Oxide Definition and Examples

Copper oxide crystals
Some oxides are gases, but others (like copper oxide) are solids.

Joao Paulo Burini / Getty Images 

An oxide is an ion of oxygen with oxidation state equal to -2 or O2-. Any chemical compound that contains O2- as its anion is also termed an oxide. Some people more loosely apply the term to refer to any compound where oxygen serves as the anion. Metal oxides (e.g., Ag2O, Fe2O3) are the most abundant form of oxides, accounting for most of the mass of the Earth's crust. These oxides form when metals react with oxygen from air or water. While metal oxides are solids at room temperature, gaseous oxides also form. Water is an oxide that is a liquid under normal temperature and pressure. Some of the oxides found in air are nitrogen dioxide (NO2), sulfur dioxide (SO2), carbon monoxide (CO), and carbon dioxide (CO2).

Key Takeaways: Oxide Definition and Examples

  • An oxide refers either to the 2- oxygen anion (O2-) or to a compound that contains this anion.
  • Examples of common oxides include silicon dioxide (SiO2), iron oxide (Fe2O3), carbon dioxide (CO2), and aluminum oxide (Al2O3).
  • Oxides tend to be solids or gases.
  • Oxides naturally form when oxygen from air or water reacts with other elements.

Oxide Formation

Most elements form oxides. Noble gases can form oxides, but do so rarely. Noble metals resist combination with oxygen, but will form oxides under lab conditions. Natural formation of oxides involves either oxidation by oxygen or else hydrolysis. When elements burn in an oxygen-rich environment (such as metals in the thermite reaction), they readily yield oxides. Metals also react with water (especially the alkali metals) to yield hydroxides. Most metal surfaces are coated with a mixture of oxides and hydroxides. This layer often passivates the metal, slowing further corrosion from exposure to oxygen or water. Iron in dry air forms iron(II) oxide, but hydrated ferric oxides (rust), Fe2O3-x(OH)2x, form when both oxygen and water are present.


A compound containing the oxide anion may simply be called an oxide. For example, CO and CO2 are both carbon oxides. CuO and Cu2O are copper(II) oxide and copper(I) oxide, respectively. Alternatively, the ratio between the cation and oxygen atoms may be used for naming. The Greek numerical prefixes are used for naming. So, water or H2O is dihydrogen monoxide. CO2 is carbon dioxide. CO is carbon dioxide.

Metal oxides may also be named using the -a suffix. Al2O3, Cr2O3, and MgO are, respectively, alumina, chromia, and magnesia.

Special names are applied to oxides based on comparing lower and higher oxygen oxidation states. Under this naming, O22- is peroxide, while O2- is superoxide. For example, H2O2 is hydrogen peroxide.


Metal oxides often form structures similar to polymers, where the oxide links three or six metal atoms together. Polymeric metal oxides tend to be insoluble in water. Some oxides are molecular. These include all of the simple oxides of nitrogen, as well as carbon monoxide and carbon dioxide.

What Is Not an Oxide?

In order to be an oxide, the oxidation state of oxygen must be -2 and the oxygen must act as an anion. The following ions and compounds are not technically oxides because they don't meet these criteria:

  • Oxygen difluoride (OF2): Fluorine is more electronegative than oxygen, so it acts as the cation (O2+) rather than the anion in this compound.
  • Dioxygenyl (O2+) and its compounds: Here, the oxygen atom is in the +1 oxidation state.


  • Chatman, S.; Zarzycki, P.; Rosso, K. M. (2015). "Spontaneous Water Oxidation at Hematite (α-Fe2O3) Crystal Faces". ACS Applied Materials & Interfaces. 7 (3): 1550–1559. doi:10.1021/am5067783
  • Cornell, R. M.; Schwertmann, U. (2003). The Iron Oxides: Structure, Properties, Reactions, Occurrences and Uses (2nd ed.). doi:10.1002/3527602097. ISBN 9783527302741.
  • Cox, P.A. (2010). Transition Metal Oxides. An Introduction to Their Electronic Structure and Properties. Oxford University Press. ISBN 9780199588947.
  • Greenwood, N. N.; Earnshaw, A. (1997). Chemistry of the Elements (2nd ed.). Oxford:Butterworth-Heinemann. ISBN 0-7506-3365-4.
  • IUPAC (1997). Compendium of Chemical Terminology (2nd ed.) (the "Gold Book"). Compiled by A. D. McNaught and A. Wilkinson. Blackwell Scientific Publications, Oxford. 
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Helmenstine, Anne Marie, Ph.D. "Oxide Definition and Examples." ThoughtCo, Aug. 29, 2020, thoughtco.com/definition-of-oxide-605457. Helmenstine, Anne Marie, Ph.D. (2020, August 29). Oxide Definition and Examples. Retrieved from https://www.thoughtco.com/definition-of-oxide-605457 Helmenstine, Anne Marie, Ph.D. "Oxide Definition and Examples." ThoughtCo. https://www.thoughtco.com/definition-of-oxide-605457 (accessed April 18, 2021).