Oxide

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An oxide is a chemical compound of oxygen with other chemical elements. In the 18th century, oxides were named calxes or calces after the calcination process used to produce oxides. Calx was later replaced by oxyd.

The oxide anion, O2−, is the conjugate base of the hydroxide ion, OH−, and is encountered in ionic oxides such as calcium oxide. O2− is unstable in aqueous solution − its affinity for H+ is so great that it always abstracts a proton from a solvent H2O molecule immediately and irreversibly:

O2− + H2O → 2OH−

Unlike compounds composed of ions that are stable in aqueous solution, ionic oxides are either insoluble in water or react with it.

For example, the Na+ and Cl− ions of sodium chloride dissolve readily in water without further chemical reaction. Iron(III) oxide, Fe2O3, is stable to water because it does not dissolve, rendering its O2− ions unavailable for protonation by H2O. Calcium oxide, CaO, dissolves in water as it reacts to form calcium hydroxide, Ca(OH)2.

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[edit] Current naming

Oxides can be named after the number of oxygen atoms in the oxide. Oxides containing only one oxygen are called oxide or monoxide, those containing two oxygen atoms dioxide, three trioxide, four tetroxide, and so on following the Greek numerical prefixes.

There are two other types of oxide — peroxide, O22−, and superoxide, O2−. Both count as oxides but have different oxidation states and react in different ways in comparison to other oxides.

[edit] 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 acids. 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 ozone 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

[edit] Common oxides sorted by oxidation state

[edit] See also

[edit] 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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