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.

Certain organic compounds (organic compound) react with oxygen or other oxidizing agents to produce substances called oxides. Thus, amines (amine), phosphines (phosphine), and sulfides (sulfide) 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 (ether).

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

The alkali metals (alkali metal) and alkaline-earth metals (alkaline-earth metal) form three different types of binary oxygen compounds: (1) oxides, containing oxide ions, O²⁻, (2) peroxides (peroxide), 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.

Periodic (periodic law) 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 (transition element) 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,

All nonmetals form covalent oxides with oxygen, which react with water to form acids or with bases to form salts. Most nonmetal oxides are acidic and form oxyacids (oxyacid), which in turn yield hydronium ions (H₃O⁺) in aqueous solution. There are two general statements that describe the behaviour of acidic oxides. First, oxides such as sulfur trioxide (SO₃) and dinitrogen pentoxide (N₂O₅), in which the nonmetal exhibits one of its common oxidation numbers, are known as acid anhydrides. These oxides react with water to form oxyacids, with no change in the oxidation number of the nonmetal; for example,

When a mixture of equal parts of nitric oxide and nitrogen dioxide, NO₂, is cooled to −21 °C (−6 °F), the gases form dinitrogen trioxide, a blue liquid consisting of N₂O₃ molecules. This molecule exists only in the liquid and solid states. When heated, it forms a mixture of NO and NO₂. Nitrogen dioxide is prepared commercially by oxidizing NO with air, but it can be prepared in the laboratory by heating the nitrate of a heavy metal, as in the following equation,

Dinitrogen pentoxide, N₂O₅, is a white solid formed by the dehydration of nitric acid by phosphorus(V) oxide.

Nitrogen dioxide reacts with water in one of two ways. In cold water NO₂ disproportionates to form a mixture of HNO₂ and HNO₃, whereas at higher temperatures HNO₃ and NO are formed. In their chemical activity, the nitrogen oxides undergo extensive oxidation-reduction reactions (oxidation–reduction reaction). Nitrous oxide resembles oxygen in its behaviour when heated with combustible materials. It is a strong oxidizing agent that decomposes upon heating to form nitrogen and oxygen. Because one-third of the gas liberated is oxygen, nitrous oxide supports combustion better than air. All the nitrogen oxides are, in fact, good oxidizing agents. Dinitrogen pentoxide reacts violently with metals, nonmetals, and organic materials, as in the following reactions with potassium (K) and iodine gas (I₂).

Carbon monoxide is produced when graphite (one of the naturally occurring forms of elemental carbon) is heated or burned in a limited amount of oxygen. The reaction of steam with red-hot coke also produces carbon monoxide along with hydrogen gas (H₂). (Coke is the impure carbon residue resulting from the burning of coal.) This mixture of CO and H₂ is called water gas and is used as an industrial fuel. In the laboratory, carbon monoxide is prepared by heating formic acid, HCOOH, or oxalic acid, H₂C₂O₄, with concentrated sulfuric acid, H₂SO₄. The sulfuric acid removes the elements of water (i.e., H₂O) from the formic or oxalic acid and absorbs the water produced. Because carbon monoxide burns readily in oxygen to produce carbon dioxide,

Carbon monoxide is an extremely dangerous poison (carbon monoxide poisoning). Because it is an odourless and tasteless gas, it gives no warning of its presence. It binds to the hemoglobin in blood to form a compound that is so stable that it cannot be broken down by body processes. When the hemoglobin is combined with carbon monoxide, it cannot combine with oxygen; this destroys the ability of hemoglobin to carry essential oxygen to all parts of the body. Suffocation can occur if sufficient amounts of carbon monoxide are present to form complexes with the hemoglobin.

Carbon dioxide is produced when any form of carbon or almost any carbon compound is burned in an excess of oxygen. Many metal carbonates (carbonate) liberate CO₂ when they are heated. For example, calcium carbonate (CaCO₃) produces carbon dioxide and calcium oxide (CaO).

