α-Sulfur 1: introduction

Sulfur is unreactive enough to exist uncombined in nature. It has a distinct yellow colour and emits an unpleasant odour when wet. This is because there a slight reaction with the water to produce hydrogen sulfide, H₂S, which smells of rotten eggs. H₂S is toxic above about 300 ppm (parts per million); beneath this level enzymes in the body can oxidise it to sulfate indefinitely. Fortunately perhaps, its odour is incredibly powerful and so one could hardly be unaware of its presence: it can be smelt at a concentration as low as 5 parts per billion, 60 thousand times less than the toxic level. (Despite being less toxic, carbon monoxide is more dangerous as it is odourless.)

Sulfur exists in many allotropes with different lengths and ring sizes. Of these the most stable is the S₈ ring. This allotrope of sulfur can form two possible crystal structures, or polymorphs, known as α- and β-sulfur. Of these two, α-sulfur is more stable up to 95°. What happens above this temperature is complicated. β-Sulfur is more thermodynamically stable but the transformation to it is slow. If the temperature is raised further the S₈ ring begins to decompose to form allotropes with smaller molecular units. These smaller units have lower melting points than S₈ and so melting then usually occurs.

A fragment of the bulk structure of α-sulfur is shown to the left, with a black wireframe indicating the orthorhombic unit cell. In this type of crystal system the three types of cell length are different but the three types of bond angle are all 90°, which makes the unit cell a rectangular prism.

Go to page 2 to focus on the S₈ ring.

Other elements/compounds with an orthorhombic unit cell:

I₂, Br₂, black phosphorus, Ga, GaAs, PBr₅, PbCl₂, HgCl₂, HgBr₂, CsClO₄, FeS, GeS, SnS, SiS₂, MnP, Ag₂Se, CuTe, CdTe, TlF, XeO₃, CrB, YBa₂Cu₃O_7-x, benzene (C₆H₆), aragonite (CaCO₃), barite (BaSO₄), brookite (TiO₂), massicot (PbO), nitre (KNO₃), chalcocite (Cu₂S), cementite (Fe₃C), anglesite (PbSO₄), anhydrite (CaSO₄), valentinite (Sb₂O₃), stibnite (Sb₂S₃), bismuthinite (Bi₂S₃), strontianite (SrCO₃), andalusite (Al₂SiO₅), celestite (SrSO₄), cerussite (PbCO₃), dyscrasite (Ag₃Sb), enstatite (MgSiO₃), goethite (FeO(OH)), hopeite (Zn₃(PO₄)₂.4H₂O), marcasite (FeS₂), purpurite (MnPO₄), sanbornite (BaSi₂O₅), shattuckite (Cu₅(SiO₃)₄(OH)₂), sillimanite (Al₂SiO₅), strengite (FePO₄.2H₂O), tephroite (Mn₂SiO₄), thenardite (Na₂SO₄), variscite (AlPO₄.2H₂O) and witherite (BaCO₃).

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