All three ions are stabilized by delocalization, which makes the
acids stronger than we might expect. Silicic acid, lacking this
delocalization, is extremely weak.
Acidity increases in the series from P to S to Cl, because of the
increasing electronegativity of the central atom. In aqueous solutions
of the two strongest acids, perchloric and sulfuric, one proton
is completely dissociated:
The bisulfate ion can dissociate again, but does so to a smaller
extent because this requires the removal of a positive charge from
an entity that already has one negative charge:
The weaker phosphoric acid loses its first proton with roughly
the same reluctance as sulfuric acid loses its second proton:
The second and third dissociations of phosphoric acid are even
weaker. Like carbonic acid, phosphoric acid is weak enough to be
used in soft drinks.
In an earlier era, every soda fountain offered "phosphates,"
in which a small amount of phosphoric acid was added to the carbonic
acid to give a special tang to the drink.
Orange phosphates, regrettably, have become one of the casualties
The second- and third-row oxyacids show "diagonality"
in their chemical properties, with each acid in the second row being
most like the one below it and to the right:
Boric and silicic acids are not strengthened by delocalization
of electrons in their ions, and are so weak that they seldom are
thought of as acids at all.
Carbonic and phosphoric acids are so weak that only the first proton
dissociation is important, and both are weak enough for use in beverages.
Nitric and sulfuric are very strong, and are the two most common
laboratory acids. Perchloric acid is the strongest of all.
The elements that form strong oxyacids, N, P, S, and Cl, also can
form weaker acids that have fewer oxygen atoms present in the molecules.