Diamond andgraphite are two allotropes of carbon: pure forms of the same element that differ in crystalline structure.
Allotropy orallotropism (from Ancient Greek ἄλλος (allos)'other' and τρόπος (tropos)'manner, form') is the property of somechemical elements to exist in two or more different forms, in the same physicalstate, known asallotropes of the elements. Allotropes are different structural modifications of an element: theatoms of the element arebonded together in different manners.[1]For example, theallotropes of carbon includediamond (the carbon atoms are bonded together to form acubic lattice oftetrahedra),graphite (the carbon atoms are bonded together in sheets of ahexagonal lattice),graphene (single sheets of graphite), andfullerenes (the carbon atoms are bonded together in spherical, tubular, or ellipsoidal formations).
The termallotropy is used for elements only, not forcompounds. The more general term, used for any compound, ispolymorphism, although its use is usually restricted to solid materials such as crystals. Allotropy refers only to different forms of an element within the same physicalphase (the state of matter, i.e.plasmas,gases,liquids, orsolids). The differences between these states of matter would not alone constitute examples of allotropy. Allotropes of chemical elements are frequently referred to aspolymorphs or asphases 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, twoallotropes of oxygen (dioxygen, O2, andozone, 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 hasnumerous solid allotropes, which all revert to the same P4 form when melted to the liquid state.
The concept of allotropy was originally proposed in 1840 by the Swedish scientist BaronJöns Jakob Berzelius (1779–1848).[2][3] The term is derived from Greek άλλοτροπἱα (allotropia)'variability, changeableness'.[4] After the acceptance ofAvogadro'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.[3] 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 ofpolymorphism known for compounds, and proposed that the terms allotrope and allotropy be abandoned and replaced by polymorph and polymorphism.[5][3] Although many other chemists have repeated this advice,IUPAC and most chemistry texts still favour the usage of allotrope and allotropy for elements only.[6]
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, andtemperature. Therefore, the stability of the particular allotropes depends on particular conditions. For instance,iron changes from abody-centered cubic structure (ferrite) to aface-centered cubic structure (austenite) above 906 °C, andtin undergoes a modification known astin pest from ametallic form to asemimetallic 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).
Typically, elements capable of variablecoordination number and/oroxidation states tend to exhibit greater numbers of allotropic forms. Another contributing factor is the ability of an element tocatenate.
Diamond – an extremely hard, transparent crystal, with the carbon atoms arranged in a tetrahedral lattice. A poor electrical conductor. An excellent thermal conductor.
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.[dubious –discuss]
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.
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 °C
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 toWerner Heisenberg for quantum mechanics and singled out the "allotropic forms of hydrogen" as its most notable application.[7]
Among the metallic elements that occur in nature in significant quantities (56 up to U, without Tc and Pm), almost half (27) are allotropic at ambient pressure: Li, Be, Na, Ca, Ti, Mn, Fe, Co, Sr, Y, Zr, Sn, La, Ce, Pr, Nd, Sm, Gd, Tb, Dy, Yb, Hf, Tl, Th, Pa and U. Somephase transitions between allotropic forms of technologically relevant metals are those of Ti at 882 °C, Fe at 912 °C and 1,394 °C, Co at 422 °C, Zr at 863 °C, Sn at 13 °C and U at 668 °C and 776 °C.
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. Thetransuranium metalsNp,Am, andCm are also allotropic.
In 2017, the concept of nanoallotropy was proposed.[21] Nanoallotropes, or allotropes ofnanomaterials, are nanoporous materials that have the same chemical composition (e.g., Au), but differ in their architecture at the nanoscale (that is, on a scale 10 to 100 times the dimensions of individual atoms).[22] Such nanoallotropes may help create ultra-small electronic devices and find other industrial applications.[22] The different nanoscale architectures translate into different properties, as was demonstrated forsurface-enhanced Raman scattering performed on several different nanoallotropes of gold.[21] A two-step method for generating nanoallotropes was also created.[22]
Berzelius, Jac. (1841).Årsberättelse om Framstegen i Fysik och Kemi afgifven den 31 Mars 1840. Första delen [Annual Report on Progress in Physics and Chemistry submitted March 31, 1840. First part.] (in Swedish). Stockholm, Sweden: P.A. Norstedt & Söner. p. 14. 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 (afαλλότροπος, som betyder: af olika beskaffenhet) ellerallotropiskt tillstånd." (If it [i.e., the wordisomer] 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 wordisomers] is not suitable for different conditions of simple substances, where these [substances] transform to have different properties, and [therefore the wordisomers] should be replaced, in their case, by a better chosen name; for example,Allotropy (fromαλλότροπος, which means: of different nature) orallotropic condition.)
Republished in German:Berzelius, Jacob; Wöhler, F. (1841)."Jahres-Bericht über die Fortschritte der physischen Wissenschaften" [Annual Report on Progress of the Physical Sciences].Jahres Bericht Über die Fortschritte der Physischen Wissenschaften (in German).20. Tübingen, (Germany): Laupp'schen Buchhandlung: 13. 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. durchAllotropie (vonαλλότροπος, welches bedeutet: von ungleicher Beschaffenheit), oder durchallotropischen Zustand." (Even if it [i.e., the wordisomer] 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 wordisomer] may be replaced with a better chosen designation, e.g., withAllotropy (fromαλλότροπος, which means: of distinct character), or withallotropic condition.)
^"allotropy",A New English Dictionary on Historical Principles, vol. 1, Oxford University Press, 1888, p. 238.
^Ostwald, Wilhelm; Taylor, W.W. (1912).Outlines of General Chemistry (3rd ed.). London, England: Macmillan and Co., Ltd. p. 104. 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."
^Jensen 2006, citing Addison, W. E. The Allotropy of the Elements (Elsevier 1964) that many have repeated this advice.
^Hanfland, M.; Loa, I.; Syassen, K. (2002-05-13). "Sodium under pressure: bcc to fcc structural transition and pressure-volume relation to 100 GPa".Physical Review B.65 (18) 184109. American Physical Society (APS).Bibcode:2002PhRvB..65r4109H.doi:10.1103/physrevb.65.184109.ISSN0163-1829.
^de la Peña O'Shea, Víctor Antonio; Moreira, Iberio de P. R.; Roldán, Alberto; Illas, Francesc (8 July 2010). "Electronic and magnetic structure of bulk cobalt: The α, β, and ε-phases from density functional theory calculations".The Journal of Chemical Physics.133 (2): 024701.doi:10.1063/1.3458691.PMID20632764.
^abcdDeffrennes, Guillaume; Faure, Philippe; Bottin, François; Joubert, Jean-Marc; Oudot, Benoit (2022). "Tin (Sn) at high pressure: Review, X-ray diffraction, DFT calculations, and Gibbs energy modeling".Journal of Alloys and Compounds.919 165675.arXiv:2203.16240.doi:10.1016/j.jallcom.2022.165675.
^Benedict, U.; Haire, R. G.; Peterson, J. R.; Itie, J. P. (1985). "Delocalisation of 5f electrons in curium metal under high pressure".Journal of Physics F: Metal Physics.15 (2):L29–L35.Bibcode:1985JPhF...15L..29B.doi:10.1088/0305-4608/15/2/002.
