Allotrope

ALLOTROPES AND COMPOUNDS INVOLVING ELEMENT-ELEMENT BONDS 

The major allotropes of sulfur in the solid, liquid, and gaseous phases. 

Plastic sulfur (dark, blackish) (stretchable, long fibers containing helical chains) 

Carbon Allotropes.—Thermodynamic functions of single-crystal graphite have been assessed in the t emperature range 0—3000 K.7 The experimental specific heats have been described by a computer-fitted single equation enthalpies, entropies, and free energies have also been calculated. 

Galimov s cavitation hypothesis8 for the formation of natural diamonds has been refuted by Frank et al.9 on the basis of the kinetics of crystal 

Inorganic Chemistry of the Main-group Elements growth and martensitic conversion. It is conceded, however, that although it is inconceivable that the recognized diamonds for which mines are worked were produced by Galimov s process, the formation of microdiamonds by this process is possible. 

The postulate10 that natural diamonds were formed by a reduction of C02 by pyrrhotite in reactions such as (1) has been tested experimentally by oxidizing natural diamonds by pure oxygen at high temperature.11 

Diamonds formed by this route would contain inclusions containing either free sulphur, or sulphur compounds, or both on oxidation, gaseous sulphur oxy-compounds would be formed, which could be detected mass spec-trometrically. No evidence of sulphur was found, and so this proposed route to natural diamond is not thought to be responsible for the formation of all natural diamonds. The data are limited by the amount of diamond oxidized (16 g), and so the possibility of some diamond formation by this route cannot be ruled out.11 

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. 

A variety of crystalline modifications (tetragonal red, triclinic red, cubic red), possibly with similar polymeric structures can also be prepared by heating amorphous red phosphorus at over 500°C. 

The most thermodynamically stable, and least reactive, form of phosphorus is black phosphorus, which exists as three crystalline (orthorhombic-, rhombohedral- and metallic, or cubic, and one amorphous, allotrope. All are polymeric solids and are practically nonflammable. Both orthorhombic and rhombohedral phosphorus appear black and graphitic, consistent with their layered structures. 

A violet crystalline allotrope, monoclinic phosphorus, or Hittorf s phosphorus, after its discoverer, can be produced by a complicated thermal and electrolytic procedure. The structure is very complex, consisting of tubes of 

At least six forms of sohd arsenic have been reported, of which three are amorphous. The most stable and most common form of arsenic at room temperature is a brittle, steel-gray solid (a-As) with a structure analogous to that of rhombohedral black phosphorus. Arsenic vapor contains tetrahedral As4 molecules, which are thought to be present in the yellow tmstable arsenic formed by condensation of the vapor. Arsenic occurs naturally as a-As and also as the mineral arsenolamprite, which may have the same structure as orthorhombic black phosphorus. 

Some elements can exist in different physical forms called allotropes alio means different, trap means form ). Different forms of the same element may have distinctly different chemical and physical properties. For example, carbon has several allotropes that include graphite and diamonds. Oxygen exists in the atmosphere mostly as diatomic molecules, O, but can also exist as the triatomic molecule ozone, O3. Ozone is significantly more reactive than O whereas O is essential to Ufe, O3 is harmful. Elements that are solids under standard conditions may have allotropes that differ primarily in their crystalline structures. 

Unlike the number of protons in atoms of an element, the number of neutrons is not fixed for a particular element. Neutrons have an important function in atoms they provide the glue that holds nuclei together. 

Remember that protons all possess a positive charge. Charges with the same sign (that is, both are positive or both are negative) tend to repel each other. Without neutrons, no atom could exist that had two or more protons. By means of an extremely strong force of nature called the 

Isotopes are atoms of the same element that have different numbers of neutrons. 

A hydrogen atom contains only one proton, so there is no repulsive force in the nucleus. 

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 attotropes. 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. 

In the lower layers of the atmosphere, ozone occurs only In trace quantities. Some ozone, however, is also created In the lower atmosphere by lightning, during electric storms, accounting for the smell of rain that 

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

Another element that exhibits allotropy because of variations in the crystal structure is tin. The common allotrope is tin metal, also known as a alpha) tin, which is stable at ambient temperatures. The other allotrope, which generally occurs as a gray powder and is known as p beta) tin, but also as tin pest, is formed only at very low temperatures when tin cools down to temperatures below -18°C, the ordinary allotrope, a tin, is converted to p tin, and the transformation is irreversible under ordinary temperatures. Tin objects exposed to temperatures below -18°C in very cold regions of the world, for example, are generally severely damaged when part of the tin converts to tin pest. In extreme cases, when exposure to low temperatures extends for long periods of time, the allotropic conversion may result in the transformation of tin objects into heaps of gray p-tin powder. 

