Dioxides

Dioxides are a class of chemical compounds in which two oxygen atoms are bonded to a central element, commonly written with the general formula MO2. In many cases these oxides occur as discrete molecules, as with carbon dioxide (CO2), while in others they form extended networks or crystalline solids, such as silicon dioxide (SiO2). The central element can be a nonmetal or a metal, and the oxidation state is often high in dioxides of metals (for example TiO2, SnO2, ZrO2).

Common examples include carbon dioxide (CO2), a gaseous product of respiration and combustion that also dissolves in water to form carbonic acid; sulfur dioxide (SO2), a gas released by volcanic activity and during burning of sulfur-containing fuels; titanium dioxide (TiO2) and silicon dioxide (SiO2), which occur as solids and have wide industrial uses. Tin(IV) oxide (SnO2) and zirconium dioxide (ZrO2) are important in sensors, catalysts, ceramics, and pigments. Germanium dioxide (GeO2) also exists as a solid oxide. SiO2 is a major component of quartz, sand, and glass, whereas TiO2 is valued as a white pigment and in photocatalysis.

Properties and chemistry vary across dioxides. Some form volatile molecular species (e.g., CO2, SO2); others are high-melting, network or crystalline solids (e.g., SiO2, TiO2). Acid-base behavior is common in solution for those that react with water to form acids or acid anhydrides. Environmental and industrial relevance differs widely: CO2 is a greenhouse gas and a carbon source in biology, while SO2 is a pollutant and preservative in food and drink.