Allotropes of sulfur

Allotropes of sulfur are distinct structural forms of elemental sulfur, including ring molecules, chains, and different crystal arrangements. Their structures give them different physical properties and stability.

Sulfur: Sulfur is a chemical element with atomic number 16 that forms diverse compounds and elemental structures. Its ability to form bonds with itself underlies its allotropes.
Rhombic sulfur: Rhombic sulfur, or α-sulfur, is the orthorhombic crystalline form composed chiefly of S₈ rings. It is the stable crystal form below about 95.5 °C.
S₆ sulfur: S₆ sulfur is an elemental sulfur allotrope whose molecules are six-membered sulfur rings. It demonstrates that sulfur rings need not contain eight atoms.
Enantiotropic phase transition: An enantiotropic phase transition is a reversible transition between forms, each stable over a different temperature range. Rhombic and monoclinic sulfur exchange stability near 95.5 °C.
Vulcanization: Vulcanization is the treatment of rubber with sulfur or other agents to create cross-links between polymer chains. Sulfur’s ability to form chains and bonds enables durable cross-linking in rubber.
Chemical allotropy: Chemical allotropy is the existence of multiple structural forms of the same element in the same physical state. Sulfur is a classic example, with unusually many documented forms.
Monoclinic sulfur: Monoclinic sulfur, or β-sulfur, is a crystalline form composed chiefly of S₈ rings. It becomes stable above the rhombic-to-monoclinic transition temperature.
S₇ sulfur: S₇ sulfur is an elemental sulfur allotrope built from seven-membered sulfur rings. Its ring size offers another molecular structure alongside the dominant S₈ form.
Gibbs free energy: Gibbs free energy is a thermodynamic potential used to determine equilibrium and spontaneous change at constant temperature and pressure. The lowest-free-energy sulfur structure is the stable form under specified conditions.
Rubber: Rubber is an elastic material made from natural or synthetic polymers that can undergo large reversible deformation. Sulfur allotrope chemistry supplies the cross-links that strengthen many rubber products.

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