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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.

History Jensen W.B., "The Origin of the Term Allotrope", Journal of Chemical Education, 2006, 83, 838-9 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, Wilhelm Ostwald noted that the allotropy of elements is just a special case of the phenomenon of Polymorphism (materials science) 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 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 (iron) 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 catenation. Allotropes are typically more noticeable in non-metals and metalloids.

Examples of allotropes include:

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:

• Red phosphorus - polymeric solid
• White phosphorus - crystalline solid
• Black phosphorus - semiconductor, analogous to graphite

Oxygen:

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

Sulfur:

• Plastic (amorphous) sulfur - polymeric solid
• Rhombic sulfur - large crystals composed of S8 molecules
• Monoclinic sulfur - fine needle-like crystals
• Molecular sulfur - sulphur tends to form ring molecules such as S7 and S12

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.

External links

• Allotrope in IUPAC Compendium of Chemical Terminology, Electronic version, http://goldbook.iupac.org/A00243.html. Accessed March 2007.

References

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.

History Jensen W.B., "The Origin of the Term Allotrope", Journal of Chemical Education, 2006, 83, 838-9 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, Wilhelm Ostwald noted that the allotropy of elements is just a special case of the phenomenon of Polymorphism (materials science) 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 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 (iron) 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 catenation. Allotropes are typically more noticeable in non-metals and metalloids.

Examples of allotropes include:

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:

• Red phosphorus - polymeric solid
• White phosphorus - crystalline solid
• Black phosphorus - semiconductor, analogous to graphite

Oxygen:

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

Sulfur:

• Plastic (amorphous) sulfur - polymeric solid
• Rhombic sulfur - large crystals composed of S8 molecules
• Monoclinic sulfur - fine needle-like crystals
• Molecular sulfur - sulphur tends to form ring molecules such as S7 and S12

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.

External links

• Allotrope in IUPAC Compendium of Chemical Terminology, Electronic version, http://goldbook.iupac.org/A00243.html. Accessed March 2007.

References

Definition: allotrope from Online Medical Dictionary
The Online Medical Dictionary is a searchable dictionary of definitions from medicine, science and technology.

Allotropy - Wikipedia, the free encyclopedia
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 ...

Allotropes of carbon - Wikipedia, the free encyclopedia
Effect of heat: it is the most stable allotrope of carbon. At a temperature of 2500 degree Celsius, it can be transformed into diamond. At about 700 degree Celsius it burns in pure ...

AskOxford: allotrope
allotrope / al trop/ • noun Chemistry each of two or more different physical forms in which an element can exist (e.g. graphite, charcoal, and diamond as forms of carbon).

allotrope - definition of allotrope by the Free Online Dictionary ...
A structurally differentiated form of an element that exhibits allotropy.

allotrope - definition of allotrope in the Medical dictionary - by the ...
A structurally differentiated form of an element that exhibits allotropism.?

allotrope definition of allotrope in the Free Online Encyclopedia.
Called linear acetylenic carbon, the new allotrope, or form, of the otherwise sooty element shows up as amber-colored, "gossamerlike" threads, whose texture resembles "angel ...

allotrope - Wiktionary
Definition from Wiktionary, a free dictionary

allotrope - Hutchinson encyclopedia article about allotrope
Property whereby an element can exist in two or more forms (allotropes), each possessing different physical properties but the same state of matter (gas, liquid, or solid).

Safety Glossary Definition - Allotrope
Safety Glossary Definition - Allotrope ... An element has Allotropes if it is found in two (or more) different forms.