Allotropic element forms

We don t normally think of elemental carbon as an organic compound. Historically, carbon was known to exist as three allotropes (elemental forms with different properties) amorphous carbon, diamond, and graphite. 

Elements along a rough diagonal from boron to polonium are intermediate in behavior, in some cases having both metallic and nonmetallic allotropes (elemental forms) these elements are designated as metalloids or semimetals. As described in Chapter 7, some elements, such as silicon and germanium, are capable of having their conductivity finely tuned by the addition of small amounts of impurities and are consequently of enormous importance in the manufacture of semiconductors in the computer industry. 

Pencil lead" is almost pure graphite. Graphite is the stable elemental form of carbon at 25°C and 1 atm. Diamond is an allotrope of graphite. Given... 

Allotrope An allotrope is formed when an element or compound exists in more than one form. Carbon is an example of an element found in different forms (e.g., carbon black, graphite, and diamonds). Oxygen has three alio tropes monoatomic or nascent oxygen (O) diatomic oxygen (O ), the gas we breathe and triatomic oxygen (O ), which is known as ozone. 

There are two allotropic (crystal forms) of terbium, both of which are dependent on its temperature. The alpha ((a) form exists at room temperatures and up to temperamres of 1,298°C, and the beta ( 3) form exists beyond these temperamres. Although terbium is a silvery metal that resembles aluminum and feels hke lead, it is much heavier than either of these two elements. It is placed in the yttrium subgroup (lanthanide series) of the rare-earths. It is also resistant to corrosion. 

With the exception of a few allotropic elements, the necessary input parameters to Eqs (6.1) or (6.2) are not available to establish the lattice stabilities of metastable structures. Therefore an alternative solution has to be found in order to achieve the desired goal. This has evolved into a standard format where the reference or ground state Gibbs energy is expressed in the form of genera] polynomials which reproduce assessed experimental Cp data as closely as possible. An example of such a standard formula is given below (Dinsdale 1991) ... 

Allotrope different forms of an element characterized by different structures Alloy a mixture of two or more metals, for example, zinc + copper = brass Alpha Decay nuclear process in which an alpha particle is emitted by the nucleus... 

The uniqueness of boron is clearly seen in its elemental forms, the number and structural complexity of which exceed those of any other element. At least five distinct allotropes are known, all of which contain icosahedral B12 cluster units that in most cases are accompanied by other boron atoms lying outside the icosahedral cages. The most thermodynamically stable form, j8-rhombohedral boron, has 105 B atoms in its unit cell, while the /3-tetragonal phase has 192 atoms and is still not completely elucidated despite years of study ... 

Cyclosulfanes (Elemental Sulfur Allotropes). Elemental sulfur displays a great propensity for catenation and polysnlfhr rings (cyclosulfanes) of various ring sizes (13-22) are known as allotropic forms of elemental sulfur. ... 

Most of the carbon in the Allende C3V chondrite is present in elemental form, rather than as polymer or extractable organic compounds (Breger et al., 1972). It was originally called amorphous carbon , since it is amorphous to x-rays. However, recent work shows it to be carbyne (Whittaker et al., 1980 Hayatsu et al., 1980b) a triply bonded, linear allotrope of elemental carbon. Carbyne exists in at least 10 varieties, ranging between graphite and diamond in hardness and density (Whittaker, 1978 and references therein). 

Boron s chemistry is so different from that of the other elements in this group that it deserves separate discussion. Chemically, boron is a nonmetal in its tendency to form covalent bonds, it shares more similarities with carbon and silicon than with aluminum and the other Group 13 elements. Like carbon, boron forms many hydrides like silicon, it forms oxygen-containing minerals with complex structures (borates). Compounds of boron have been used since ancient times in the preparation of glazes and borosilicate glasses, but the element itself has proven extremely difficult to purify. The pure element has a wide diversity of allotropes (different forms of the pure element), many of which are based on the icosahedral Bj2 unit. 

