oxide

oxide, any of a large and important class of chemical compounds in which oxygen is combined with another element. With the exception of the lighter inert gases (helium [He], neon [Ne], argon [Ar], and krypton [Kr]), oxygen (O) forms at least one binary oxide with each of the elements.

Both metals and nonmetals can attain their highest oxidation states (i.e., donate their maximum number of available valence electrons) in compounds with oxygen. The alkali metals and the alkaline earth metals, as well as the transition metals and the posttransition metals (in their lower oxidation states), form ionic oxides—i.e., compounds that contain the O²⁻ anion. Metals with high oxidation states form oxides whose bonds have a more covalent nature. Nonmetals also form covalent oxides, which are usually molecular in character. A smooth variation from ionic to covalent in the type of bonding in oxides is observed as the periodic table is traversed from the metals on the left to the nonmetals on the right. This same variation is observed in the reaction of oxides with water and the resulting acid-base character of the products. Ionic metal oxides react with water to give hydroxides (compounds containing the OH⁻ ion) and resultant basic solutions, whereas most nonmetal oxides react with water to form acids and resultant acidic solutions (see the table).

Certain organic compounds react with oxygen or other oxidizing agents to produce substances called oxides. Thus, amines, phosphines, and sulfides form amine oxides, phosphine oxides, and sulfoxides, respectively, in which the oxygen atom is covalently bonded to the nitrogen, phosphorus, or sulfur atom. The so-called olefin oxides are cyclic ethers.

Metal oxides

Metal oxides are crystalline solids that contain a metal cation and an oxide anion. They typically react with water to form bases or with acids to form salts.

The alkali metals and alkaline earth metals form three different types of binary oxygen compounds: (1) oxides, containing oxide ions, O²⁻, (2) peroxides, containing peroxide ions, O₂²⁻, which contain oxygen-oxygen covalent single bonds, and (3) superoxides, containing superoxide ions, O₂⁻, which also have oxygen-oxygen covalent bonds but with one fewer negative charge than peroxide ions. Alkali metals (which have a +1 oxidation state) form oxides, M₂O, peroxides, M₂O₂, and superoxides, MO₂. (M represents a metal atom.) The alkaline earth metals (with a +2 oxidation state) form only oxides, MO, and peroxides, MO₂. All the alkali metal oxides can be prepared by heating the corresponding metal nitrate with the elemental metal. 2MNO₃ + 10M + heat → 6M₂O + N₂ A general preparation of the alkaline earth oxides involves heating the metal carbonates. MCO₃ + heat → MO + CO₂ Both alkali metal oxides and alkaline earth metal oxides are ionic and react with water to form basic solutions of the metal hydroxide. M₂O + H₂O → 2MOH (where M = group 1 metal)
MO + H₂O → M(OH)₂ (where M = group 2 metal) Thus, these compounds are often called basic oxides. In accord with their basic behaviour, they react with acids in typical acid-base reactions to produce salts and water; for example, M₂O + 2HCl → 2MCl + H₂O (where M = group 1 metal). These reactions are also often called neutralization reactions. The most important basic oxides are magnesium oxide (MgO), a good thermal conductor and electrical insulator that is used in firebrick and thermal insulation, and calcium oxide (CaO), also called quicklime or lime, used extensively in the steel industry and in water purification.

Periodic trends of the oxides have been thoroughly studied. In any given period, the bonding in oxides progresses from ionic to covalent, and their acid-base character goes from strongly basic through weakly basic, amphoteric, weakly acidic, and finally strongly acidic. In general, basicity increases down a group (e.g., in the alkaline earth oxides, BeO < MgO < CaO < SrO < BaO). Acidity increases with increasing oxidation number of the element. For example, of the five oxides of manganese, MnO (in which manganese has an oxidation state of +2) is the least acidic and Mn₂O₇ (which contains Mn⁷⁺) the most acidic. Oxides of the transition metals with oxidation numbers of +1, +2, and +3 are ionic compounds consisting of metal ions and oxide ions. Those transition metal oxides with oxidation numbers +4, +5, +6, and +7 behave as covalent compounds containing covalent metal-oxygen bonds. As a general rule, the ionic transition metal oxides are basic. That is, they will react with aqueous acids to form solutions of salts and water; for example, CoO + 2H₃O⁺ → Co²⁺ + 3H₂O. The oxides with oxidation numbers of +5, +6, and +7 are acidic and react with solutions of hydroxide to form salts and water; for example, CrO₃ + 2OH^- → CrO₄²⁻ + H₂O. Those oxides with +4 oxidation numbers are generally amphoteric (from Greek amphoteros, “in both ways”), meaning that these compounds can behave either as acids or as bases. Amphoteric oxides dissolve not only in acidic solutions but also in basic solutions. For example, vanadium oxide (VO₂) is an amphoteric oxide, dissolving in acid to give the blue vanadyl ion, [VO]²⁺, and in base to yield the yellow-brown hypovanadate ion, [V₄O₉]²⁻. Amphoterism among the main group oxides is primarily found with the metalloidal elements or their close neighbours.