An oxide is a compound in which one or more oxygen atoms are bonded to another type of atom, often a metal. In oxidative reactions, oxygen combines chemically with another substance.

In nonmetal oxides, the bonding is primarily covalent (characterized by the sharing of electrons)rather than ionic (accomplished as a result of opposite charges). Many familiar nonmetallic oxides, including carbon monoxide, carbon dioxide, and sulfur dioxide, exist as molecules. Most nonmetals can form more than one oxide. Normally the more highly oxidized oxide forms when the nonmetal burns at ordinary temperatures in an abundance of oxygen, allowing the maximum number of oxygens to bond, e.g., CO₂. High temperatures and/or little oxygen favor formation of the less oxidized oxide, e.g., CO. Many nonmetal oxides react with water to form acids; these substances are called acid anhydrides.

When oxygen reacts with metal, the reaction product is...