Black pigments of vegetable origin have generally been made from various kinds of charred plant matter, mostly wood, but also leaves or seeds the charcoal formed during the charring process is then washed, to remove soluble matter, and finally ground to powder. Over 95% of well-burned char- 

AUotropes.— The chemistry of the allotropic forms of carbon which has been abstracted for this Report concentrates on their structural, spectroscopic, and intrinsic chemical properties those papers which describe solely their catalytic, adsorption, diffusion, and other similar properties have been excluded. 

Vapour-phase Species. The chemical properties of carbon vapour have been reviewed by Skell et al The report describes the preparation of and compositions of the vapour and discusses the chemistry of the constituent species, particularly C, Cg, C3, and C4. Several papers describing detailed aspects of the chemistry of these vapour-phase species have also been published. Chemical reactions of C, Cg, and C3, formed by laser-induced vaporization of either graphite or tantalum carbide, with oxygen, hydrogen, or methane have been studied by means of time-resolved mass spectrometry and gas-phase titrations in an attempt to determine the relative abundances of these three species in the vapour phase (Table 1). The techniques developed and 

Skell et have investigated the reactions of diatomic carbon, produced in a carbon arc under high vacuum, with simple organic compounds at surfaces cooled by liquid nitrogen. The reaction with alkanes and ethers gives rise to allenes as well as acetylene, whereas the reaction with propylene (two moles) yields 5,6-methano-l,4-heptadienes. 

Diamond. The synthesis of both natural and synthetic diamond has been the subject of several investigations. In a discussion of the possible natural routes to diamond, Galimov suggests that synthesis may occur under conditions of cavitation occurring in a fast-moving magmatic melt flowing in kimberlite pipes.  

The growth of diamond in metal-carbon systems under superhigh pressures has been observed on a substrate (seed crystals of synthetic diamond) in the form of single crystallites, as discrete linear series of crystals, or in groups of 

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Many substances exist in two or more solid allotropic fomis. At 0 K, the themiodynamically stable fomi is of course the one of lowest energy, but in many cases it is possible to make themiodynamic measurements on another (metastable) fomi down to very low temperatures. Using the measured entropy of transition at equilibrium, the measured heat capacities of both fomis and equation (A2.1.73) to extrapolate to 0 K, one can obtain the entropy of transition at 0 K. Within experimental... 

The scope of tire following article is to survey the physical and chemical properties of tire tliird modification of carbon, namely [60]fullerene and its higher analogues. The entluisiasm tliat was triggered by tliese spherical carbon allotropes resulted in an epidemic-like number of publications in tire early to mid-1990s. In more recent years tire field of fullerene chemistry is, however, dominated by tire organic functionalization of tire highly reactive fullerene... 

EttI R, Chao I, Diederich F and Whetten R L 1991 Isolation of C-g, a chiral (Dg) allotrope of carbon Nature 353... 

Arsenic and antimony resemble phosphorus in having several allotropic modifications. Both have an unstable yellow allotrope. These allotropes can be obtained by rapid condensation of the vapours which presumably, like phosphorus vapour, contain AS4 and Sb4 molecules respectively. No such yellow allotrope is known for bismuth. The ordinary form of arsenic, stable at room temperature, is a grey metallic-looking brittle solid which has some power to conduct. Under ordinary conditions antimony and bismuth are silvery white and reddish white metallic elements respectively. 

White phosphorus is very reactive. It has an appreciable vapour pressure at room temperature and inflames in dry air at about 320 K or at even lower temperatures if finely divided. In air at room temperature it emits a faint green light called phosphorescence the reaction occurring is a complex oxidation process, but this happens only at certain partial pressures of oxygen. It is necessary, therefore, to store white phosphorus under water, unlike the less reactive red and black allotropes which do not react with air at room temperature. Both red and black phosphorus burn to form oxides when heated in air, the red form igniting at temperatures exceeding 600 K,... 

None of the common allotropic forms of these metals is affected by air unless they are heated, when aU burn to the (III) oxide. 

White and red phosphorus combine directly with chlorine, bromine and iodine, the red allotrope reacting in each case at a slightly higher temperature. The reactions are very vigorous and white phosphorus is spontaneously inflammable in chlorine at room temperature. Both chlorine and bromine first form a trihalide ... 

Oxygen, sulphur and selenium are known to exist in more than one allotropic form. 

This exists in two allotropic forms, oxygen, O2 and ozone, O3. 

Like sulphur, selenium exists in a number of allotropic forms. These include both crystalline, rhombic and monoclinic modifications... 

The metal looks like iron it exists in four allotropic modifications, stable over various temperature ranges. Although not easily attacked by air. it is slowly attacked by water and dissolves readily in dilute acids to give manganese(II) salts. The stable form of the metal at ordinary temperatures is hard and brittle—hence man ganese is only of value in alloys, for example in steels (ferroalloys) and with aluminium, copper and nickel. 

