Protons, Hydrogen Ions and Hydronium Ions

The hydrogen atom is the first element in the Periodic Table of the Elements. Although there are traces of other isotopes, most hydrogen consists of one proton with a single orbiting electron. Ionization occurs when the electron is stripped, leaving a naked proton. This proton is considered to be an ion of hydrogen. It carries a charge of +1. Cosmic rays include a high percentage of hydrogen ions, accelerated to great speeds. Naked protons are well suited for particle-acceleration experiments.

Another Form of Hydrogen Ion

Although it is technically the case that the term hydrogen ion refers only to a naked proton, when the chemist speaks of a hydrogen ion or a proton, he is usually referring to the complex formed by combining a proton with a molecule of water--actually an hydronium ion (or more appropriately, an oxonium ion), H₃O⁺. The hydronium ion possesses trigonal pyramid symmetry, which means it is shaped like a tripod. The angles between the hydrogen legs are approximately 113 degrees, so the "legs" are pretty far apart and the tripod is nearly flat. Intriguingly, the single positive charge is not restricted to the one hydrogen atom, but is shared by all three available hydrogen atoms.

What Stabilizes the Hydronium Ion

The size of the hydronium ion is much larger than the radius of a proton. By spreading the charge out over the entire ion, charge density is effectively decreased, since charge density may be defined as the charge divided by either the total surface area or the total volume of the particle. Decreasing the charge density stabilizes the structure. Besides, the hydronium ion coordinates with additional surrounding water molecules, adding yet further stabilization.

Hydrogen and Acids

Acids are substances that react with bases to form more neutral compounds called salts. The mineral acids, including hydrochloric and sulfuric acids, are among the acids containing ionizable hydrogen. When added to a solution of, for instance, sodium hydroxide, the hydrogen ion (hydronium ion) departs to combine hydroxyl group. That is, H₃O⁺ reacts with OH^- to form two molecules of water. Also, the sodium ion combines with the chloride ion to form ordinary table salt NaCl. Mineral acids also react with metals and other substances. When an acid reacts with a metal, the metal's valence electrons react with the hydrogen ion to form elemental hydrogen gas.

Pittsburgh Super Computing Center: Getting the Jump on Superacids

Elmhurst College -Virtual Chembook: Acids

City Collegiate: Hydrides

California Institute of Technology: Cosmic Rays