06.Periodicity of Behavior;
       Sodium Through Argon
  Previous PageNext Page
       The Acidic Nonmetals: P, S, and Cl

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 of progress.

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.


  Page 29 of 48 HomeGlossary