Allotropy or allotropism () is the property of some chemical elements to exist in two or more different forms, in the same physical state, known as allotropes of the elements. Allotropes are different structural modifications of an element: the atoms of the element are bonded together in different manners. For example, the allotropes of carbon include diamond (the carbon atoms are bonded together to form a cubic lattice of tetrahedra), graphite (the carbon atoms are bonded together in sheets of a hexagonal lattice), graphene (single sheets of graphite), and fullerenes (the carbon atoms are bonded together in spherical, tubular, or ellipsoidal formations).
The term allotropy is used for elements only, not for compounds. The more general term, used for any compound, is polymorphism, although its use is usually restricted to solid materials such as crystals. Allotropy refers only to different forms of an element within the same physical phase (the state of matter, i.e. plasmas, gases, liquids, or solids). The differences between these states of matter would not alone constitute examples of allotropy. Allotropes of chemical elements are frequently referred to as polymorphs or as phases of the element.
For some elements, allotropes have different molecular formulae or different crystalline structures, as well as a difference in physical phase; for example, two allotropes of oxygen (dioxygen, O2, and ozone, O3) can both exist in the solid, liquid and gaseous states. Other elements do not maintain distinct allotropes in different physical phases; for example, phosphorus has numerous solid allotropes, which all revert to the same P4 form when melted to the liquid state.
+ Comparison of Isomerism, Allotropy, and Polymorphism Feature Isomerism (Isomers) Allotropy (Allotropes) Polymorphism (Polymorphs) Core Concept Molecules with identical molecular formulas but distinct structural arrangements or spatial orientations. Distinct structural modifications of a single element within the same physical state. The capacity of a solid compound or element to manifest in multiple distinct crystal lattices. Scope of Applicability Compounds and molecules (organic and inorganic). Elements exclusively. Crystalline solids (elements, ionic salts, or molecular solids). Structural Variation Variations in chemical bonding, atom connectivity, or three-dimensional spatial geometry. Variations in bonding patterns, molecular formulas, or crystal structures of an element. Variations in the packing arrangements or space groups of atoms or molecules in a solid. Chemical Properties Significant variances in reactivity and functionality (e.g., ethanol versus dimethyl ether). Marked divergence in chemical reactivity (e.g., the inertness of diamond versus the reactivity of graphite). Identity in liquid or gaseous phases; minor variations in solid-state surface reactivity. Physical Properties Variances in boiling points, melting points, and densities. Radical divergence in characteristics (e.g., the transparency and hardness of diamond versus the opacity and softness of graphite). Distinct variations in solubility, hardness, density, and melting points. Physical State Gas, liquid, and solid phases. Predominantly solid phases, with occurrences in liquid and gas phases (e.g., dioxygen versus ozone gas). Crystalline solids exclusively. Primary Mechanism Alternative modes of atomic linkage, functional group placement, or molecular rotation. Alternative modes of homonuclear bonding and molecular aggregation. Thermodynamic variations in temperature and pressure during the process of crystallization. Examples Butane and isobutane Ethanol and dimethyl ether Carbon allotropes: Diamond, graphite, graphene, and fullerenes Oxygen allotropes: Dioxygen and ozone Calcium carbonate forms: Calcite and aragonite Titanium dioxide forms: Rutile, anatase, and brookite Carbon systems: Diamond and graphite (classified concurrently as allotropes and polymorphs)
History
The concept of allotropy was originally proposed in 1840 by the Swedish scientist Baron Jöns Jakob Berzelius (; 1779–1848).See: From p. 14: "Om det ock passar väl för att uttrycka förhållandet emellan myrsyrad ethyloxid och ättiksyrad methyloxid, så är det icke passande för de olika tillstånd hos de enkla kropparne, hvari dessa blifva af skiljaktiga egenskaper, och torde för dem böra ersättas af en bättre vald benämning, t. ex. Allotropi (af αλλότροπος, som betyder: af olika beskaffenhet) eller allotropiskt tillstånd." (If it [i.e., the word isomer] is also well suited to express the relation between formic acid ethyl oxide [i.e., ethyl formate] and acetic acid methyloxide [i.e., methyl acetate], then it [i.e., the word isomers] is not suitable for different conditions of simple substances, where these [substances] transform to have different properties, and [therefore the word isomers] should be replaced, in their case, by a better chosen name; for example, Allotropy (from αλλότροπος, which means: of different nature) or allotropic condition.) Republished in German: From p. 13: "Wenn es sich auch noch gut eignet, um das Verhältniss zwischen ameisensaurem Äthyloxyd und essigsaurem Methyloxyd auszudrücken, so ist es nicht passend für ungleiche Zustände bei Körpern, in welchen diese verschiedene Eigenschaften annehmen, und dürfte für diese durch eine besser gewählte Benennung zu ersetzen sein, z. B. durch Allotropie (von αλλότροπος, welches bedeutet: von ungleicher Beschaffenheit), oder durch allotropischen Zustand." (Even if it [i.e., the word isomer] is still well suited to express the relation between ethyl formate and methyl acetate, then it is not appropriate for the distinct conditions in the case of substances where these [substances] assume different properties, and for these, [the word isomer] may be replaced with a better chosen designation, e.g., with Allotropy (from αλλότροπος, which means: of distinct character), or with allotropic condition.) Merriam-Webster online dictionary: Allotropy. The term is derived .. After the acceptance of Avogadro's hypothesis in 1860, it was understood that elements could exist as polyatomic molecules, and 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. From p. 104: "Substances are known which exist not only in two, but even in three, four or five different solid forms; no limitation to the number is known to exist. Such substances are called polymorphous. The name allotropy is commonly employed in the same connexion, especially when the substance is an element. There is no real reason for making this distinction, and it is preferable to allow the second less common name to die out." Although many other chemists have repeated this advice, IUPAC and most chemistry texts still favour the usage of allotrope and allotropy for elements only.Jensen 2006, citing Addison, W. E. The Allotropy of the Elements (Elsevier 1964) that many have repeated this advice.
