The Allotropes of Carbon

Distinguish the allotropes of carbon by their structures and show how their structures affect their properties. 

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. 

To conclude are diamonds forever No. They convert slowly into graphite, which is the stablest form of carbon. Graphite has the lowest energy for any of the allotropes of carbon, and will not convert to diamond without the addition of energy. 

The limitations of the simple Zintl-Klemm concept can be illustrated by differences in the two [MT1] intermetallics (M = Na [79] and Cs [80]). Complete electron transfer from M to T1 leads to [ M TI, where the Tl anion with four valence electrons is isoelectronic with a neutral group 14 atom and four bonds and needed to attain the octet configuration. Hence, the Tl- anion should form structures similar to allotropes of carbon or heavier group 14 elements. Indeed, [NaTl] has a stuffed diamond structure [79] with internal Na and an anionic (Tl-) lattice similar to diamond. However, the Tl- anions in [CsTl] form tetragonally compressed octahedra [80] unlike any structures of the allotropes of carbon or its heavier congeners. 

In 1991, scientists at AT T Bell Laboratories discovered a new class of high-temperature superconductors based on fullerene, the allotrope of carbon that contains Cgo molecules (Sections 10.10 and 19.6). Called "buckyballs," after the architect R. Buckminster Fuller, these soccer ball-shaped Cgo molecules react with potassium to give K3C6o- This stable crystalline solid contains a face-centered cubic array of buckyballs, with K+ ions in the cavities between the Cgo molecules (Figure 21.16). At room temperature, K3Q,o is a metallic conductor, but it becomes a superconductor at 18 K. The rubidium fulleride, Rb C o, and a rubidium— thallium-Cfio compound of unknown stoichiometry have higher Tc values of 30 K and 45M8 K, respectively. 

Because the allotropes of carbon (graphite, diamond, fullerenes) and closely related silicon-carbide solids will not dissolve in strong acids, they are left as a solid... 

The next level of complexity for "polymers" occurs when there is an infinite cross-linking between the various modules so that the mathematics model requires a higher dimension for its description. The simplest examples of this, even though they traditionally are not considered as "polymers", are two of the allotropes of carbon — graphite and diamond. Before naming these, however, the focus is returned to the above section (1) and a third and fourth allotrope of carbon are examined. The simplest theoretically-possible example of a one-dimensional polymer is the repetitive cumulene. The systemic name for this allotrope is ... 

Some other free (uncombined) elements also occur in different forms. Different forms of the same element are called allotropes of each other. Except for oxygen, the elements that form diatomic molecules when uncombined do not form allotropes, but many other nonmetals do. The allotropes of carbon— diamond and graphite—are perhaps best known to the general public. Sulfur and phosphorus are also notable for forming allotropes. 

The allotropes of carbon have very different chemical and physical properties. For example, diamond is the hardest namral substance known. It has a rating of 10 on the Mohs scale. The Mohs scale is a way of expressing the hardness of a material. It runs from 0 (for talc) to 10 (for diamond). The melting point of diamond is about 6,700°F (3,700°C) and its boiling point is about 7,600°F (4,200°C). Its density is 3.50 grams per cubic centimeter. 

It is interesting to note that the allotropes of carbon include one very hard substance and one very soft substance. They differ only in the arrangement and bonding of the C atoms. 

The relationship between the allotropes of carbon is shown in Figure 2.1. 

The allotropes of carbon comprise diamond, which is three dimensional graphite with its graphene planes that are two dimensional Cgo nanotubes, which can be considered as a unit cell of hexagons crystallized in one direction can be termed uni-directional and finally, spheroidal fullerenes, which are zero dimensional, when considered as a carbon sheet wrapped around a point. 

As was noted earlier in this chapter, the heat of combustion used for carbon in Hess s law (Equation 1.6) computations is a function of the allotrope of carbon used. The difference in allotropic forms is a function of the bonding present. Thus, as shown in Figure 1.24, the idealized lattice network of flawless diamond is that of a continuous and endless array of sp -hybridized carbons about 154 pm apart (AH° of C... 

I n order to understand the formation of the allotropes of carbon from these precursors and the reasons for their behavior and properties, it is essential to have a clear picture of the atomic configuration of the carbon atom and the various ways in which it bonds to other carbon atoms. These are reviewed in this chapter. 

When an element has more than one form, it shows allotropy. The different forms are called allotropes. Diamond and graphite are therefore the allotropes of carbon. 

Note The allotropes of carbon include graphite, diamond, and buckminsterfullerene (Ceo) the latter, discovered in 1985, is composed of soccer-ball-shaped molecules. The thermod5mamic stability of buckminsterfullerene has not yet been determined and the validity of its inclusion on the C phase diagram is, therefore, uncertain. (Metastable phases, such as supercooled water, do not appear on phase diagrams.) The crystal structure is face-centered cubic with Ceo molecules at the comers and faces of a cubic unit cell. The unit cell is shown on the next page. 

Relative Standard Entropies Allotropes As mentioned previously, some elements can exist in two or more forms—called allotropes— in the same state of matter. For example, the allotropes of carbon include diamond and graphite— both solid forms of carbon. Since the arrangement of atoms within these forms is different, their standard molar entropies are different ... 

Carbon atoms form strong single bonds with each other and are also able to form multiple bonds via pic-pic bonding. Both types of bonding appear in the allotropes of carbon. 

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