Oxide
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An oxide () is a chemical compound containing at least one oxygen atom and one other element in its chemical formula. "Oxide" itself is the dianion (anion bearing a net charge of −2) of oxygen, an O2− ion with oxygen in the oxidation state of −2. Most of the Earth's crust consists of oxides. Even materials considered pure elements often develop an oxide coating. For example, aluminium foil develops a thin skin of (called a passivation layer) that protects the foil from further oxidation.Greenwood, N. N.; & Earnshaw, A. (1997). Chemistry of the Elements (2nd Edn.), Oxford:Butterworth-Heinemann. .


Stoichiometry

Oxides are extraordinarily diverse in terms of stoichiometries (the measurable relationship between reactants and chemical equations of an equation or reaction) and in terms of the structures of each stoichiometry. Most elements form oxides of more than one stoichiometry. A well known example is carbon monoxide and carbon dioxide.Greenwood, N. N.; & Earnshaw, A. (1997). Chemistry of the Elements (2nd Edn.), Oxford:Butterworth-Heinemann. . This applies to ''binary'' oxides, that is, compounds containing only oxide and another element. Far more common than binary oxides are oxides of more complex stoichiometries. Such complexity can arise by the introduction of other cations (a positively charged ion, i.e. one that would be attracted to the cathode in electrolysis) or other anions (a negatively charged ion). Iron silicate, Fe2SiO4, the mineral fayalite, is one of many examples of a ternary oxide. For many metal oxides, the possibilities of polymorphism and nonstoichiometry exist as well. The commercially important dioxides of titanium exists in three distinct structures, for example. Many metal oxides exist in various nonstoichiometric states. Many molecular oxides exist with diverse ligands as well. For simplicity sake, most of this article focuses on binary oxides.


Formation

Oxides are associated with all elements except a few noble gases. The pathways for the formation of this diverse family of compounds are correspondingly numerous.


Metal oxides

Many metal oxides arise by decomposition of other metal compounds, e.g. carbonates, hydroxides, and nitrates. In the making of calcium oxide, calcium carbonate (
limestone Limestone is a type of carbonate rock, carbonate sedimentary rock which is the main source of the material Lime (material), lime. It is composed mostly of the minerals calcite and aragonite, which are different Polymorphism (materials science) ...
) breaks down upon heating, releasing carbon dioxide: :CaCO3 -> CaO + CO2 The reaction of elements with oxygen in air is a key step in corrosion relevant to the commercial use of iron especially. Almost all elements form oxides upon heating with oxygen atmosphere. For example, zinc powder will burn in air to give
zinc oxide Zinc oxide is an inorganic compound with the Chemical formula, formula . It is a white powder which is insoluble in water. ZnO is used as an additive in numerous materials and products including cosmetics, Zinc metabolism, food supplements, rubbe ...
: :2 Zn + O2 -> 2 ZnO The production of metals from ores often involves the production of oxides by roasting (heating) metal sulfide minerals in air. In this way, ( molybdenite) is converted to molybdenum trioxide, the precursor to virtually all molybdenum compounds: :2 MoS2 + 7 O2 -> 2MoO3 + 4 SO2 Noble metals (such as
gold Gold is a chemical element; it has chemical symbol Au (from Latin ) and atomic number 79. In its pure form, it is a brightness, bright, slightly orange-yellow, dense, soft, malleable, and ductile metal. Chemically, gold is a transition metal ...
and
platinum Platinum is a chemical element; it has Symbol (chemistry), symbol Pt and atomic number 78. It is a density, dense, malleable, ductility, ductile, highly unreactive, precious metal, precious, silverish-white transition metal. Its name origina ...
) are prized because they resist direct chemical combination with oxygen.


Non-metal oxides

Important and prevalent nonmetal oxides are carbon dioxide and carbon monoxide. These species form upon full or partial oxidation of carbon or hydrocarbons. With a deficiency of oxygen, the monoxide is produced: :2 CH4 + 3 O2 -> 2 CO + 4 H2O :2 C + O2 -> 2 CO With excess oxygen, the dioxide is the product, the pathway proceeds by the intermediacy of carbon monoxide: :CH4 + 2 O2 -> CO2 + 2 H2O :C + O2 -> CO2 Elemental nitrogen () is difficult to convert to oxides, but the combustion of ammonia gives nitric oxide, which further reacts with oxygen: :4 NH3 + 5 O2 -> 4 NO + 6 H2O :2 NO + O2 -> 2 NO2 These reactions are practiced in the production of
nitric acid Nitric acid is an inorganic compound with the formula . It is a highly corrosive mineral acid. The compound is colorless, but samples tend to acquire a yellow cast over time due to decomposition into nitrogen oxide, oxides of nitrogen. Most com ...
, a commodity chemical. The chemical produced on the largest scale industrially is sulfuric acid. It is produced by the oxidation of sulfur to
sulfur dioxide Sulfur dioxide (IUPAC-recommended spelling) or sulphur dioxide (traditional Commonwealth English) is the chemical compound with the formula . It is a colorless gas with a pungent smell that is responsible for the odor of burnt matches. It is r ...
, which is separately oxidized to sulfur trioxide: :S + O2 -> SO2 :2 SO2 + O2 -> 2 SO3 Finally the trioxide is converted to sulfuric acid by a hydration reaction: :SO3 + H2O -> H2SO4


Structure

Oxides have a range of structures, from individual molecules to
polymer A polymer () is a chemical substance, substance or material that consists of very large molecules, or macromolecules, that are constituted by many repeat unit, repeating subunits derived from one or more species of monomers. Due to their br ...
ic and crystalline structures. At standard conditions, oxides may range from solids to gases. Solid oxides of metals usually have polymeric structures at ambient conditions.