Carbon dioxide is a colourless and essentially odourless gas that is 1.5 times as dense as air. It is not toxic, although a large concentration could result in suffocation simply by causing a lack of oxygen in the body. All carbonated beverages contain dissolved CO₂, hence the name carbonated. One litre (1.06 quarts) of water at 20 °C (68 °F) dissolves 0.9 litre of CO₂ at one atmosphere, forming carbonic acid (H₂CO₃), which has a mildly acidic (sour) taste. Solid CO₂ sublimes at normal atmospheric pressure. Thus, solid CO₂, called Dry Ice, is a valuable refrigerant that is always free of the liquid form. Carbon dioxide is also used as a fire extinguisher, because most substances do not burn in it, and it is readily available and inexpensive. Air containing as little as 2.5 percent CO₂ extinguishes a flame.

The Earth's atmosphere contains approximately 0.04 percent carbon dioxide by volume and serves as a huge reservoir of this compound. The carbon dioxide content of the atmosphere has significantly increased in the last several years largely because of the burning of fossil fuels (fossil fuel). A so-called greenhouse effect can result from increased carbon dioxide and water vapour in the atmosphere. These gases allow visible light from the Sun to penetrate to the Earth's surface, where it is absorbed and reradiated as infrared radiation. This longer-wavelength radiation is absorbed by the carbon dioxide and water and cannot escape back into space. There is a growing concern that the resulting increased heat in the atmosphere could cause the Earth's average temperature to rise over a period of time. This change would have a serious impact on the environment, affecting climate, ocean levels, and agriculture. The solubility of carbon dioxide in water makes oceans and lakes significant sources of this gas. Atmospheric carbon dioxide is in dynamic equilibrium with that dissolved in water and with that bound primarily as carbonates in the Earth's crust. With sunlight and chlorophyll serving as a catalyst (i.e., a compound that increases the rate of a reaction without being consumed itself), green plants convert carbon dioxide and water into sugar and oxygen. This process is called photosynthesis and uses the energy of light.

Carbon suboxide, C₃O₂, is a foul-smelling lacrimatory (tear-stimulating) gas produced by the dehydration of malonic acid, CH₂(COOH)₂, with P₄O₁₀ in a vacuum at 140 to 150 °C (284 to 302 °F). Carbon suboxide is a linear symmetrical molecule whose structure can be represented as O=C=C=C=O. At 25 °C (77 °F) the compound is unstable and polymerizes to highly coloured solid products, but it is a stable molecule at −78 °C (−108.4 °F). Under the influence of ultraviolet light (in the process known as photolysis), C₃O₂ decomposes to the very reactive molecule ketene, C₂O. Since carbon suboxide is the acid anhydride of malonic acid, it reacts slowly with water to produce that acid.

The two common oxides of sulfur are sulfur dioxide, SO₂, and sulfur trioxide, SO₃. The pungent odour of burning sulfur is actually due to the sulfur dioxide that is produced. It occurs in volcanic gases and in the atmosphere near industrial plants that burn coal or oil containing sulfur compounds. Sulfur dioxide forms when these compounds react with oxygen during combustion. It is produced commercially by burning elemental sulfur and by roasting (heating in air) sulfide ores (ore) such as zinc sulfide, ZnS, iron(IV) sulfide, FeS₂, and copper(I) sulfide, Cu₂S. In the laboratory sulfur dioxide is conveniently prepared by the action of sulfuric acid (H₂SO₄) on either sulfite salts, which contain the SO₃²⁻ ion, or hydrogen sulfite salts, which contain the HSO₃⁻ ion. Sulfurous acid, H₂SO₃, is formed first, but it quickly decomposes into SO₂ and H₂O. Sulfur dioxide is also formed when many reducing agents react with hot, concentrated sulfuric acid. Sulfur trioxide slowly forms when SO₂ and O₂ are heated together.

