oxides

Oxides are chemical compounds composed of oxygen atoms bonded to another element. They form a large and diverse class of inorganic substances with general formulas such as MO or MxOy for binary oxides, and they include derivatives such as peroxides (containing O–O bonds) and superoxides. Oxides occur across the periodic table, including metals and nonmetals, and arise from oxidation processes in nature and industry.

Classification of oxides is often based on their acidic or basic character. Basic oxides, typically formed by metals, react with water to yield metal hydroxides (for example CaO + H2O → Ca(OH)2) and react with acids to form salts. Acidic oxides, usually nonmetals, react with water to produce acids (for example CO2 + H2O → H2CO3; SO3 + H2O → H2SO4). Amphoteric oxides, such as aluminum oxide (Al2O3) and zinc oxide (ZnO), react with both acids and bases. Some oxides, including certain transition-metal oxides, are amphoteric or display redox activity. Peroxides and superoxides form when oxygen is present as O2^2− or O2−, often reacting to release reactive oxygen species.

Properties of oxides vary widely. The oxygen often has an oxidation state of −2; bonding ranges from predominantly ionic in simple metal oxides to covalent in many nonmetal or network oxides. Structures include ionic lattices (Na2O), covalent networks (SiO2), and mixed oxides with complex crystal chemistry.

Occurrence and applications are broad. Oxides exist in minerals such as hematite (Fe2O3), quartz (SiO2), and corundum (Al2O3). Industrial uses include cement (CaO), glass and ceramics (SiO2), pigments and catalysts (TiO2, ZnO, SnO2), and semiconductor applications. Safety concerns vary with oxide dusts and specific compounds, as some oxides can be irritants or toxic.