Carbon allotropy

FIGURE 16.5 Models of carbon allotropies. Clockwise from (a) diamond, (b) graphite... 

The eommonest erystalline forms of earbon, cubie diamond and hexagonal graphite, are elassical examples of allotropy that are found in every chemistry textbook. Both diamond and graphite also exist in two minor crystallographie forms hexagonal diamond and rhombohedral graphite. To these must be added earbynes and Fullerenes, both of which are crystalline earbon forms. Fullerenes are sometimes referred to as the third allotrope of carbon. However, sinee Fullerenes were diseovered more recently than earbynes, they are... 

A number of chemical elements, mainly oxygen and carbon but also others, such as tin, phosphorus, and sulfur, occur naturally in more than one form. The various forms differ from one another in their physical properties and also, less frequently, in some of their chemical properties. The characteristic of some elements to exist in two or more modifications is known as allotropy, and the different modifications of each element are known as its allotropes. The phenomenon of allotropy is generally attributed to dissimilarities in the way the component atoms bond to each other in each allotrope either variation in the number of atoms bonded to form a molecule, as in the allotropes oxygen and ozone, or to differences in the crystal structure of solids such as graphite and diamond, the allotropes of carbon. 

The allotropy of carbon is due to variations in the crystal structure of the element. There are three allotropes of carbon graphite, diamond, and... 

All five of the Group VI elements have a property called allotropy, which means they exist in more than one solid form. Like carbon, which could be black graphite or a sparkling diamond, these elements can exist with their atoms arranged in more than one way. 

Many elements including sulphur, carbon and oxygen can exist in two or more forms with different physical, and often chemical, properties such elements are said to exhibit allotropy and the different forms are known as allotropes or allotropic forms. 

A table of crystal structures for the elements can be found in Table 1.11 (excluding the Lanthanide and Actinide series). Some elements can have multiple crystal structures, depending on temperature and pressure. This phenomenon is called allotropy and is very common in elemental metals (see Table 1.12). It is not unusual for close-packed crystals to transform from one stacking sequence to the other, simply through a shift in one of the layers of atoms. Other common allotropes include carbon (graphite at ambient conditions, diamond at high pressures and temperature), pure iron (BCC at room temperature, FCC at 912°C and back to BCC at 1394°C), and titanium (HCP to BCC at 882°C). 

Arsenic exhibits allotropy, which is characteristic of non-metals the usual, more stable, metallic form resembles the typical metals in appearance and in being a fairly good conductor of electricity. Under atmospheric pressure it begins to volatilise at about 450° C. and passes into a vapour containing complex molecules, As4, which at higher temperatures dissociate to As2 this complexity is not unusual in non-metals. The yellow allotrope, which is stable at low temperatures, resembles white phosphorus in being soluble in carbon disulphide—a property which emphasises the non-metallic character of this variety. The reactivity of the allotropes, as in the case of phosphorus, differs considerably. 

When an element can exist in more than one physical form in the same state it is said to exhibit allotropy (or polymorphism). Each of the different physical forms is called an allotrope. Allotropy is actually quite a common feature of the elements the Periodic Table (p. 136). Some examples of elements which show allotropy are sulfur, tin, iron and carbon. 

Allotropy. Particularly among the nonmetals that are solids under ordinary atmospheric conditions, it is frequently found that the same element is capable of existing in different physical forms. Thus, the element carbon may exist as the crystalline diamond or the amorphous... 

TSome elements and compounds occur in more than one form. Each form is then known as an allotrope the phenomenon is called allotropy. For example, diamond and graphite are allotropes of carbon. 

Allotropy.—Dimorphism apart, a few substances are known to exist in more than one solid form. These varieties of the same substance exhibit different physical properties, while their chemical qualities are the same in kind. Such modifications are said to be allotropic. One or more allotropic modifications of a substance are usually crystalline, the other or others amorphous or vitreous. Sulfur, for example, exists not only in two dimor-j)hous varieties of crystals, but also in a third,. allotropic form, in which it is flexible, amorphous, and transparent. Carbon exists in three allotropic forms two crystalline, the diamond and graphite the third amorphous. 

In 1833 Berzelius used the names empirical (empirische) and rational (rationelle) formulae for those giving the result of analysis (e.g. alcohol C H 0) and those showing the electrochemical division of the atoms (e.g. C H + H 0 or C H + 0), respectively. In 1840 he introduced the name allotropy for the existence of different varieties of an element (sulphur, carbon, silicon) different forms of a compound may contain different allotropic forms of an element, e.g. Sa and Sj3 in pyrites and marcasite. 

Allotropy 9- la-tr9-pe (1850) n. (1) The existence of a substance in two or more sohd, liquid or gaseous forms due to differences in the arrangement of atoms of molecules. Examples are amorphous, graphite, and diamond forms of carbon NO2 and N2O4. (2) Property which an element or compound possesses, of existing in different forms which in themselves have different characteristics. These various forms are described as allotropic modifications. Carbon, for example, is found in an amorphous form as carbon black, and in the crystalline form as graphite, and as a diamond. 

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