Sulphur

Sulphur is a chemical element with the symbol S and atomic number 16. It is a non‑metal and occurs in the Earth's crust in relatively large amounts. Native sulphur appears as a bright yellow, odourless solid at room temperature and is essential in various biological and industrial processes.

Allotropes and properties: The most common allotrope at ambient conditions is orthorhombic sulphur, composed of S8 molecules. A monoclinic form exists briefly around 96–119 °C. When molten and cooled slowly, plastic sulphur forms long polymer chains. Sulphur melts at 115.21 °C and boils at 444.6 °C; the solid has a density of about 2.07 g/cm3. It is insoluble in water but dissolves in carbon disulfide. Elemental sulphur is typically odourless, but burning produces sulfur dioxide, which has a sharp, characteristic smell.

Occurrence and production: Sulphur occurs naturally near volcanic activity and in sulfide minerals such as pyrite and galena, as well as in sulfate minerals. It is mined from native sulfur deposits and obtained as a byproduct of refining natural gas and petroleum. Historically, the Frasch process was used to extract underground sulfur; today, sulfur is also recovered from hydrogen sulfide streams in refining. A large portion of produced sulphur is used to manufacture sulfuric acid.

Uses: The principal use is in the production of sulfuric acid, essential for fertilizers and many chemicals. Other applications include vulcanisation of rubber, production of pesticides, dyes, pharmaceuticals, matches, and various industrial processes.

Safety and environment: Inhalation of sulfur dioxide during combustion is toxic and contributes to air pollution and acid rain. Fine sulphur dust can pose an explosion risk; appropriate handling and emissions controls are required to minimise hazards.