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 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.

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History

The concept of allotropy was originally proposed in 1841 by the Swedish scientist Baron Jons Jakob Berzelius (1779-1848) who offered no explanation.^[1] 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.

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. pressure, light, and temperature. Therefore the stability of the particular allotropes depends on particular 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.

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

• 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

• dioxygen, O₂ - colorless
• ozone, O₃ - blue
• tetraoxygen, O₄ - red

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