Sulfur allotropy

F. Tuinstra, Structural Aspects of the Allotropy of Sulfur and the Other Divalent Elements, Waltman, Delft, 1967... 

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. 

Elemental sulfur exhibits complicated allotropy, that is, it exists in many modifications.4 The stable, prismatic crystal form at room temperature, a-S or orthorhombic sulfur, is built up of stacks of Ss rings (Section 3.4). If heated quickly, it melts at 112.8 °C. If it is heated slowly, however, it changes to needlelike crystals of /3-S or monoclinic sulfur, which is the stable form above 95.5 °C and which melts at 119 °C. Both 0-S and the yellow mobile melt (below 160 °C) are composed exclusively of Ss rings. Solids containing S7, Sg, S10, S12, and other rings are known, but all slowly revert to Sg below 160 °C. 

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. 

When an element can exist in more than one physical form in the same state it is said to exhibit allotropy (or polymorphism). Each of the different physical forms is called an allotrope. Allotropy is actually quite a common feature of the elements the Periodic Table (p. 136). Some examples of elements which show allotropy are sulfur, tin, iron and carbon. 

The allotropy of sulfur is far more extensive and complex than that of any other element. This arises from the following factors. 

Tuinstra published his Structural Aspects of the Allotropy of Sulfur and the Other Divalent Elements in 1967. They are not discussed here in detail but are sketched only very roughly. In these considerations all excited or ionized states of the molecules are excluded, and only molecules in which all bonds are equal are taken into consideration—that is molecules with identical bond length, angles, and dihedral angles. Still the number of conformations for a distinct number of atoms remains large. 

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. 

Sulfur exhibits allotropy and its structure in all phases is quite complex. The common crystalline modification, rhombic sulfur, is in equilibrium with a triclinic modification above 96°C. Both have structures based on Sg-rings but the crystals are quite different. If molten sulfur is poured into water a dark red plastic form is obtained in a semielastic form. The structure appears to be a helical chain of S atoms. Selenium and tellurium both have a gray metal-like modification but sulfur does not have this form. 

The difference and the similarity between allotropy and polymorphism can be illustrated by considering sulfur. The rhombic and monoclinic crystalline forms both consist of puckered Ss rings, and these two modifications can interconvert by heating and cooling. It is tempting to call this relationship crystal allotropy, but the correct term is polymorphism because both structures involve the same compound (i.e., atomic connectivity). When heated to above 160 °C, the Ss rings open by means of a free radical reaction to form polymeric chains. In contrast to crystal allotropy, the relationship between the polymeric chains and the Sg rings can be termed chemical allotropy. However, since polymorphism is the preferred term for crystal allotropy, chemical allotropy can be shortened to allotropy. 

The allotropy of carbon, oxygen, phosphorus, and sulfur results from the versatility of their covalent bonding. Carbon occurs as diamond and as graphite (Fig. 21.1). Diamond is extremely hard, in consequence of its stable network covalent structure, which is entirely o--bonded. Graphite is relatively soft, in part because of the ease with which its TT-bonded atomic layers can slip past one another. At ordinary temperatures and pressures, both forms are quite unreactive, and graphite is the form with lower free energy (more stable) by about 0.7 kcal/mole. 

The early chemistry of sulfur was quite confused. Jabir (10th Century), a most noted alchemist, insisted that all metals were compounded of mercury and sulfur. Such theoretical bias was long extant. In some aspects the present chemistry of sulfur resembles the qualitative descriptions of observers several thousand years ago. The allotropy of sulfur recognized by Pliny (1st Century A.D.) still presents formidable problems of interpretation. (Donohue, 1961 Pauling, 1949 and Prins et al., 1957). 

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