Structure of Acids

The simplest type of acid has a molecule consisting of one atom of hydrogen and one atom of a nonmetallic element. A molecule of hydrochloric acid, for example, contains one atom of hydrogen and one atom of chlorine. Hydrochloric acid can be obtained by dissolving hydrogen chloride, a gas, in water. Both fndrochloric acid and hydrogen chloride have the chemical formula HCl, H being the symbol for hydrogen and Cl the symbol for chlorine.

Sodium chloride, one of the many chemical compounds classified as a salt, can be formed by combining metallic sodium (Na) with hydrochloric acid. The sodium replaces the hydrogen, as indicated by the chemical equation for this reaction:

2 Na + 2 HCl>2 NaCl + H2

Sulfur, nitrogen, carbon, phosphorus, fluorine, and arsenic are among the other common elements that combine with hydrogen, or with hydrogen and oxygen, to form acids. Most of the common acids can be prepared by the reaction of oxides of nonmetals with water. In contrast to hydrochloric acid and other simple acids, some acids contain dozens of atoms in each of their molecules. The largest nucleic-acid molecules each contain millions of atoms.