Sulphur

Sulphur is a chemical element, an abundant, multivalent non-metal. Under normal conditions, sulfur atoms form cyclic octatomic molecules with chemical formula S₈. Elemental sulphur is a bright yellow crystalline solid when at room temperature. Chemically, sulphur can react as either an oxidant or reducing agent. It oxidizes most metals and several nonmetals, including carbon, which leads to its negative charge in most organosulfur compounds, but it reduces several strong oxidants, such as oxygen and fluorine.

Characteristics

When burned, sulphur melts to a blood-red liquid and emits a blue flame that is best observed in the dark.

Physical properties

Sulphur forms polyatomic molecules with different chemical formulas, with the best-known allotrope being octasulfur, cyclo-S8. Octasulphur is a soft, bright-yellow solid with only a faint odor, similar to that of matches. It melts at 115.21 °C, boils at 444.6 °C and sublimes easily. At 95.2 °C, below its melting temperature, cyclo-octasulphur changes from α-octasulfur to the β-polymorph. The structure of the S₈ ring is virtually unchanged by this phase change, which affects the intermolecular interactions. Between its melting and boiling temperatures, octasulphur changes its allotrope again, turning from β-octasulphur to γ-sulphur, again accompanied by a lower density but increased viscosity due to the formation of polymers. At even higher temperatures, however, the viscosity decreases as depolymerization occurs. Molten sulphur assumes a dark red color above 200 °C. The density of sulphur is about 2 g·cm⁻³, depending on the allotrope; all of its stable allotropes are excellent electrical insulators.

Chemical properties

Sulphur burns with a blue flame concomitant with formation of sulphur dioxide, notable for its peculiar suffocating odor. Sulphur is insoluble in water but soluble in carbon disulfide and, to a lesser extent, in other nonpolar organic solvents, such as benzene and toluene. The first and the second ionization energies of sulphur are 999.6 and 2252 kJ·mol⁻¹, respectively.