Chemical elements allotropes

Elemental sulfur is one of the best investigated chemical elements but it represents also one of the most complex systems. The large number of its allotropes (ca. 30 [1]) and their peculiar behavior on melting, vaporization... 

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. 

Carbon is unique among chemical elements since it exists in different forms and microtextures transforming it into a very attractive material that is widely used in a broad range of electrochemical applications. Carbon exists in various allotropic forms due to its valency, with the most well-known being carbon black, diamond, fullerenes, graphene and carbon nanotubes. This review is divided into four sections. In the first two sections the structure, electronic and electrochemical properties of carbon are presented along with their applications. The last two sections deal with the use of carbon in polymer electrolyte fuel cells (PEFCs) as catalyst support and oxygen reduction reaction (ORR) electrocatalyst. 

The extraordinary ability of carbon to combine with itself and other chemical elements in different ways is the basis of organic chemistry. As a consequence, there is a rich diversity of structural forms of solid carbon because it can exist as any of several allotropes. It is found abundantly in nature as coal, as natural graphite and also in much less abundant form as diamond. 

Since the oxides do not have to be isolated, the sulfur solution after addition of the peroxyacid solution is simply kept in the refrigerator until S,(, has formed which is then isolated by cooling and recrystallization When both Sg and S g are dissolved in CS and the solution is cooled, then, under special concentration conditions, a new sulfur allotrope crystallizes out as orange-yellow opaque crystals of m.p. 92 °C. This compound has been shown by vibrational spectroscopy and X-ray structural analysis to consist of equal amounts of Sg and molecules in their usual conformations. In solution the mean molecular weight of 258 corresponding to 8 atoms per molecule indicates complete dissociation This is the first example of an allotrope of a chemical element consisting of molecules of different sizes. 

POLYMORPHISM. 1, A phenomenon in which a substance exhibits different forms. Dimorphic substances appear in two solid forms, whereas trimorphic exist in three, as sulfur., carbon, tin, silver iodide, and calcium carbonate. Polymorphism is usually restricted to the solid state, Polymorphs yield identical solutions and vapors (if vaporizable). The relation between them has been termed physical isomerism. See Allotropes under Chemical Elements, See also Mineralogy,... 

URANIUM. [CAS 7440-61-1], Chemical element symbol. U, at. no. 92, at. wt, 238,03, periodic table group (Actinides), mp 1,131 to i. 33°C, bp 3,818°C, density 18.9 g/cm3 (20UC). Uranium metal is found in three allotropic forms (1) alpha phase, stable below 668°C, orthorhombic (2) beta phase, existing between 668 and 774°C. tetragonal and (3) gamma phase, above 774°C, body-centered cubic crystal structure. The gamma phase behaves most nearly that of a true metal. The alpha phase has several nonmetallic features in its crystallography. The beta phase is brittle. See also Chemical Elements. 

ZIRCONIUM. [CAS 7440-67-7]. Chemical element symbol Zr. at. no. 40, at. wt. 91.22, periodic table group 4. mp l,853°C, bp 4,376°C, density 6.44 g/cm3, 6.47 g/cm3 (single crystal). Metallic zirconium is allotropic. Up to about 863°C, the alpha phase (hexagonal close-packed) is stable above this temperature, the metal assumes the beta phase (body-centered cubic). The most common impurity, oxygen, tends to stabilize the alpha phase. 

The crystal structures of the chemical elements provide a simple starting point for illustrating what can be learned from listed atomic coordinates. Tin has both a nonmetallic and a metallic allotrope (polymorph). 

The particular advantage of diffraction analysis is that it discloses the presence of a substance as that substance actually exists in the sample, and not in terms of its constituent chemical elements. For example, if a sample contains the compound A By, the diffraction method will disclose the presence of A B as such, whereas ordinary chemical analysis would show only the presence of elements A and B. Furthermore, if the sample contained both A B, and Aj Bjy, both of these compounds would be disclosed by the diffraction method, but chemical analysis would again indicate only the presence of A and B. To consider another example, chemical analysis of a plain carbon steel reveals only the amounts of iron, carbon, manganese, etc., which the steel contains, but gives no information regarding the phases present. Is the steel in question wholly martensitic, does it contain both martensite and austenite, or is it composed only of ferrite and cementite Questions such as these can be answered by the diffraction method. Another rather obvious application of diffraction analysis is in distinguishing between different allotropic modifications of the same substance solid silica, for example, exists in one amorphous and six crystalline modifications, and the diffraction patterns of these seven forms are all different. 

An allotrope of a chemical element is defined as a solid phase (of the pure element) which differs by its crystal structure and therefore by its X-ray diffraction pattern from the other allotropes of that element. This definition can be extended to microcrystalline and amorphous phases which may be characterized either by their diffraction pattern or by suitable molecular spectra. 

The wide variations in the properties of iron and iron alloys must be related to the existence of pure soHd iron in more than one phase, i.e., several crystallographic structures. This characteristic of many chemical elements including iron is called allotropism. Allotropism must not be confused with the term applied to a pure compound (e.g., a molecule or an alloy) that exhibits several crystal lattices and is calledpofy/wo/p/i/s i. The temperature at which a change in... 

Investigations of the chemical properties of plutonium have continued in many laboratories throughout the world as it has become available. This has led to the situation where the chemistry of this relative newcomer is as well understood as is that of most of the well-studied elements. The four oxidation states of plutonium—III, IV, V, and VI—lead to a chemistry which is as complex as that of any other element. It is unique among the elements in that these four oxidation states can all exist simultaneously in aqueous solution at appreciable concentration. As a metal, also, its properties are unique. Metallic plutonium has six allotropic forms, in the temperature range from room temperature to its melting point (640 C), and some of these have properties not found in any other known metal. 

Many elements including sulphur, carbon and oxygen can exist in two or more forms with different physical, and often chemical, properties such elements are said to exhibit allotropy and the different forms are known as allotropes or allotropic forms. 

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