Molecular oxides

File:Carbon-dioxide-2D-dimensions.svg,
Carbon dioxide Carbon dioxide is a chemical compound with the chemical formula . It is made up of molecules that each have one carbon atom covalent bond, covalently double bonded to two oxygen atoms. It is found in a gas state at room temperature and at norma ...
is the main product of fossil fuel combustion. File:Carbon monoxide 2D.svg, Carbon monoxide is the product of the incomplete combustion of carbon-based fuels and a precursor to many useful chemicals. File:Nitrogen-dioxide-2D-dimensions-vector.svg, Nitrogen dioxide is a problematic pollutant from internal combustion engines. File:Sulfur-dioxide-2D.svg,
Sulfur dioxide Sulfur dioxide (IUPAC-recommended spelling) or sulphur dioxide (traditional Commonwealth English) is the chemical compound with the formula . It is a colorless gas with a pungent smell that is responsible for the odor of burnt matches. It is r ...
, the principal oxide of sulfur, is emitted from volcanoes. File:Nitrous-oxide-2D-dimensions.png, Nitrous oxide ("laughing gas") is a potent greenhouse gas produced by soil bacteria.
Although most metal oxides are crystalline solids, many non-metal oxides are molecules. Examples of molecular oxides are carbon dioxide and carbon monoxide. All simple oxides of nitrogen are molecular, e.g., NO, N2O, NO2 and N2O4. Phosphorus pentoxide is a more complex molecular oxide with a deceptive name, the real formula being P4O10. Tetroxides are rare, with a few more common examples being ruthenium tetroxide, osmium tetroxide, and xenon tetroxide.


Reactions


Reduction

Reduction of metal oxide to the metal is practiced on a large scale in the production of some metals. Many metal oxides convert to metals simply by heating ( thermal decomposition). For example, silver oxide decomposes at 200 °C: : 2 Ag2O -> 4 Ag + O2 Most often, however, metal oxides are reduced by a chemical reagent. A common and cheap reducing agent is carbon in the form of coke. The most prominent example is that of iron ore smelting. Many reactions are involved, but the simplified equation is usually shown as: : 2 Fe2O3 + 3 C -> 4 Fe + 3 CO2 Some metal oxides dissolve in the presence of reducing agents, which can include organic compounds. Reductive dissolution of ferric oxides is integral to
geochemical Geochemistry is the science that uses the tools and principles of chemistry to explain the mechanisms behind major geological systems such as the Earth's crust and its oceans. The realm of geochemistry extends beyond the Earth, encompassing the ...
phenomena such as the iron cycle.


Hydrolysis and dissolution

Because the M-O bonds are typically strong, metal oxides tend to be insoluble in solvents, though they may be attacked by aqueous acids and bases. Dissolution of oxides often gives oxyanions. Adding aqueous base to gives various
phosphate Phosphates are the naturally occurring form of the element phosphorus. In chemistry, a phosphate is an anion, salt, functional group or ester derived from a phosphoric acid. It most commonly means orthophosphate, a derivative of orthop ...
s. Adding aqueous base to gives polyoxometalates. Oxycations are rarer, some examples being nitrosonium (), vanadyl (), and uranyl (). Many compounds are known with both oxides and other groups. In
organic chemistry Organic chemistry is a subdiscipline within chemistry involving the science, scientific study of the structure, properties, and reactions of organic compounds and organic matter, organic materials, i.e., matter in its various forms that contain ...
, these include ketones and many related carbonyl compounds. For the transition metals, many oxo complexes are known as well as oxyhalides.


Nomenclature and formulas

The chemical formulas of the oxides of the
chemical element A chemical element is a chemical substance whose atoms all have the same number of protons. The number of protons is called the atomic number of that element. For example, oxygen has an atomic number of 8: each oxygen atom has 8 protons in its ...
s in their highest oxidation state are predictable and are derived from the number of valence electrons for that element. Even the chemical formula of O4, tetraoxygen, is predictable as a group 16 element. One exception is copper, for which the highest oxidation state oxide is copper(II) oxide and not copper(I) oxide. Another exception is fluoride, which does not exist as one might expect—as F2O7—but as OF2.


See also

* Other oxygen ions: ozonide (), superoxide (), peroxide () and dioxygenyl (). * Suboxide * Oxohalide * Oxyanion * Complex oxide * See :Template:Oxides for a list of oxides. *
Salt In common usage, salt is a mineral composed primarily of sodium chloride (NaCl). When used in food, especially in granulated form, it is more formally called table salt. In the form of a natural crystalline mineral, salt is also known as r ...
* Wet electrons


References

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