Allotropy

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Allotropy

Allotropy (Gr. allos, other, and tropos, manner) or allotropism is a behavior exhibited by certain chemical element

Chemical element

A chemical element is a type of atom that is distinguished by its atomic number; that is, by the number of protons in its atomic nucleus. The term is also used to refer to a pure chemical Chemical substance composed of atoms with the same number of protons....
s: these elements can exist in two or more different forms, known as allotropes of that element. In each allotrope, the element's atoms are bonded

Chemical bond

A chemical bond is the physical process responsible for the attractive interactions between atoms and molecules, and that which confers stability to diatomic and polyatomic chemical compounds....
together in a different manner. Allotropes are different structural modifications of an element.

For example, the element carbon

Carbon

Carbon is a chemical element with chemical symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalence?making four electrons available to form covalent bond chemical bonds....
has 2 common allotropes: diamond

Diamond

In mineralogy, diamond is the Allotropes of carbon where the carbon atoms are arranged in an isometric-hexoctahedral crystal lattice. After graphite, diamond is the second most stable form of carbon....
, where the carbon atoms are bonded together in a tetrahedral lattice arrangement, and Graphite

Graphite

The mineral graphite is one of the allotropes of carbon. It was named by Abraham Gottlob Werner in 1789 from the Greek language ??afe?? : "to draw/write", for its use in pencils, where it is commonly called lead, as distinguished from the actual metallic element lead....
, 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

State of matter

States of matter are the distinct forms that different phase take on. Historically, the distinction is made based on qualitative differences in bulk properties....
(i.e.

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Encyclopedia

Allotropy (Gr. allos, other, and tropos, manner) or allotropism is a behavior exhibited by certain chemical element

Chemical element

Chemical bond

Carbon

Diamond

Graphite

State of matter

States of matter are the distinct forms that different phase take on. Historically, the distinction is made based on qualitative differences in bulk properties....
(i.e. different solid

Solid

A solid object is in the states of matter characterized by resistance to deformation and changes of volume. In other words, it has high values both of Young's modulus and of shear modulus; this contrasts e.g....
, liquid

Liquid

Liquid is one of the principal states of matter. A liquid is a fluid that has the particles loose and can freely form a distinct surface at the boundaries of its bulk material....
or gas

Gas

In physics, a gas is a state of matter, consisting of a collection of particles without a definite shape or volume that are in more or less random motion....
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

Oxygen

Oxygen no O2 produced; 2) O2 produced, but absorbed in oceans & seabed rock; 3) O2 starts to gas out of the oceans, but is absorbed by land surfaces and formation of ozone layer; 4-5) O2 sinks filled and the gas accumulates]]...
(dioxygen, O₂ and ozone

Ozone

Ozone or trioxygen is a triatomic molecule, consisting of three oxygen atoms. It is an allotrope of oxygen that is much less stable than the diatomic O2....
, 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

Phosphorus

Phosphorus is the chemical element that has the symbol P and atomic number 15. The name comes from the and . A Valency nonmetal of the nitrogen group, phosphorus is commonly found in inorganic phosphate minerals....
has numerous solid allotropes, which all revert to the same P₄ form when melted to the liquid state.

History

The concept of allotropy was originally proposed in 1841 by the Swedish scientist Baron Jöns Jakob Berzelius

Jöns Jakob Berzelius

Friherre J?ns Jacob Berzelius was a Sweden chemist. He worked out the modern technique of chemical formula, and is together with John Dalton, Antoine Lavoisier, and Robert Boyle considered a father of modern chemistry....
(1779-1848) who offered no explanation. After the acceptance of Avogadro's hypothesis

Avogadro's law

Avogadro's law is a gas law named after Amedeo Avogadro who, in 1811, hypothesized that:Thus, the number of molecules in a specific volume of gas is independent of the size or mass of the gas molecules....
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

Wilhelm Ostwald

Friedrich Wilhelm Ostwald was a Baltic German chemist. He received the Nobel Prize in Chemistry in 1909 for his work on catalysis, chemical equilibria and reaction velocities....
noted that the allotropy of elements is just a special case of the phenomenon of polymorphism

Polymorphism (materials science)

Polymorphism in materials science is the ability of a solid material to exist in more than one form or crystal structure. Polymorphism can potentially be found in any crystalline material including polymers, minerals, and metals, and is related to allotropy, which refers to chemical elements....
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

Pressure

Pressure is the force per unit area applied to an object in a direction surface normal to the surface. Gauge pressure is the pressure relative to the local atmospheric or ambient pressure....
, light

