H₃PO₃ does not comprise the structure P(OH)₃ that its formula suggests, except is tetrahedral with a PH bond: HPO(OH)₂. So It is diprotic with a identical pK₁ to phosphoric acid. The tendency is constant with hypophosphorous acid H₂PO(OH). Both the acids are strong reducing agents.

Arsenic acid H₃AsO₄ is identical to phosphoric acid but is a comparatively strong oxidizing agent. Sb^V oxo compounds have dissimilar structures and are based on the octahedral [Sb(OH)₆]^- ion. An Aqueous As^III and Sb^III species are tough to characterize; they are very much more weakly acidic than phosphorous acid and are possibly derived from As(OH)₃ and Sb (OH)₃. The subsequent salts tend to have polymeric structures, for instance, NaAsO₂ with oxygen linked [-As(O^-) -O]_∞ chains isoelectronic with SeO₂.