oxide

 

chemical compound containing oxygen and one other chemical element. Oxides are widely and abundantly distributed in nature. Water is the oxide of hydrogen. Silicon dioxide is the major component of sand and quartz. Carbon dioxide is given off during respiration by animals and plants. Carbon monoxide, sulfur dioxide, and oxides of nitrogen are among the waste gases of gasoline-burning internal-combustion engines. Nitrous oxide is an oxide of nitrogen often called laughing gas. Many of the metals form oxides. Some metal oxides, e.g., those of iron, aluminum, tin, and zinc, are important as ores. Litharge and red lead are lead oxides used as pigments in paint. A number of elements, e.g., arsenic, carbon, manganese, nitrogen, phosphorous, and sulfur, combine with oxygen to form more than one oxide. The inert gases do not form oxides. The halogens and inactive metals do not combine directly with oxygen, but their oxides can be formed by indirect methods. Oxides are usually named according to the number of oxygen atoms present in a molecule, e.g., monoxide (or simply oxide), dioxide, trioxide. In a molecule of carbon monoxide, CO, for example, there is one oxygen atom; in carbon dioxide, CO₂, there are two; and in phosphorus pentoxide, P₂O₅, there are five. Oxides are commonly classified as acidic or basic oxides or anhydrides. Sulfur trioxide is an acid anhydride; it reacts with water to form sulfuric acid. Phosphorus pentoxide reacts vigorously with water to form phosphoric acid. Many metal oxides react with water to form alkaline hydroxides, e.g., calcium oxide (lime) reacts with water to form calcium hydroxide (slaked lime). Some metal oxides do not react with water but are basic in that they react with an acid to form a salt and water. Others exhibit amphoterism; i.e., they react with both acids and bases. Still others are neutral and nonreactive.

An oxide is a chemical compound containing at least one oxygen atom and other elements. Most of the earth's crust consists of oxides. Oxides result when elements are oxidized by air. Combustion of hydrocarbons affords the two principal oxides of carbon, carbon monoxide and carbon dioxide. Even materials that are considered to be pure elements often contain a coating of oxides. For example, aluminium foil has a thin skin of Al₂O₃ that protects the foil from further corrosion.

Most oxides are insoluble in water

The oxide ion, O²⁻, is the conjugate base of the hydroxide ion, OH⁻, and is encountered in ionic solid such as calcium oxide. O²⁻ is unstable in aqueous solution − its affinity for H⁺ is so great (pKb ~ -22) that it abstracts a proton from a solvent H₂O molecule:

O²⁻ + H₂O → 2 OH⁻

Although many anions are stable in aqueous solution, ionic oxides are not. For example, sodium chloride dissolves readily in water to give a solution containing the constituent ions, Na⁺ and Cl^-. Oxides do not behave like this. If an ionic oxide dissolves, the O²⁻ ions become protonated. Although Calcium oxide, CaO, is said to "dissolve" in water, the products include hydroxide:

CaO + H₂O → Ca²⁺ + 2 OH^-

In fact, no monoatomic dianion is known to dissolve in water - all are so basic that they undergo hydrolysis. Concentrations of oxide ion in water are too low to be detectable with current technology.

Authentic soluble oxides do exist of course, but they release oxyanions, not O^2-. Well known soluble salts of oxyanions include sodium sulfate (Na₂SO₄), potassium permanganate (KMnO₄), and sodium nitrate (NaNO₃).

Nomenclature

In the 18th century, oxides were named calxes or calces after the calcination process used to produce oxides. Calx was later replaced by oxyd.

Oxides are usually named after the number of oxygen atoms in the oxide. Oxides containing only one oxygen are called oxides or monoxides, those containing two oxygen atoms are dioxides, three oxygen atoms makes it a trioxide, four oxygen atoms are tetroxides, and so on following the Greek numerical prefixes.

Two other types of oxide are peroxide, O₂²⁻, and superoxide, O₂⁻. In such species, oxygen is assigned higher oxidation states than oxide.

Types of oxides

Oxides of more electropositive elements tend to be basic. They are called basic anhydrides; adding water, they may form basic hydroxides. For example, sodium oxide is basic; when hydrated, it forms sodium hydroxide.

Oxides of more electronegative elements tend to be acidic. They are called acid anhydrides; adding water, they form oxoacids. For example, dichlorine heptoxide is acid; perchloric acid is a more hydrated form.

Some oxides can act as both acid and base at different times. They are amphoteric. An example is aluminium oxide. Some oxides do not show behavior as either acid or base.

The oxides of the chemical elements in their highest oxidation state are predictable and the chemical formula can be 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 that does not exist as expected as F₂O₇ but as OF₂ with the least electronegative element given priority. ^[1]. Phosphorus pentoxide, the third exception is not properly represented by the chemical formula P₂O₅ but by P₄O₁₀

Common oxides sorted by oxidation state

• Element in (I) state
  • Copper(I) oxide (Cu₂O)
  • Dicarbon monoxide (C₂O)
  • Dichlorine monoxide (Cl₂O)
  • Lithium oxide (Li₂O)
  • Nitrous oxide (N₂O)
  • Potassium oxide (K₂O)
  • Rubidium oxide (Rb₂O)
  • Silver(I) oxide (Ag₂O)
  • Thallium oxide (Tl₂O)
  • Sodium oxide (Na₂O)
  • Water (H₂O)