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 modification known as tin pest from a metallic form to a semimetallic form below 13.2 °C (55.8 °F). As an example of allotropes having different chemical behaviour, ozone (O3) is a much stronger oxidizing agent than dioxygen (O2).
List 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.
Examples of allotropes include:
Non-metals Element AllotropesCarbon Diamond – an extremely hard, transparent crystal, with the carbon atoms arranged in a tetrahedral lattice. A poor electrical conductor. An excellent thermal conductor. Lonsdaleite – also called hexagonal diamond. Graphene – is the basic structural element of other allotropes, nanotubes, charcoal, and fullerenes. Q-carbon – a ferromagnetic, tough, and brilliant crystal structure that is harder and brighter than diamonds. Graphite – a semimetallic, soft, black, flaky solid, a good electrical conductor. The C atoms are bonded in flat hexagonal lattices (graphene), which are then layered in sheets. Linear acetylenic carbon (carbyne) Amorphous carbon Fullerenes, including buckminsterfullerene, also known as "buckyballs", such as C60. Carbon nanotubes – allotropes having a cylindrical nanostructure. Schwarzites Cyclocarbon Glassy carbon Superdense carbon allotropes – proposed allotropesNitrogen Dinitrogen – by far the most common and stable form of nitrogen, found in the air. Hexazine Hexanitrogen Octaazacubane Tetranitrogen Trinitrogen Solid nitrogenPhosphorus White phosphorus – crystalline solid of tetraphosphorus (P4) molecules Red phosphorus – amorphous polymeric solid Scarlet phosphorus Violet phosphorus with monoclinic crystalline structure Black phosphorus – semiconductor, analogous to graphite Diphosphorus – gaseous form composed of P2 molecules, stable between 1200 °C and 2000 °C; created e.g. by dissociation of P4 molecules of white phosphorus at around 827 °COxygen Dioxygen, O2 – colorless (faint blue liquid and solid) Ozone, O3 – blue Tetraoxygen, O4 – metastable Octaoxygen, O8 – redSulfur Cyclo-Pentasulfur, Cyclo-S5 Cyclo-Hexasulfur, Cyclo-S6 Cyclo-Heptasulfur, Cyclo-S7 Cyclo-Octasulfur, Cyclo-S8Selenium "Red selenium", cyclo-Se8 Gray selenium, polymeric Se Black selenium, irregular polymeric rings up to 1000 atoms long Monoclinic selenium, dark red transparent crystalsSpin isomers of hydrogen Orthohydrogen, H2 with nuclear spins aligned parallel Parahydrogen, H2 with nuclear spins aligned antiparallel These nuclear spin isomers have sometimes been described as allotropes, notably by the committee which awarded the 1932 Nobel prize to Werner Heisenberg for quantum mechanics and singled out the "allotropic forms of hydrogen" as its most notable application.Werner Heisenberg – Facts Nobelprize.org
Metalloids Element AllotropesBoron Amorphous boron – brown powder – B12 regular icosahedra α-rhombohedral boron β-rhombohedral boron γ-orthorhombic boron α-tetragonal boron β-tetragonal boron Cubic boron High-pressure superconducting phase Borophene Borospherene, B40Silicon Amorphous silicon α-silicon, a semiconductor, diamond cubic structure β-silicon - metallic, with the BCC similar to molybdenum and beta-tin (High Pressure Phase) Q-Silicon - a ferromagnetic (Similar to Q-Carbon) and highly conductive phase of silicon (similar to graphite) Silicene, buckled planar single layer Silicon, similar to GrapheneGermanium Amorphous germanium α-germanium – semimetallic element or semiconductor, with the same structure as diamond (similar chemical properties with sulfur and silicon) β-germanium – metallic, with the same structure as beta-tin Germanene – Buckled planar Germanium, similar to grapheneArsenic Yellow arsenic – molecular non-metallic As4, with the same structure as white phosphorus (Similar chemical properties with nitrogen and phosphorus) Gray arsenic, polymeric As (metallic, though heavily anisotropic) (similar to aluminum and antimony in chemical properties)