Allotropic forms of sulphur

Sulphur exists in both crystalline and amorphous allotropic forms. The crystalline forms are rhombic sulphur (also called alpha-sulphur) and monoclinic sulphur (also called beta-sulphur); the amorphous forms include plastic sulphur (gamma-sulphur), milk of sulphur and colloidal sulphur. Rhombic (alpha) sulphur is the thermodynamically stable form at ordinary temperature and pressure; it forms characteristic yellow crystals composed of puckered, crown-shaped S8 rings. On slow heating above 96 °C it converts to monoclinic (beta) sulphur, and on cooling below 96 °C the beta form reverts to the alpha form — the two forms are thus enantiotropic, interconverting reversibly at the 96 °C transition temperature. Monoclinic sulphur also consists predominantly of S8 rings, together with a small proportion of S6 rings; it forms long, needle-like prisms (hence it is also called prismatic sulphur) and is stable in the range 96-119 °C, slowly reverting to rhombic sulphur outside that range. When molten sulphur is poured rapidly into cold water, a soft, rubbery, yellow ribbon of plastic sulphur is obtained, made of long disordered polymeric sulphur chains; it is easily stretched, but on standing it slowly hardens and converts to the stable rhombic form. Sulphur also exists in liquid and vapour states: around 140 °C, monoclinic sulph …