trioxide

A trioxide is a chemical oxide that contains three oxygen atoms per formula unit. The term is used for a variety of inorganic compounds in which three oxygens are bound in the oxide portion of the formula, though their structures and physical states can differ widely. Notable examples include sulfur trioxide (SO3), chromium trioxide (CrO3), dinitrogen trioxide (N2O3), and arsenic trioxide (As2O3). Some phosphorus oxides are also described as trioxides, but their stoichiometry is often represented as P2O3 or P4O6.

Sulfur trioxide is a highly reactive oxide that exists in several forms and as a gas at elevated temperatures. It readily reacts with water to form sulfuric acid (H2SO4) and is a key intermediate in the industrial production of sulfuric acid through the contact process. Chromium trioxide is a dense, orange solid and a very strong oxidizer. It is used commercially as an oxidizing agent and in chrome plating, but it is toxic and corrosive.

Dinitrogen trioxide is an unstable oxide of nitrogen that forms by association of NO and NO2 and exists in equilibrium with NO2 and N2O4. It is the anhydride of nitrous acid (HNO2) and is not appreciably stable at room temperature. Arsenic trioxide is a solid that occurs naturally as arsenolite; it is highly toxic and has historically been used as a poison, while today it also has medical applications in certain cancer therapies.

In summary, trioxides span diverse chemistries and applications, illustrating how three oxygen atoms can define a family of distinct compounds.