Octasulfur

Octasulfur
Systematic name Octathiocane
Identifiers
CAS number	10544-50-0 Y
PubChem	66348
ChemSpider	59726 Y
MeSH	Cyclooctasulfur
ChEBI	CHEBI:29385
ChEMBL	CHEMBL1235452 Y
Gmelin Reference	2973
Jmol-3D images	Image 1
SMILES S1SSSSSSS1
InChI InChI=1S/S8/c1-2-4-6-8-7-5-3-1 Y Key: JLQNHALFVCURHW-UHFFFAOYSA-N Y
Properties
Molecular formula	S8
Molar mass	256.52 g mol−1
Appearance	Vivid, yellow, translucent crystals
Density	2.07 g cm-3
Melting point	119 °C, 392 K, 246 °F
Boiling point	159 °C, 432 K, 318 °F (decomposes)
log P	6.117
Thermochemistry
Std enthalpy of formation ΔfHo298	102 kJ·mol−1
Standard molar entropy So298	431 J·mol−1·K−1
Related compounds
Related compounds	Hexathiane
Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
Infobox references

Octasulfur is a cyclosulfane with the molecular formula S₈. It is a simple, yellow coloured, standard allotrope of sulfur. It is also the final member of the thiocane heterocylic series, where every carbon is substituted with a sulfur atom.

Octasulfur exists as three distinct polymorphs, rhombohedral, and two monoclinic forms, of these only two are stable at standard conditions. The rhombohedral crystal form is the accepted standard. The remaining polymorph is only stable between 96 °C and 115 °C at 100 kPa, above 115 octasulfur starts to slowly disproportionate. However, if heated fast enough, with minimal degradation, octasulfur will melt at 119 °C, before being completely degraded above 159 °C.

Octasulfur forms several sulfur allotropes:

α-Sulfur
β-Sulfur
γ-Sulfur
λ-Sulfur

λ-Sulfur is the liquid form of octasulfur, from which γ-sulfur can be crystallised by quenching. If λ-sulfur is crystallised slowly, it will revert to β-sulfur. Since it must have been heated over 115 °C, neither crystallised β-sulfur, or γ-sulfur will be pure. The only known method of obtaining pure γ-sulfur, is by crystallising from solution.

Octasulfur easily forms large sized crystals, these crystals are typically vivid yellow in colour, and are somewhat translucent. As is typical of other crystalline compounds, pulverised sulfur is completely different in appearance - it is a paler colour, and opaque as is shown in the image.