The element has several allotropes. The form stable below 98° is a-sulphur, whose rhombic crystals are built up from 8-membered rings of S atoms with S—S—S bond lengths of 2.12 A and bond angles of 105.4°. They are packed into crankshafts stacked in crossed layers (Warren and Burwell, 1935) as shown in Fig. 183a. 

Arsenic occurs in two allotropic forms. Allotropes are forms of an element with different physical and chemical properties. The more common form of arsenic is a shiny, gray, brittle, metallic-looking solid. The less common form is a yellow crystalline solid. It is produced when vapors of arsenic are cooled suddenly. 

One of the unusual properties of boron is the many physical forms, called allotropes, in which it occurs. Allotropes are forms of an element with different physical and chemical properties. One form of boron consists of clear red crystals with a density of 2.46 grams per cubic centimeter. A second form consists of black crystals with a metallic appearance and a density of 2.31 grams per cubic centimeter. Boron can also occur as a brown powder with no crystalline stmcmre. The density of this powder is 2.350 grams per cubic centimeter. 

Carbon is the 17th most common element in Earth s crust. Its abundance has been estimated to be between 180 and 270 parts per million. It rarely occurs as a diamond or graphite. Both allotropes are formed in the earth over millions of years, when dead plant materials are squeezed together at very high temperamres. Diamonds are usually found hundreds or thousands of feet beneath the earth s surface. Africa has many diamond mines. 

Praseodymium is a soft, malleable, ductile metal with a yellowish, metallic shine. Malleable means capable of being hammered into a thin sheet. Ductile means capable of being made into thin wires. Praseodymium has a melting point of 1,710°F (930°C) and a boiling point of about 5,800°F (3,200°C). Its density is 6.78 to 6.81 grams per cubic centimeter. Two allotropes of praseodymium exist. Allotropes are forms of an element with different physical and chemical properties. One allotrope, the alpha form, changes into a second allotrope, the beta form, at about 1,472°F (800 C). 

Sulfur exists in two allotropic forms. Allotropes are forms of an element with different physical and chemical properties. The two forms of sulfur... 

Several elements form homocyclic rings. Rhombic sulfur, the thermodynamically stable form at room temperature, consists of Sjj rings in the crown conformation. Unstable modifications, S . are known which include n — 6 through n = 36, In fact, sulfur has more allotropes than any other element. 3 Selenium also forms five-, six-, seven-, and eight-membered rings, but they are unstable with respect to the chain form. 

Network solids formed from one element only, such as the allotropes graphite and diamond in which the carbon atoms are connected differently Allotropes are forms of an element with different solid structures. 

Elemental sulfur is a yellow, tasteless, almost odorless, insoluble, nonmetallic molecular solid of crownlike Sg rings. The more stable allotrope, rhombic sulfur, forms beautiful yellow crystals, whereas the less stable allotrope, monoclinic sulfur, forms needlelike crystals. The two allotropes differ in the manner in which the Sg rings are stacked together. For simplicity, the elemental form of sulfur is often represented as S(s) rather than Sg(s). 

Allotropic modifications (allotropes) Different forms of the same element in the same physical state. 

No other element forms more solid allotropes than sulfur. At present, about 30 well characterized sulfur allotropes are known. These can be divided into ambient pressure allotropes and high-pressure allotropes depending on the conditions during preparation. While the molecular and crystal structures of the ambient pressure allotropes are known in most cases, this does not apply to all of the high-pressure forms. Therefore, in the following the two groups are described in separate sections of this chapter. 

Selenium is a naturally occurring mineral element in the earth s crust. It is distributed widely in nature and is found in most rocks and soils at concentrations between 0.1 and 2.0 ppm. However, selenium is seldom found in its elemental form in the environment, but is obtained primarily as a byproduct of copper refining. Selenium exists in several allotropic forms. The primary factor determining the fate of selenium in the environment is its oxidation state. Selenium is stable in four valence states (-2, 0, +4, and +6) and forms chemical compounds similar to those of sulfur. The heavy metal selenide compounds (-2) are insoluble in water, as is elemental selenium. The inorganic alkali selenites (+4) and selenates (+6) are soluble in water and are, therefore, more bioavailable. 

---