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An oxide is a type of chemical which contains oxygen bonded to another element. The terminology is usually used to refer to inorganic compounds, where the oxygen is not part of a polyatomic anion.


All metals form oxides. Lower oxides of transition metals tend to have color to them, while higher transition metal oxides are black due to a nonstoichiometric composition, or white due to the lack of d-shell electrons. Other metal oxides are often white due to the complete lack of d-shell electrons. Most metal oxides are basic, and will combine with water to form hydroxides. Few of these, however, are soluble in water, save for the oxides of alkali metals and thallium(I). Some metal oxides are amphoteric, meaning that they can be both acidic and basic. Aluminum oxide is the most common example. Some higher oxides of metals are acidic and will form oxoacids or oxoacid anions. Some common examples are chromium(VI) oxide, which forms chromic acid when dissolved, and chromates with many other metal ions, and manganese heptoxide, which forms permanganates. More commonly, however, they form with heavier transition metals, such as tantalum and tungsten.

Some nonmetals form oxides, which tend to be a gas at normal temperatures, though some are solid or liquid. Most nonmetal oxides are acidic, and many will combine with water to form an acid. Most nonmetal oxides are soluble in water. A few, however, are insoluble, notably carbon dioxide and sulfur dioxide. These can both be captured by basic solutions and stored as carbonates and sulfites, respectively.

When considered 'ions', an oxide has -2 charge per oxygen, except in peroxides, where both oxygen atoms have a charge of -1 each. Superoxide ions contain two oxygen atoms with an overall charge of -1, forming a radical. One example of a metal oxide is copper(II) oxide, with the formula CuO. All metals can form oxides, but some form oxides more easily than others. For example: rust forms on iron easily, but gold is not normally oxidized by the oxygen in the air.