Allotropy

An allotrope is a particular form of a chemical element distinguished by the physical arrangement of that element's atoms, when multiple are possible. For example, red phosphorous and white phosphorous are two allotropes of phosphorous. Likewise, graphite and diamond are two allotropes of carbon. Allotropes often have differing chemical properties such as stability, density, hardness, and reactivity.

Definition

Allotropes are different structural modifications of an element, where the atoms of the element are bonded together in a different configuration. For each allotrope of carbon, the structure differs: for diamond the carbon atoms are bonded in a tetrahedral lattice arrangement, in graphite the carbon atoms are bonded together in parallel sheets of a hexagonal lattice, graphene consists of a single sheet of graphite, and fullerenes are spherical structures.

The term allotropy is used for elements only, not for compounds. The more general term, used for any crystalline material, is polymorphism. Allotropy refers only to different forms of an element within the same phase (i.e. different solid, liquid or gas forms).

List of allotropes

Non-metals

Element	Allotrope	Description and properties
Carbon	Amorphous carbon	Black solid, most reactive form of carbon
	Atomic carbon	Exists is two forms, monoatomic and diatomic carbon. They only exist at temperatures over 3,642 °C.
	Carbyne	Also known as linear acetylenic carbon, it consists of repeated chains of (−C≡C−)n. It is considered to be much stronger than most of carbon's allotropes.
	Colossal carbon tubes	Also wrote as CCT, they are similar to carbon nanotubes, although their diameter is much larger, micrometer size. CCTs slightly weaker than their nanotube counterparts, they have a tensile strength of 7 GPa, high specific strength (tensile strength per density), and a breaking length of 6.0×103 km.
	Diamond	Extremely hard, transparent crystal, with the carbon atoms arranged in a tetrahedral lattice. It is a poor electrical conductor, but has the highest thermal conductivity of all natural compounds.
	Fullerenes	An allotrope of carbon in the form of a hollow sphere, ellipsoid, tube, and many other shapes. Spherical fullerenes are also called buckyballs, as they resemble the classic balls used in football (soccer). The most known form is the Buckminsterfullerene or C60.
	Graphene	A single layer of graphite recently discovered, it has extraordinary electrical, thermal, and physical properties. It can be produced by epitaxy on an insulating or conducting substrate or by mechanical exfoliation (repeated peeling) from graphite.
	Graphite	The most common allotropes of carbon. Unlike diamond, it is opaque, has a black-grey color with a metallic luster, it is an electrical conductor, though it has poor thermal conductivity. It is the most stable thermodinamically form of carbon. Graphite has a Mohs hardness of 1, compared to that of diamond, which is 10.
	Graphydine	A form of graphyne with diacetylene groups, has been successfully been synthesized on copper substrates. It exhibits a nanoweb-like structure characterized by triangular and regularly distributed pores, which make it look like a nanoporous membrane. In fact, due to the effective size of its pores, which almost matches the van der Waals diameter of the helium atom, graphdiyne could behave as an ideal two-dimensional membrane for helium chemical and isotopic separation.
	Graphyne	Graphyne is a theorized allotrope of carbon, with a structure consisting of one-atom-thick planar sheets of sp and sp2-bonded carbon atoms arranged in crystal lattice. It can be seen as a lattice of benzene rings connected by acetylene bonds. Calculations indicate this form is stable.
	Lonsdaleite	Also known as diamond-like carbon or hexagonal diamond, it is an allotrope of carbon with a hexagonal lattice, that can be found in nature in the impact sites of meteorites containing graphite. It is considered to be slightly stronger than cubic diamond.
	Nanofoam	An allotrope of carbon discovered in 1997 by a team led by Andrei V. Rode. It consists of a cluster-assembly of carbon atoms, 6 nanometers wide, consisting of about 4000 carbon atoms linked in graphite-like sheets, that are given negative curvature by the inclusion of heptagons among the regular hexagonal pattern, the clusters are strung together in a loose three-dimensional web. The material is extremely light, with a density of 2–10 mg/cm3. Large samples of carbon nanofoam are similar to aerogels, but with 1% of the density of previously produced carbon aerogels or only a few times the density of air at sea level. Unlike carbon aerogels however, carbon nanofoam is a poor electrical conductor. The nanofoam contains numerous unpaired electrons, which Rode and colleagues propose is due to carbon atoms with only three bonds that are found at topological and bonding defects. This gives rise to what is perhaps carbon nanofoam's most unusual feature: it is attracted to magnets, and below −183 °C can itself be made magnetic.
