Allotropy

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Diamond and graphite are two allotropes of carbon: pure forms of the same element that differ in structure.

Allotropy (Gr. allos, other, and tropos, manner) is a behavior exhibited by certain chemical elements: these elements can exist in two or more different forms, known as allotropes of that element. In each different allotrope, the element's atoms are bonded together in a different manner.

For example, the element carbon has two common allotropes: diamond, where the carbon atoms are bonded together in a tetrahedral lattice arrangement, and graphite, where the carbon atoms are bonded together in sheets of a hexagonal lattice.

Note that allotropy refers only to different forms of an element within the same phase or state of matter (i.e. different solid, liquid or gas forms) - the changes of state between solid, liquid and gas in themselves are not considered allotropy. For some elements, allotropes have different molecular formulae which can persist in different phases - for example, the two allotropes of oxygen (dioxygen, O2 and ozone, O3), can both exist in the solid, liquid and gaseous states. Conversely, some elements do not maintain distinct allotropes in different phases: for example phosphorus has numerous solid allotropes, which all revert to the same P4 form when melted to the liquid state.

Contents

  • 1 History
  • 2 Differences in properties of an element's allotropes
  • 3 Examples of allotropes
  • 4 External links
  • 5 References

[edit] History

[1] The concept of allotropy was originally proposed in 1841 by the Swedish scientist Baron Jons Jakob Berzelius (1779-1848) who offered no explanation. After the acceptance of Avogadro's hypothesis in 1860 it was understood that elements could exist as polyatomic molecules, and the two allotropes of oxygen were recognized as O2 and O3. In the early 20th century it was recognized that other cases such as carbon were due to differences in crystal structure.

By 1912, Ostwald noted that the allotropy of elements is just a special case of the phenomenon of polymorphism known for compounds, and proposed that the terms allotrope and allotropy be abandoned and replaced by polymorph and polymorphism. Although many other chemists have repeated this advice, IUPAC and most chemistry texts still favour the usage of allotrope and allotropy for elements only.

[edit] Differences in properties of an element's allotropes

Allotropes of the same element can typically exhibit quite different physical properties and chemical behaviour, even though they contain nothing else but atoms of that element. They may have different colors, odors, hardnesses, electrical and thermal conductivities, etc.

The change between different allotropic forms of an element is often triggered by pressure and temperature, and many allotropes are only stable in the correct conditions. For instance, iron changes from a body-centered cubic structure (ferrite) to a face-centered cubic structure (austenite) above 906°C, and tin undergoes a transformation known as tin pest from a metallic phase to a semiconductor phase below 13.2°C.

[edit] Examples of allotropes

Typically, elements capable of variable coordination number and/or oxidation states tend to exhibit greater numbers of allotropic forms. Another contributing factor is the ability of an element to catenate. Allotropes are typically more noticeable in non-metals and metalloids.

Examples of allotropes include:

Carbon:

Main article: Allotropes of carbon

Phosphorus:

Oxygen:

Sulfur:

Plutonium has six distinct solid allotropes under "normal" pressures. Their densities vary within a ratio of some 4:3, which vastly complicates all kinds of work with the metal (particularly casting, machining, and storage). A seventh Plutonium allotrope exists at very high pressures, which adds further difficulties in exotic applications.