Oxide, a chemical compound consisting of oxygen and one other element. Oxygen will combine with all other elements with the exception of some of the noble gases. Many elements form more than one oxide; for example, both carbon monoxide (which has the chemical formula CO) and carbon dioxide (CO2) are oxides of carbon. A monoxide has one oxygen atom in each of its molecules, and a dioxide has two oxygen atoms per molecule. Other common oxides are trioxides (three oxygen atoms per molecule), tetroxides (four oxygen atoms per molecule), and pentoxides (five oxygen atoms per molecule).

Oxides vary in stability, form, abundance, and characteristics such as color, odor, and taste. Water (H2O) is a monoxide of hydrogen and is a stable liquid. Nitrogen dioxide (NO2) is highly poisonous gas. Magnesium oxide (MgO) is a solid that can withstand temperatures of more than 5,000 F. (2,760 C.) without melting.

Some oxides are extremely common among the rocks and minerals of the earth; for example, aluminum oxide (Al2O3) is a major constituent of clay, and silicon dioxide (SiO2) occurs as quartz. Other oxides are formed only in the laboratory.

The term oxide is also used to designate a compound consisting of oxygen and a radical. (A radical is a group of atoms of two or more elements that acts as a single atom during a series of chemical reactions.) Diethyl oxide, an ether that has the formula (C2H5)2O, is a compound of this type.


In every oxide mentioned thus far, the oxygen atom gains two electrons from the other atom or radical. An element whose atoms gain two electrons in a chemical reaction is said to have a valence of —2.

There are certain oxides in which two oxygen atoms are bound together and exhibit a combined valence of —2, so that the oxygen, in effect, has a valence of —1. Oxides of this type are called peroxides. The best-known peroxide is hydrogen peroxide, which has the formula H2O2. Sodium peroxide (Na2O2) and barium peroxide (BaO2) are other examples of peroxides.