Allotropy



 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 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 phosphorus has numerous solid allotropes, which all revert to the same P4 form when melted to the liquid state.

History

The concept of allotropy was originally proposed in 1841 by the Swedish scientist Baron 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, 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.

Differences in properties of an element's allotropes

Allotropes are different structural forms of the same element and can exhibit quite different physical properties and chemical behaviours. The change between allotropic forms is triggered by the same forces that affect other structures, i.e. semiconductor phase below 13.2 °C.

Examples of allotropes

Typically, elements capable of variable coordination number and/or metalloids.

Examples of allotropes include:

Carbon:

Main article: Allotropes of carbon
  • diamond - an extremely hard, transparent crystal, with the carbon atoms arranged in a tetrahedral lattice. A poor electrical conductor. An excellent thermal conductor.
  • graphite - a soft, black, flaky solid, a moderate electrical conductor. The C atoms are bonded in flat hexagonal lattices, which are then layered in sheets.
  • fullerene - (including the "buckyball", C60)

Phosphorus:

Main article: Allotropes of phosphorus
  • Red phosphorus - polymeric solid
  • White phosphorus - crystalline solid
  • Black phosphorus - semiconductor, analogous to graphite

Oxygen:

Main article: Allotropes of oxygen

Sulfur:

Main article: Allotropes of sulfur
  • Plastic (amorphous) sulfur - polymeric solid
  • Rhombic sulfur - large crystals composed of S8 molecules
  • Other ring molecules such as S7 and S12

Selenium:

  • "Red selenium," cyclo-Se8
  • Gray selenium, polymeric Se

Arsenic:

  • Yellow arsenic, As4
  • Gray arsenic, polymeric As

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.

References

  1. ^ Jensen W.B., "The Origin of the Term Allotrope", Journal of Chemical Education, 2006, 83, 838-9
 
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