	Nanotubes	Allotropes of carbon with a cylindrical nanostructure. They have unusual properties, which are valuable for nanotechnology, electronics, optics and other fields of materials science and technology. In particular, owing to their extraordinary mechanical properties, they can be used to create very light ultrastrong materials. Nanotubes are categorized as single-walled nanotubes (SWNTs) and multi-walled nanotubes (MWNTs). Individual nanotubes naturally align themselves into "ropes" held together by van der Waals forces, more specifically, pi-stacking.
	Pyrolytic carbon	A form of carbon not found in nature, which posseses covalent bonding between its graphene sheets as a result of imperfections in its production. Pyrolytic carbon is produced by heating a hydrocarbon nearly to decomposition temperature, and allowing the graphite to crystallise (pyrolysis). It has a single cleavage plane, similar to mica, and due to this property it exhibits several unusual anisotropic properties. Pyrolytic carbon is well known for it's strong diamagnetism.
	Vitreous carbon	Also known as glass-like carbon or glassy carbon A non-graphitizing (or nongraphitizable) form of carbon which combines glassy and ceramic properties with those of graphite. It posses high temperature resistance, good hardness (7 Mohs), low density, low electrical resistance, low friction, low thermal resistance, extreme resistance to chemical attack and impermeability, to gases and liquids, more so than graphite. Glassy carbon is widely used as an electrode material in electrochemistry, as well as for high temperature crucibles.
Phosphorus	Diphosphorus	A gaseous form of phosphorus, with the chemical formula P2. Diphosphorus is very reactive and is only stable at temperatures above 1100 K. As it cools, it will turn into white phosphorus.
	Black phosphorus	Also known as β-metallic phosphorus, it is the least reactive allotrope and the thermodynamically stable form below 550 °C. As its name suggests it is black and is not soluble in any solvents. Black phosphorus is a semiconductor and has a structure resembling that of graphite.
	Red phosphorus	It is an amorphous network or phosphorus atoms. Can be obtained by heating white phosphorus or exposing it to light for some time. If further heated, the amorphous red phosphorus will crystallize. Red phosphorus does not ignite in air at temperatures below 240 °C and is insoluble in any solvents.
	Scarlet phosphorus	Can be obtained by allowing a solution of white phosphorus in carbon disulfide to evaporate in sunlight
	Violet phosphorus	A form of phosphorus, also known as monoclinic phosphorus, or Hittorf's metallic phosphorus, that can be produced by day-long annealing of red phosphorus above 550 °C. It can also be made by dissolving white phosphorus in molten lead in a sealed tube at 500 °C for 18 hours. Upon slow cooling, Hittorf's allotrope crystallises out. The crystals can be purified by dissolving the lead in dilute nitric acid, followed by boiling in concentrated hydrochloric acid.
	White phosphorus	Also known as yellow phosphorus, Willie Pete, WP or simply tetraphosphorus (P4), is the most useful form of phosphorus. It is white, though exposed to light becomes yellow. It is pyrophoric and is the most reactive from of phosphorus at standard conditions. It exists as molecules made up of four atoms in a tetrahedral structure. WP has two different crystalline forms: α form, has a body-centered cubic crystal structure and is stable under standard conditions; the β form, believed to have a hexagonal crystal structure, is stable at temperatures below 195.2 K. Unlike most forms of phosphorus, the white form is soluble in many organic solvents, such as carbon disulfide, or toluene.
Oxygen	Dioxygen	A colorless gas, slightly bluish in liquid form, it is the most common form of oxygen. It is paramagnetic.
	Octaoxygen	A deep red solid, that forms at high pressures (10 GPa).
	Ozone	Known as trioxygen it is an unstable blueish gas (deep blue as liquid), extremely reactive, that forms when normal oxygen (dioxygen) is exposed to electric discharges. Unlike dioxygen, ozone is diamagnetic.
	Tetraoxygen	A metastable form of oxygen, that quickly converts into octaoxygen. Also known as oxozone.
Selenium	Black selenium	A form consisting of irregular polymeric rings up to 1000 atoms long. It is slightly soluble in carbon disulfide. Upon heating to 180 °C turns into gray selenium.
	Gray selenium	A polymeric form of selenium, with a hexagonal structure, the most stable form. It is a semiconductor.
	Red selenium	Similar to octasulfur, it is a cyclic molecule consisting of 8 atoms of selenium. It is the amorphous form of selenium.
Sulfur1	Amorphous sulfur	A quenched form of sulfur, that melts at 160 °C.
	Hexasulfur	A cyclic form of sulfur containing 6 atoms of sulfur.
	Octasulfur	The most common form of sulfur. Has at least 4 forms.
	Polymeric sulfur	Also known as insoluble sulfur

¹As there are at least 30 known allotropes of sulfur, only the most common and best characterized are listed.