Both SO₂ and SO₃ are gases at room temperature. In the vapour state, SO₃ exists as single molecules (monomers (monomer)), but in the solid state it can occur in several polymeric (polymer) forms. As expected, both of the sulfur oxides (sulfur oxide) are acidic oxides that react with water to form oxyacids (oxyacid). The moderately strong sulfurous acid is produced when sulfur dioxide reacts with water, and sulfuric acid, a strong acid, is formed in the reaction of sulfur trioxide with water. Sulfur trioxide dissolves readily in concentrated sulfuric acid to form pyrosulfuric acid, H₂S₂O₇, which is also called fuming sulfuric acid or oleum. The sulfur oxides react with many ionic metal oxides and hydroxides to form sulfites or hydrogen sulfites and sulfates or hydrogen sulfates, respectively. The sulfur atom in sulfur trioxide exhibits its maximum oxidation number of +6 and thus cannot be oxidized, while sulfur dioxide, whose sulfur atom has an oxidation number of +4, can be both oxidized and reduced.

Air pollution by sulfur oxides is a major environmental problem, with millions of tons of sulfur dioxide emitted into the atmosphere each year. This compound itself is harmful to plant and animal life, as well as to many building materials. Another problem of great concern is acid rain. Both sulfur oxides dissolve in atmospheric water droplets to form acidic solutions that can be very damaging when distributed in the form of rain. It is thought that sulfuric acid is the major cause of the acidity in acid rain, which can damage forests and cause fish to die off in many lakes. Acid rain is also corrosive to metals, limestone, and other materials. The possible solutions to this problem are expensive because of the difficulty of removing sulfur from coal and oil before they are burned.

As discussed previously, the alkali metals (alkali metal) as well as the alkaline-earth metals (alkaline-earth metal) form peroxides (peroxide). A number of other electropositive metals, such as the lanthanides, also form peroxides. These are intermediate in character between the ionic peroxides and the essentially covalent peroxides formed by metals such as zinc (Zn), cadmium (Cd), and mercury (Hg). The peroxide ion, O₂²⁻, has a single oxygen-oxygen covalent bond and an oxidation state of −1 on the oxygen atoms. The peroxide ion is a powerful hydrogen ion acceptor, making the peroxides of the alkali metals and alkaline-earth metals strong bases. Solutions of these peroxides are basic because of the reaction of the peroxide ion with water, which functions as a weak acid in this case.

The most important covalent peroxide is hydrogen peroxide, H₂O₂. When pure, this syrupy viscous liquid has a pale blue colour, although it appears almost colourless. Many of its physical properties resemble those of water. It has a larger liquid range than water, melting at −0.43 °C (31.2 °F) and boiling at 150.2 °C (302.4 °F), and it has a higher density (1.44 grams per cubic centimetre at 25 °C 【77 °F】) than water. The dielectric constant of pure H₂O₂ is, like that of water, quite high—70.7 at 25 °C, compared with a value of 78.4 for water at 25 °C. However, adding water, which is miscible in all proportions, causes the dielectric constant to increase to a maximum value of 121 at about 35 percent H₂O₂ and 65 percent H₂O. World production of H₂O₂ is well over one-half million tons per year, making it a major industrial chemical. Most industrial hydrogen peroxide is prepared by a well-conceived process introduced originally by IG Farbenindustrie (IG Farben) of Germany that uses only hydrogen and oxygen as raw materials. The process involves oxidation of 2-ethylanthraquinol to 2-ethylanthraquinone by passage of air through a solution of the quinol in an organic solvent. The hydrogen peroxide that is produced is extracted into water. The quinone is then reduced back to the quinol by hydrogen in the presence of palladium metal on an inert support. The process is thus a cyclic one. It can be shown by an examination of reduction potentials that aqueous solutions of hydrogen peroxide or the pure liquid should spontaneously decompose to water and oxygen.

In the superoxide ion, O₂⁻, the oxygen has an oxidation number of −¹/₂. The stability of metal superoxides depends on the size and the electropositive character of the metal. The larger the metal and the more electropositive it is, the greater the stability of its superoxide. Thus, potassium (K), rubidium (Rb), cesium (Cs), strontium (Sr), and barium (Ba) form stable superoxides when burned in oxygen. These compounds are yellow to orange paramagnetic solids. They are strong oxidizing agents that vigorously hydrolyze (hydrolysis) (react with water) to produce oxygen gas and hydroxide ions.