• Element in (II) state
  • Aluminium monoxide (AlO)
  • Barium oxide (BaO)
  • Beryllium oxide (BeO)
  • Cadmium oxide (CdO)
  • Calcium oxide (CaO)
  • Carbon monoxide (CO)
  • Cobalt(II) oxide (CoO)
  • Copper(II) oxide (CuO)
  • Iron(II) oxide (FeO)
  • Lead(II) oxide (PbO)
  • Magnesium oxide (MgO)
  • Mercury(II) oxide (HgO)
  • Nickel(II) oxide (NiO)
  • Nitrogen oxide (NO)
  • Palladium(II) oxide (PdO)
  • Silver(II) oxide (AgO)
  • Strontium oxide (SrO)
  • Sulfur monoxide (SO)
  • Tin(II) oxide (SnO)
  • Titanium(II) oxide (TiO)
  • Vanadium(II) oxide (VO)
  • Zinc oxide (ZnO)

• Element in (III) state
  • Aluminium oxide (Al₂O₃)
  • Antimony trioxide (Sb₂O₃)
  • Arsenic trioxide (As₂O₃)
  • Bismuth trioxide (Bi₂O₃)
  • Boron oxide (B₂O₃)
  • Chromium(III) oxide (Cr₂O₃)
  • Dinitrogen trioxide (N₂O₃)
  • Erbium(III) oxide (Er₂O₃)
  • Gadolinium(III) oxide (Gd₂O₃)
  • Gallium(III) oxide (Ga₂O₃)
  • Holmium(III) oxide (Ho₂O₃)
  • Indium(III) oxide (In₂O₃)
  • Iron(III) oxide (Fe₂O₃)
  • Lanthanum(III) oxide (La₂O₃)
  • Lutetium(III) oxide (Lu₂O₃)
  • Nickel(III) oxide (Ni₂O₃)
  • Phosphorus trioxide (P₄O₆)
  • Promethium(III) oxide (Pm₂O₃)
  • Rhodium(III) oxide (Rh₂O₃)
  • Samarium(III) oxide (Sm₂O₃)
  • Scandium(III) oxide (Sc₂O₃)
  • Terbium(III) oxide (Tb₂O₃)
  • Thallium(III) oxide (Tl₂O₃)
  • Thulium(III) oxide (Tm₂O₃)
  • Titanium(III) oxide (Ti₂O₃)
  • Tungsten(III) oxide (W₂O₃)
  • Vanadium(III) oxide (V₂O₃)
  • Ytterbium(III) oxide (Yb₂O₃)
  • Yttrium(III) oxide (Y₂O₃)

• Element in (IV) state
  • Carbon dioxide (CO₂)
  • Cerium(IV) oxide (CeO₂)
  • Chlorine dioxide (ClO₂)
  • Chromium(IV) oxide (CrO₂)
  • Dinitrogen tetroxide (N₂O₄)
  • Germanium dioxide (GeO₂)
  • Hafnium(IV) oxide (HfO₂)
  • Lead(I) peroxide (PbO₂)
  • Manganese(IV) oxide (MnO₂)
  • Nitrogen dioxide (NO₂)
  • Ozone (O₃)
  • Plutonium dioxide (PuO₂)
  • Ruthenium(IV) oxide (RuO₂)
  • Selenium dioxide (SeO₂)
  • Silicon dioxide (SiO₂)
  • Sulfur dioxide (SO₂)
  • Tellurium dioxide (TeO₂)
  • Thorium dioxide (ThO₂)
  • Tin dioxide (SnO₂)
  • Titanium dioxide (TiO₂)
  • Tungsten(IV) oxide (WO₂)
  • Uranium dioxide (UO₂)
  • Vanadium(IV) oxide (VO₂)
  • Zirconium dioxide (ZrO₂)

• Element in (V) state
  • Antimony pentoxide (Sb₂O₅)
  • Arsenic pentoxide (As₂O₅)
  • Dinitrogen pentoxide (N₂O₅)
  • Phosphorus pentoxide (P₂O₅)
  • Tantalum pentoxide (Ta₂O₅)
  • Vanadium(V) oxide (V₂O₅)

• Element in (VI) state
  • Carbon trioxide (CO₃)
  • Chromium trioxide (CrO₃)
  • Molybdenum(VI) oxide (MoO₃)
  • Rhenium trioxide (ReO₃)
  • Selenium trioxide (SeO₃)
  • Sulfur trioxide (SO₃)
  • Tellurium trioxide (TeO₃)
  • Tetraoxygen (O₄)
  • Tungsten trioxide (WO₃)
  • Uranium trioxide (UO₃)
  • Xenon trioxide (XeO₃)

• Element in (VII) state
  • Dichlorine heptoxide (Cl₂O₇)
  • Manganese(VII) oxide (Mn₂O₇)
  • Rhenium(VII) oxide (Re₂O₇)

• Element in (VIII) state
  • Osmium tetroxide (OsO₄)
  • Ruthenium tetroxide (RuO₄)
  • Xenon tetroxide (XeO₄)

See also

• Other oxygen ions ozonide, O₃⁻, superoxide, O₂⁻, peroxide, O₂²⁻ and dioxygenyl, O₂⁺.
• Suboxide

• See Category:Oxides for a list of oxides.

References

1. ^ Fully Exploiting the Potential of the Periodic Table through Pattern Recognition Schultz, Emeric. J. Chem. Educ. 2005 82 1649.

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