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Allotropes of carbon - Wikipedia, the free encyclopedia

Diamond and graphite are two allotropes of carbon: pure forms of the same ...

Allotropy

Allotropy or allotropism is the property of some chemical elements to exist in two or more different forms, known as allotropes of these elements. Allotropes are different structural modifications of an element; the atoms of the element are bonded together in a different manner.

For example, carbon has 3 common allotropes: diamond, where the carbon atoms are bonded together in a tetrahedral lattice arrangement, graphite, where the carbon atoms are bonded together in sheets of a hexagonal lattice, and fullerenes, where 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 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); 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, 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

The concept of allotropy was originally proposed in 1841 by the Swedish scientist Baron Jöns Jakob Berzelius (1779–1848) who offered no explanation. The term is derived from the Greekάλλοτ�οπἱα (allotropia; variation, changeableness). 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, 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. 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, 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 transformation known as tin pest from a metallic phase to a semiconductor phase below 13.2 °C. As an example of 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. Allotropes are typically more noticeable in non-metals (excluding the halogens and the noble gases) and metalloids. Nevertheless, metals tend to have many allotropes.

Examples of allotropes include:

Non-metals and metalloids

Metals

Among the naturally occurring 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 882ËšC, Fe at 912ËšC and 1394ËšC, Co at 422ËšC, Zr at 863ËšC, Sn at 13ËšC and U at 668ËšC and 776ËšC.

Tin

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

Iron

  • ferrite (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 1,394°C; face centred cu


From Yahoo Answers

Question:Is the hardest allotrope form of carbon an average dimond? Some one told me they found a medtiorite in australia with a super hard form of something which they thought was a super hard dimond.

Answers:As far as I know diamond iis it. Super hard diamond, would only be diamond. It would have to have something coating it to make it harder, in which case the coating would be harder than diamond, not the diamond itself. But the only other thing I can think of is that it was a near pure carbon diamond. The lesser amount of imperfections in the diamond, the harder it will get.

Question:

Answers:it's just C (diamond). the difference is the shape of its structure. each C atom is attached to 4 other C atoms, forming a covalent network solid with super strong bonds.

Question:i hv checked wikipedia, but nothing is clear. and what is non-graphitizing carbon?

Answers:I can't be of much help but I recall that glassy carbon is produced through the heat decomposition of carbon polymers that are cross linked in all three dimensions. It appears to be a carbon allotrope. Non graphitising carbon sounds like carbon allotropes that do not have the graphite structure of parallel sheets.

Question:

Answers:i only kno of 2 which are diamond and graphite

From Youtube

Allotropes Of Carbon :The different forms or allotropes of carbon (see below) include the hardest naturally occurring substance, diamond, and also one of the softest known substances, graphite. Moreover, it has an affinity for bonding with other small atoms, including other carbon atoms, and is capable of forming multiple stable covalent bonds with such atoms. As a result, carbon is known to form almost ten million different compounds; the large majority of all chemical compounds. Carbon also has the highest melting and sublimation point of all elements. At atmospheric pressure it has no melting point as its triple point is at 10.8 0.2 MPa and 4600 300 K, so it sublimates at about 3900 K Carbon sublimes in a carbon arc which has a temperature of about 5800 K. Thus, irrespective of its allotropic form, carbon remains solid at higher temperatures than the highest melting point metals such as tungsten or rhenium. Although thermodynamically prone to oxidation, carbon resists oxidation more effectively than elements such as iron and copper that are weaker reducing agents at room temperature. Carbon compounds form the basis of all known life on Earth, and the carbon-nitrogen cycle provides some of the energy produced by the Sun and other stars. Although it forms an extraordinary variety of compounds, most forms of carbon are comparatively unreactive under normal conditions. At standard temperature and pressure, it resists all but the strongest oxidizers. It does not react with sulfuric acid ...

Carbon Allotropes :A discussion of carbon allotropes - including diamond, lonsdaleite, graphite, and the fullerenes.