Allotropes

Chemistry: Foundations and Applications, (2004) by Anthony F. Masters

Allotropes

Figure 1. Elements that exist as allotropes.

B C N O Al Si P S Ga Ge As Se In Sn Sb Te Tl Pb Bi Po

Allotropes are different forms of the same element. Different bonding arrangements between atoms result in different structures with different chemical and physical properties. Allotropes occur only with certain elements, in Groups 13 through 16 in the Periodic Table. This distribution of allotropic elements is illustrated in Figure 1.

Group 13

Boron (B), the second hardest element, is the only allotropic element in Group 13. It is second only to carbon (C) in its ability to form element bonded networks. Thus, in addition to amorphous boron, several different allotropes of boron are known, of which three are well characterized. These are red crystalline α -rhombohedral boron, black crystalline β -rhombohedral boron (the most thermodynamically stable allotrope), and black crystalline β -tetragonal boron. All are polymeric and are based on various modes of condensation of the B 12 icosahedron (Figure 2).

Group 14

In Group 14, only carbon and tin exist as allotropes under normal conditions. For most of recorded history, the only known allotropes of carbon were diamond and graphite. Both are polymeric solids. Diamond forms hard, clear, colorless crystals, and was the first element to have its structure determined by x-ray diffraction. It has the highest melting point and is the hardest of the naturally occurring solids. Graphite, the most thermodynamically stable form of carbon, is a dark gray, waxy solid, used extensively as a lubricant. It also comprises the "lead" in pencils.

The diamond lattice (Figure 3a) contains tetrahedral carbon atoms in an infinite three-dimensional network. Graphite is also an infinite three-dimensional network, but it is made up of planar offset layers of trigonal carbons forming fused hexagonal rings (Figure 3b). The C-C bonds within

Figure 2. B 12 icosahedron.

Figure 3a. Portion of the structure of diamond. This structure repeats infinitely in all directions.

a layer are shorter than those of diamond, and are much shorter than the separation between the graphite layers. The weak, nonbonding, interaction between the layers, allowing them to easily slide over each other, accounts for the lubricating properties of graphite.

Diamond and graphite are nonmolecular allotropes of carbon. A range of molecular allotropes of carbon (the fullerenes) has been known since the discovery in 1985 of C 60 (Figure 4). The sixty carbon atoms approximate a sphere of condensed five- and six-membered rings. Although initially found in the laboratory, fullerenes have since been shown to occur in nature at low concentrations. C 60 and C 70 are generally the most abundant and readily isolated fullerenes.

In 1991 carbon nanotubes were discovered. They are more flexible and stronger than commercially available carbon fibers, and can be conductors or semiconductors. Although the mechanism of their formation has not been determined, they can be thought of as the result of "rolling up" a section of a graphite sheet and capping the ends with a hemisphere of C 60 , C 70 , or another molecular allotrope fragment. Five- or seven-membered rings can be incorporated among the six-membered rings, leading to an almost infinite range of helical, toroidal, and corkscrew-shaped tubes, all with different mechanical strengths and conductivities.

Figure 3b. Portion of the structure of graphite. This structure repeats infinitely in all directions.

Figure 4. A fullerene allotrope of C 60 .

Tin is a relatively low melting (232°C) material that exists in two allotropic forms at room temperature and pressure, α -Sn (gray tin) and β -Sn (white tin). α -Sn is the stable form below 13°C and has the diamond structure (Figure 3a). White, or β -Sn is metallic and has a distorted close-packed lattice.

Group 15

There are two allotropic elements in Group 15, phosphorus and arsenic . Phosphorus exists in several allotropic forms. The main ones (and those from which the others are derived) are white, red, and black (the thermodynamically stable form at room temperature). Only white and red phosphorus are of industrial importance. Phosphorus was first produced as the common white phosphorus, which is the most volatile , most reactive, and most toxic, but the least thermodynamically stable form of phosphorus, α -P 4 . It coverts to a polymorphic form, β -P 4 , at −76.9°C. White phosphorus is a waxy, nonconductor and reacts with air—the phosphorescent reaction of oxygen with the vapor above the solid producing the yellow-green chemiluminescent light, which gives phosphorus its name (after the Greek god, Eosphoros, the morning star, the bringer of light). The phosphorus in commercial use is amorphous red phosphorus, produced by heating white phosphorus in the absence of air at about 300°C. It melts around 600°C and was long thought to contain polymers formed by breaking a P-P bond of each P 4 tetrahedron of white phosphorus then linking the "opened" tetrahedra (Figures 5a and 5b).