Photochemistry

Photochemistry, a sub-discipline of chemistry, is the study of the interactions between atoms, small molecules, and light . The pillars of photochemistry are UV/VIS spectroscopy, photochemical reactions in organic chemistry and photosynthesis in biochemistry....
, and temperature

Temperature

In physics, temperature is a physical property of a Physical system that underlies the common notions of hot and cold; something that feels hotter generally has the greater temperature....
. Therefore the stability of the particular allotropes depends on particular conditions. For instance, iron

Iron

Iron is a chemical element with the symbol Fe and atomic number 26. Iron is a Group 8 element and period 4 element. Iron is lustrous and silvery in color....
changes from a body-centered cubic structure (ferrite)

Ferrite (iron)

Ferrite or alpha iron is a materials science term for iron, or a solid solution with iron as the main constituent, with a body centred cubic crystal structure....
to a face-centered cubic structure (austenite

Austenite

Austenite is a metallic non-magnetic solid solution of iron and an alloying element. In plain-carbon steel, austenite exists above the critical eutectoid temperature of 1000 K ; other alloys of steel have different eutectoid temperatures....
) above 906 °C, and tin

Tin

Tin is a chemical element with the symbol Sn and atomic number 50. Tin is obtained chiefly from the mineral cassiterite, where it occurs as an oxide, SnO2....
undergoes a transformation known as tin pest

Tin pest

Tin pest is an autocatalysis, Allotropy transformation of the element tin, which causes deterioration of tin objects at low temperatures. Tin pest has also been called tin disease, or tin leprosy ....
from a metallic phase to a semiconductor

Semiconductor

A semiconductor is a material that has electrical conductivity between those of a Electrical conductor and an electrical insulation; it can vary over that wide range either permanently or dynamically....
phase below 13.2 °C.

List of allotropes

Typically, elements capable of variable coordination number

Coordination number

The coordination number of an atom in a molecule or a crystal is the integer number of its nearest Neighbourhood . This number is determined somewhat differently for molecules and for crystals....
and/or oxidation states tend to exhibit greater numbers of allotropic forms. Another contributing factor is the ability of an element to catenate

Catenation

Catenation is the ability of a chemical element to form a long chain-like structure via a series of covalent bonds. Catenation occurs most readily in carbon, which forms covalent bonds with other carbon atoms.Catenation is the reason for the presence of large number of organic compounds in nature....
. Allotropes are typically more noticeable in non-metals (excluding the halogen

Halogen

|}The halogens or halogen elements are a chemical series of nonmetal chemical element from Periodic table group International Union of Pure and Applied Chemistry of the periodic table, comprising fluorine, F; chlorine, Cl; bromine, Br; iodine, I; and astatine, At....
s and the noble gas

Noble gas

|}The noble gases are a group of chemical elements with very similar properties: under standard conditions, they are all odorless, colorless, monatomic gases, with a very low chemical reactivity....
es) and metalloid

Metalloid

is a term used in chemistry when classifying the chemical elements. On the basis of their general physical and chemical properties, nearly every element in the periodic table can be termed either a metal or a nonmetal....
s. Nevertheless, metal

Metal

In chemistry, a metal is a chemical element whose atoms readily lose electrons to form positive ions , and form metallic bonds between other metal atoms and ionic bonds between nonmetal atoms....
s tend to have many allotropes.

Examples of allotropes include:

