Oxygen

Oxygen is one of the most active of all chemical elements. The oxygen-oxygen bond in diatomic oxygen is relatively strong, but once broken, the atomic oxygen formed (O) reacts readily with the vast majority of elements. The noble gases and noble metals are the most important exceptions, although oxy compounds of most of these elements are also known and can be prepared by indirect methods.

The reaction between oxygen and another element generally results in the formation of a binary compound known as an oxide. The reaction itself is known as oxidation. For example, the oxidation reaction between oxygen and sodium produces sodium oxide. In many cases, an element may form more than one oxide. Copper, as an example, forms both copper(I) (cuprous) oxide and copper(II) (cupric) oxide. Nitrogen forms five oxides: nitrous oxide (N₂O), nitric oxide (NO), dinitrogen trioxide (N₂O₃), nitrogen dioxide (NO₂), and dinitrogen pentoxide (N₂O₅).

Perhaps the most important of all oxides is water, by far the most abundant compound on the planet. Water is composed of two hydrogen atoms bonded to a single oxygen atom by means of a strong covalent bond.

In many cases, the reaction between oxygen and another element is highly exothermic. One of the best known of such reactions is the one that takes place between carbon and oxygen, to form (usually) carbon dioxide and carbon monoxide. It is this reaction, which takes place when coal burns, that was responsible to a significant extent for the development of huge new energy sources during the Industrial Revolution that transformed human society.

Oxygen also reacts with a number of compounds. For example, hydrocarbons react with oxygen at high temperatures to form (primarily) carbon dioxide and water vapor. Oxidation that takes place very rapidly, usually at high temperatures, is known as combustion. The combustion of hydrocarbons in petroleum and natural gas has been another major source of energy in human civilization over the past 200 years.

Some forms of oxidation occur more slowly, without the production of noticeable heat or light. When plants and animals die, for example, the organic materials of which they are made slowly react with oxygen in the atmosphere. This form of oxidation is known as decay. The decay of organic matter is a highly complex chemical phenomenon, with a large variety of chemical products formed in the reaction.

Inorganic materials also react slowly with oxygen. When iron and certain other metals are exposed to oxygen (in the presence of water), they form oxides. The best known of all metallic oxides is probably rust, a hydrated form of iron(III) (ferric) oxide with the general formula Fe₂O₃ • nH₂O. The rusting of bridges, buildings, motor vehicles, tools, fences, and other structures is a major economic problem throughout the world. A number of techniques, such as galvanizing, tinning, painting, and enameling are used to reduce or prevent the rusting of materials.