Sulfur allotropic modifications

All allotropic forms of sulfur that occur at room temperature consist of Sn rings, with n = 6 to 20 (Table 16.4.1). Many homocyclic sulfur allotropes have been characterized by X-ray crystallography S6, S7 (two modifications), Ss (three modifications), S9, S10, S6-Sio, Sn, S12, S13, S14, Sis (two forms), S20, and polymer chain Sco. Their structural data are listed in Table 16.4.2, and the molecular structures are shown in Fig. 16.4.1. 

Regardless of the allotropic modification, solid 87 decomposes at 20 °C completely within ten days but can be stored at -78 for longer periods of time without decomposition. The first signs of the decomposition products 8g and 8 (polymeric sulfur) can be detected already after 30 min at 20 °C... 

Allotropy.—Dimorphism apart, a few substances are known to exist in more than one solid form. These varieties of the same substance exhibit different physical properties, while their chemical qualities are the same in kind. Such modifications are said to be allotropic. One or more allotropic modifications of a substance are usually crystalline, the other or others amorphous or vitreous. Sulfur, for example, exists not only in two dimor-j)hous varieties of crystals, but also in a third,. allotropic form, in which it is flexible, amorphous, and transparent. Carbon exists in three allotropic forms two crystalline, the diamond and graphite the third amorphous. 

Elemental sulfur is altered by chemical, allotropic, or me-chanical means. Of the chemical modifiers the Thiokol family of additives are some of the most effective modifiers available. By control of the time-temperature history of sulfur in the liquid and solid phases, it is possible to control the allotropic modification of sulfur and its mechanical properties. Mechanical modification of sulfur with non-chemically reactive filler materials such as aggregates and fibers yields materials with properties different from those of the parent materials. 

Sulfur is a low cost, high purity, readily available material It exists in many allotropic modifications with interesting physical and chemical properties. In 1970 world consumption of sulfur in all forms totaled 38 million tons. However, only a small percentage of the sulfur consumed depended upon or benefited from its high purity and unique chemical and physical properties, Sulfurization of asphalt, sulfur impregnation of ceramics and paper, and the use of sulfur in construction are a few potential applications which could exploit sulfur s unique properties. The present status and additional research needed to commercialize these potential applications are discussed. 

This method of detecting free selenium is decisive only when free sulfur is absent. Selenium exists in several allotropic modifications. The usual red )S-form, obtained by reducing a solution of selenious acid, is slightly soluble in carbon disulfide. The more stable dark a-form, obtained by heating the red modification, is almost insoluble in this solvent. 

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. 

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. 

Sulfur exists in several allotropic forms at ordinary temperatures it exists as thermodynamically stable alpha-cyclooctasulfur (Ss). The cyclooctasulfur also has two other modifications, the beta and the gamma forms. 

AUatropes. Some or the elements exist in two or more modifications distinct in physical properties, and usually in some chemical properties. Allotropy in solid elements is attributed to differences in the bonding of the atoms in the solid. Various types of allotropy are known. In ertuntiomorphic allotropy, the transition from one form to another is reversible and takes place at a definite temperature, above or below which only one form is stable, e.g., the alpha and beta forms of sulfur. In dynamic alloimpy. the transition from one form to another is reversible, but with no definite transition temperature. The proportions of the allotropcs depend upon the temperature. In monotropic allotropy, the transition is irreversible. One allotrope is mctastable at all temperatures, e.g.. explosive antimony. 

Allotropic forms of sulfur. Solid sulfur exists in two crystalline modifications. Rhombic sulfur consists of S8 molecules, is stable at temperatures below 95.5°C, has a specific gravity of 1.96, and is soluble in carbon disulfide. At 95.5°C, rhombic sulfur changes slowly, with absorption of heat, into the monoclinic form. Molten rhombic sulfur consists of S8 molecules and exists as a pale-yellow, thin, and limpid liquid known as X sulfur. When the temperature is raised, X sulfur is slowly converted to dark and viscous p sulfur, which consists of Ss and S4 molecules and which is considered to be the amorphous variety of... 

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. 

The only stable form of sulfur at STP conditions is the well known orthorhombic ce-Sg modification which was already known in antiquity. No wonder that this allotrope is by far the best studied. Although there is a considerable amount of knowledge on the structural and physical as well as chemical properties of a-Sg, from the experimental and theoretical point of view there are also ambiguities. For example, the thermal volume expansion below 300 K was reported contradictorily [76, 77]. 

Allotropes of an element are dilferent structural modifications of that element. Allotropes of sulfur include cyclic... 

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