Carbon is found free in nature in three allotropic forms amorphous, graphite, and diamond. A fourth form, known as "white" carbon, is now thought to exist. Ceraphite is one of the softest known materials while diamond is one of the hardest. 

In 1824 Berzelius, generally credited with the discovery, prepared amorphous silicon by the same general method and purified the product by removing the fluosilicates by repeated washings. Deville in 1854 first prepared crystalline silicon, the second allotropic form of the element. 

Phosphorus exists in four or more allotropic forms white (or yellow), red, and black (or violet). Ordinary phosphorus is a waxy white solid when pure it is colorless and transparent. White phosphorus has two modifications alpha and beta with a transition temperature at -3.8oC. 

Sulfur is pale yellow, odorless, brittle solid, which is insoluble in water but soluble in carbon disulfide. In every state, whether gas, liquid or solid, elemental sulfur occurs in more than one allotropic form or modification these present a confusing multitude of forms whose relations are not yet fully understood. 

Gobalt is a brittle, hard metal, resembling iron and nickel in appearance. It has a metallic permeability of about two thirds that of iron. Gobalt tends to exist as a mixture of two allotropes over a wide temperature range. The transformation is sluggish and accounts in part for the wide variation in reported data on physical properties of cobalt. 

Selenium exists in several allotropic forms. Three are generally recognized, but as many as that have been claimed. Selenium can be prepared with either an amorphous or crystalline structure. The color of amorphous selenium is either red, in powder form, or black, in vitreous form. Crystalline monoclinic selenium is a deep red crystalline hexagonal selenium, the most stable variety, is a metallic gray. 

Strontium is found chiefly as celestite and strontianite. The metal can be prepared by electrolysis of the fused chloride mixed with potassium chloride, or is made by reducing strontium oxide with aluminum in a vacuum at a temperature at which strontium distills off. Three allotropic forms of the metal exist, with transition points at 235 and 540oC. 

Metallic polonium has been prepared from polonium hydroxide and some other polonium compounds in the presence of concentrated aqueous or anhydrous liquid ammonia. Two allotropic modifications are known to exist. 

The metal has a bright silvery metallic luster. Neodymium is one of the more reactive rare-earth metals and quickly tarnishes in air, forming an oxide that spalls off and exposes metal to oxidation. The metal, therefore, should be kept under light mineral oil or sealed in a plastic material. Neodymium exists in two allotropic forms, with a transformation from a double hexagonal to a body-centered cubic structure taking place at 863oC. 

Ytterbium has a bright silvery luster, is soft, malleable, and quite ductile. While the element is fairly stable, it should be kept in closed containers to protect it from air and moisture. Ytterbium is readily attacked and dissolved by dilute and concentrated mineral acids and reacts slowly with water. Ytterbium has three allotropic forms with transformation points at -13oC and 795oC. The beta form is a room-temperature, face-centered, cubic modification, while the... 

The metal has a silvery appearance and takes on a yellow tarnish when slightly oxidized. It is chemically reactive. A relatively large piece of plutonium is warm to the touch because of the energy given off in alpha decay. Larger pieces will produce enough heat to boil water. The metal readily dissolves in concentrated hydrochloric acid, hydroiodic acid, or perchloric acid. The metal exhibits six allotropic modifications having various crystalline structures. The densities of these vary from 16.00 to 19.86 g/cms. 

AUotropes. Systematic names for gaseous and liquid modifications of elements are sometimes needed. Allotropic modifications of an element bear the name of the atom together with the descriptor to specify the modification. The following are a few common examples ... 

Even at the lowest temperatures, a substantial pressure is required to soHdify helium, and then the soHd formed is one of the softest, most compressible known. The fluid—soHd phase diagrams for both helium-3 and helium-4 are shown in Eigure 1 (53). Both isotopes have three allotropic soHd forms an fee stmeture at high pressures, an hep stmeture at medium and low pressures, and a bcc stmeture over a narrow, low pressure range for helium-4 and over a somewhat larger range for helium-3. The melting pressure of helium-4 has been measured up to 24°C, where it is 11.5 GPa (115 kbar) (54). 

Thin films (qv) of lithium metal are opaque to visible light but are transparent to uv radiation. Lithium is the hardest of all the alkaH metals and has a Mohs scale hardness of 0.6. Its ductiHty is about the same as that of lead. Lithium has a bcc crystalline stmcture which is stable from about —195 to — 180°C. Two allotropic transformations exist at low temperatures bcc to fee at — 133°C and bcc to hexagonal close-packed at — 199°C (36). Physical properties of lithium are Hsted ia Table 3. 

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