Allotropic Modifications

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. 

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. 

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 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 ... 

Properties. Pure thorium metal is a dense, bright silvery metal having a very high melting point. The metal exists in two allotropic modifications. Thorium is a reactive, soft, and ductile metal which tarnishes slowly on exposure to air (12). Having poor mechanical properties, the metal has no direct stmctural appHcations. A survey of the physical properties of thorium is summarized in Table 1. Thorium metal is diamagnetic at room temperature, but becomes superconducting below 1.3—1.4 K. 

The structural complexity of borate minerals (p. 205) is surpassed only by that of silicate minerals (p. 347). Even more complex are the structures of the metal borides and the various allotropic modifications of boron itself. These factors, together with the unique structural and bonding problems of the boron hydrides, dictate that boron should be treated in a separate chapter. 

Boron is unique among the elements in the structural complexity of its allotropic modifications this reflects the variety of ways in which boron seeks to solve the problem of having fewer electrons than atomic orbitals available for bonding. Elements in this situation usually adopt metallic bonding, but the small size and high ionization energies of B (p. 222) result in covalent rather than metallic bonding. The structural unit which dominates the various allotropes of B is the B 2 icosahedron (Fig. 6.1), and this also occurs in several metal boride structures and in certain boron hydride derivatives. Because of the fivefold rotation symmetry at the individual B atoms, the B)2 icosahedra pack rather inefficiently and there... 

Phosphorus (like C and S) exists in many allotropic modifications which reflect the variety of ways of achieving catenation. At least five crystalline polymorphs are known and there are also several amorphous or vitreous forms (see Fig. 12.3). All forms, however, melt to give the same liquid which consists of symmetrical P4 tetrahedral molecules, P-P 225 pm. The same molecular form exists in the gas phase (P-P 221pm), but at high temperatures (above 800°C) and low pressures P4 is in equilibrium with the diatomic form P=P (189.5 pm). At atmospheric pressure, dissociation of P4 into 2P2 reaches 50% at 1800°C and dissociation of P2 into 2P reaches 50% at 2800°. 

The substance indicated by the same symbol in two or more equations is in exactly the same state in the reactions represented by those equations. In particular, the different allotropic modifications of a solid element (e.g., charcoal, graphite, diamond or yellow and red phosphorus) have different heats of combustion, and the particular form used must be specified in every case. 

The heat of transformation is defined as the change in enthalpy that results when one mole of a substance undergoes a specific change of state such as melting, evaporation, or allotropic modification. 

The only element that was discovered in body fluids (urine). This is plausible, as P plays a main role in all life processes. It is one of the five elements that make up DNA (besides C, H, N, and 0 evolution did not require anything else to code all life). The P-O-P bond, phosphoric acid anhydride, is the universal energy currency in cells. The skeletons of mammals consists of Ca phosphate (hydroxylapatite). The element is encountered in several allotropic modifications white phosphorus (soft, pyrophoric P4, very toxic), red phosphorus (nontoxic, used to make the striking surface of matchboxes), black phosphorus (formed under high pressures). Phosphates are indispensable as fertilizer, but less desirable in washing agents as the waste water is too concentrated with this substance (eutrophication). It has a rich chemistry, is the basis for powerful insecticides, but also for warfare agents. A versatile element. 

After different allotropic modifications of carbon nanostructures (fullerenes, tubules) have been discovered, a lot of papers dedicated to the investigations of such materials, for instance [9-15] were published, determined by the perspectives of their vast application in different fields of material science. 

In the light of several allotropic modifications known for phosphorus, the relatively high single bond energies and the tendency of phosphorus to catenate it remains mysterious, and that apart from P4 and As4 only very scarce information on isolated E cluster molecules is available from hard experimental data. In contrast, a vast amount of solid theoretical work has been performed [11],... 

According to the atomic theory the differences between what are termed "allotropic modifications" are generally ascribed to differences in the number and arrangement of the atoms constituting the molecules of such "modifications," and not to any differences in the atoms themselves. But we cannot argue that two such "allotropic modifications" or elements which are transmutable into one another... 

If we wish to distinguish between two such "allotropic modifications" apart from any theoretical views concerning the nature and constitution of matter, we can say that such "modifications" are different because equal weights of them contain, or are equivalent to, different quantities of energy, since the change of one "form" to another takes place only with the evolution or absorption (as the case may be) of heat. But, according to... 

A new class of conjugated hydrocarbons is that of the fullerenes [11], which represent an allotropic modification of graphite. Their electrochemistry has been studied in great detail during the last decade [126]. The basic entity within this series is the Ceo molecule (23). Because of its high electron affinity, it can be reduced up to its hexaanion (Fig. 4) [14,127]. Solid-state measurements indicate that the radical anion of Ceo reversibly dimerizes. NMR measurements confirm a u-bond formation between two radical anion moieties [128,129]. 

Red monoclinic crystal changes into a black allotropic modification at 267°C density 3.50g/cm3 melts at 320°C boils at 565°C insoluble in water soluble in alkalies. 

Silvery white metal soft and malleable hexagonal closed pack crystal system transforms to face-centered cubic crystals at 310°C which further transforms to a body-centered cubic allotropic modification at 868°C density 6.166 g/cm3 Brinnel hardness (as cast) 37 melts at 918°C vaporizes at 3,464°C vapor pressure 1 torr at 2,192°C electrical resistivity 56.8 x 10 ohm-cm at 25°C Young s modulus 3.84 x lO- dynes/cm Poisson s ratio 0.288 thermal neutron cross section 8.9 bams. 

Black tetragonal crystal exhibits two allotropic modifications—a stable alpha phase, occurring in tetragonal crystalline form (as hausmannite) and an unstable beta modification density 4.85 g/cm Moh s hardness 5.5 melts at 1,567°C insoluble in water soluble in hydrochloric acid. 

Exists in two aUotropic forms red tetragonal allotropic modification (alpha form) and the yeUow rhombic modification (beta form). 

Trigonal crystalline solid or amorphous powder mineral millerite has a yellow metallic luster color varies from yellow to brownish black density 5.30 to 6.65 g/cm3 exhibits three allotropic modifications (1) the acid-soluble amorphous alpha form obtained from nickel salt solution by precipitation with ammonium sulfide, (2) the alpha form rapidly transforms to a crystalline beta form as a brown colloidal dispersion upon exposure to air, and (3) a rhombo-hedral gamma modification found native as mineral millerite, which also can be prepared artificially under certain conditions. 

White crystals exists in three allotropic modifications a rhombohedral gamma form that transforms to trigonal beta form at 75°C, the trigonal converting to a cubic alpha form at 145°C. 

Titanium has two allotropic modifications (1) alpha form and (2) beta modification. The alpha form has a close-packed hexagonal crystal structure density 4.54 g/cm3 at 20°C and stable up to 882°C. It converts very slowly to a body-centered cubic beta form at 882°C. The density of the beta form is 4.40 g/cm3 at 900°C (estimated). The other physical properties are as follows ... 

Crystal structures they are complex with many allotropic modifications having, some of them, low symmetries, evidencing therefore, a 6d-5f covalent bonding (see Chap. C) ... 

The lower oxidation states are more stable than those of neptunium (59). Much that is known has not been disclosed, but the information is slowly emerging. Thus, only in 1954 was it revealed that the metallurgists at Los Alamos in 1945 knew that plutonium metal had the unique property of possessing at least five allotropic modifications at atmospheric pressure (74). 

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