Non-metals and metalloids

Element	Allotropes
Carbon Allotropes of carbon This is a list of the allotropes of carbon....	diamond Diamond In mineralogy, diamond is the Allotropes of carbon where the carbon atoms are arranged in an isometric-hexoctahedral crystal lattice. After graphite, diamond is the second most stable form of carbon.... - an extremely hard, transparent crystal, with the carbon atoms arranged in a tetrahedral lattice. A poor electrical conductor. An excellent thermal conductor. graphite Graphite The mineral graphite is one of the allotropes of carbon. It was named by Abraham Gottlob Werner in 1789 from the Greek language ??afe?? : "to draw/write", for its use in pencils, where it is commonly called lead, as distinguished from the actual metallic element lead.... - a soft, black, flaky solid, a moderate electrical conductor. The C atoms are bonded in flat hexagonal lattices, which are then layered in sheets. amorphous carbon Amorphous carbon Amorphous carbon or free, reactive carbon, is an Allotropes of carbon that does not have any crystalline structure. As with all Amorphous solid materials, some short-range order can be observed.... fullerene Fullerene Fullerene are a family of carbon Allotropy, molecules composed entirely of carbon, in the form of a hollow sphere, ellipsoid, cylinder , or plane.... s, including "buckyballs", such as C₆₀, and carbon nanotube Carbon nanotube Carbon nanotubes are allotropes of carbon with a nanostructure that can have a length-to-diameter ratio of up to 28,000,000:1, which is significantly larger than any other material.... s
Phosphorus Allotropes of phosphorus Elemental phosphorus can exist in several allotropy; the most common of which are white and red. There are also violet and black phosphorus, and gaseous diphosphorus.... :	White phosphorus - crystalline solid Red phosphorus - polymeric solid Scarlet phosphorus Violet phosphorus Black phosphorus - semiconductor, analogous to graphite Diphosphorus Diphosphorus Diphosphorus, P2, is the diatomic form of phosphorus. Unlike its nitrogen group neighbour nitrogen which forms a stable N2 molecule with a nitrogen to nitrogen triple bond, phosphorus prefers a tetrahedral form P4 because P-P pi-bonds are high in energy....
Oxygen Allotropes of oxygen There are several known allotropes of oxygen:dioxygen, O2 - colorlessozone, O3 - blue*tetraoxygen, O4 - metastable... :	dioxygen, O₂ - colorless ozone Ozone Ozone or trioxygen is a triatomic molecule, consisting of three oxygen atoms. It is an allotrope of oxygen that is much less stable than the diatomic O2.... , O₃ - blue tetraoxygen Tetraoxygen The tetraoxygen molecule , also called oxozone was first predicted in 1924 by Gilbert N. Lewis, who proposed it as an explanation for the failure of liquid oxygen to obey Curie's law.... , O₄ - metastable Metastability Metastability is a general scientific concept which describes states of delicate equilibrium. A system is in a metastable state when it is in equilibrium but is susceptible to fall into lower-energy states with only slight interaction.... octaoxygen Solid oxygen Solid oxygen forms at normal atmospheric pressure at a temperature below 54.36 K . Solid oxygen O2, like liquid oxygen, is a clear substance with a light diffuse sky radiation color caused by absorption in the red .... , - red
Nitrogen Nitrogen Nitrogen is a chemical element that has the symbol N and atomic number 7 and atomic mass 14.00674?. Elemental nitrogen is a colorless, odorless, tasteless and mostly inert diatomic gas at standard conditions, constituting 78% by volume of Earth's atmosphere.... :	dinitrogen tetranitrogen Tetranitrogen As reported in the January 18, 2002 Edition of Science at the University of Rome La Sapienza, Fulvio Cacace and his colleagues created a novel form of nitrogen known as tetranitrogen .... trinitrogen two solid forms: one hexagonal close-packed and the other alpha cubic
Sulfur Allotropes of sulfur There are a large number of allotropy of sulfur. In this respect, sulfur is second only to allotropes of carbon.The most common form found in nature is yellow orthorhombic a-sulfur, which contains puckered rings of .... :	Plastic (amorphous) sulfur - polymeric solid Rhombic sulfur - large crystals composed of S₈ molecules Monoclinic sulfur - fine needle-like crystals Other ring molecules such as S₇ and S₁₂
Selenium Selenium Selenium is a chemical element with the atomic number 34, represented by the chemical symbol Se, an atomic mass of 78.96. It is a nonmetal, chemically related to sulfur and tellurium, and rarely occurs in its elemental state in nature.... :	"Red selenium," cyclo-Se₈ Gray selenium, polymeric Se Black selenium
Boron Boron Boron is a chemical element with atomic number 5 and the chemical symbol B. Boron is a trivalent metalloid element which occurs abundantly in the evaporite ores borax and ulexite....	amorphous boron - brown powder crystalline boron - black, hard (9.3 on Mohs' scale), and a weak conductor at room temperature.
Germanium Germanium Germanium is a chemical element with the symbol Ge and atomic number 32. It is a lustrous, hard, greyish-white metalloid in the carbon group, chemically similar to its group neighbors tin and silicon....	a-germanium - ß-germanium - at high pressures
Silicon Silicon Silicon is the most common metalloid. It is a chemical element, which has the symbol Si and atomic number 14. The atomic mass is 28.0855....	amorphous silicon - brown powder nanocrystalline silicon Nanocrystalline silicon Nanocrystalline silicon , sometimes also known as microcrystalline silicon , is a form of porous silicon . It is an allotropy form of silicon with paracrystalline structure?is similar to amorphous silicon , in that it has an amorphous solid phase.... - similar to the amorphous silicon crystalline silicon - has a metallic luster and a grayish color. Single crystals of crystalline silicon can be grown with a process known as the Czochralski process
Arsenic Arsenic Arsenic is a well-known chemical element that has the symbol As and atomic number 33. Arsenic was first documented by Albertus Magnus in 1250.... :	Yellow arsenic - molecular non-metallic As₄ Gray arsenic, polymeric As (metalloid) Black arsenic (metalloid) and several similar other ones.
Antimony Antimony Antimony is a chemical element with the symbol Sb and atomic number 51. A metalloid, antimony has four allotropy forms. The stable form of antimony is a blue-white metalloid.... :	blue-white antimony - the stable form (metalloid) yellow antimony (non-metallic) black antimony (non-metallic) (a fourth one too)

Metals

Among the naturally occuring metallic elements (up to U, without Tc and Pm), 28 are allotropic at ambient pressure: Li, Be, Na, Ca, Sr, Ti, Mn, Fe, Co, Sr, Y, Zr, Sn, La, Ce, Pr, Nd, (Pm), Sm, Gd, Tb, Dy, Yb, Hf, Tl, Po, Th, Pa, U. Considering only the technologically-relevant metals, six metals are allotropic: Ti at 882°C, Fe at 912 and 1394°C, Co at 422°C, Zr at 863°C, Sn at 13°C and U at 668 and 776°C.

Tin

Tin

Tin is a chemical element with the symbol Sn and atomic number 50. Tin is obtained chiefly from the mineral cassiterite, where it occurs as an oxide, SnO2....

grey tin (alpha-tin)
white tin (beta tin)
rhombic tin (gamma)

Iron

Allotropes of iron

Iron represents perhaps the best-known example for allotropy in a metal. There are three allotropic forms of iron, known as alpha, gamma, and delta....

ferrite
Ferrite (iron)

Ferrite or alpha iron is a materials science term for iron, or a solid solution with iron as the main constituent, with a body centred cubic crystal structure....
(alpha iron) - forms below 770°C (the Curie point, Tc ); the iron becomes magnetic in its alpha form; BCC
beta - forms below 912°C (BCC)
gamma - forms below 1394°C; face centred cubic (FCC) crystal structure
delta - forms from cooling down molten iron below 1538°C; has a body-centred cubic (BCC) crystal structure

Lanthanides and actinides

Cerium
Cerium

Cerium is a chemical element with the symbol Ce and atomic number 58....
, Samarium
Samarium

Samarium is a chemical element with the symbol Sm and atomic number 62....
, Terbium
Terbium

Terbium is a chemical element with the symbol Tb and atomic number 65....
,Dysprosium
Dysprosium

Dysprosium is a chemical element with the symbol Dy and atomic number 66. It is a rare earth element with a metallic silver luster. Dysprosium is never found in nature as a free element, though it is found in various minerals, such as xenotime....
and Ytterbium
Ytterbium

Ytterbium is a chemical element with the symbol Yb and atomic number 70. A soft silvery metallic element, ytterbium is a Rare earth element of the lanthanide series and is found in the minerals gadolinite, monazite, and xenotime....
have three allotropes.

Praseodymium
Praseodymium

Praseodymium is a chemical element that has the symbol Pr and atomic number 59....
, Neodymium
Neodymium

Neodymium is a chemical element with the symbol Nd and atomic number 60....
, Gadolinium
Gadolinium

Gadolinium is a chemical element that has the symbol Gd and atomic number 64....
and Terbium
Terbium

Terbium is a chemical element with the symbol Tb and atomic number 65....
have two allotropes

Plutonium
Allotropes of plutonium

Even at ambient pressure, plutonium occurs in a variety of allotropes. These allotropes differ widely in crystal structure and density; the a and d allotropes differ in density by more than 25% at constant pressure....
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. The transuranien metals Np, Am, and Cm are also allotropic.

Promethium
Promethium

Promethium is a chemical element with the symbol Pm and atomic number 61. It is notable for being the only other exclusively radioactive element besides technetium which is followed by chemical elements that have stable isotopes....
, Americium
Americium

Americium is a synthetic element that has the symbol Am and atomic number 95. A radioactive decay metallic element, americium is an actinide that was obtained in 1944 by Glenn T....
, Berkelium
Berkelium

Berkelium is a synthetic element with the symbol Bk and atomic number 97. A radioactive metallic element in the actinide series, berkelium was first synthesized by bombarding americium with alpha particles and was named after the University of California, Berkeley....
, Californium
Californium

Californium is a metallic chemical element with the symbol Cf and atomic number 98. A Radioactive decay transuranic element, californium is used in starting nuclear reactors, optimizing coal-fired power plants and cement production facilities , medical treatment of cancer, and oil exploration via down hole well logging....
have 3 allotropes

Allotropy

History

Differences in properties of an element's allotropes

List of allotropes

Non-metals and metalloids

Metals

Lanthanides and actinides

External links