page_title: Allotropes of sulfur
section_title: Cyclo-heptasulfur, cyclo-S7
hierarchical_section_title: Allotropes of sulfur / Solid cyclo-sulfur allotropes / Cyclo-heptasulfur, cyclo-S7
caption_attribution_description: English: structure of Cycloheptasulfur, S7.
context_page_description: The element sulfur exists as many allotropes. In terms of large number of allotropes, sulfur is second only to carbon. In addition to the allotropes, each allotrope often exists in polymorphs, delineated by Greek prefixes.
  Furthermore, because elemental sulfur has been an item of commerce for centuries, its various forms are given traditional names. Early workers identified some forms that have later proved to be single or mixtures of allotropes. Some forms have been named for their appearance, e.g. "mother of pearl sulfur", or alternatively named for a chemist who was pre-eminent in identifying them, e.g. "Muthmann's sulfur I" or "Engel's sulfur".
  The most commonly encountered form of sulphur is the orthorhombic polymorph of S
  ₈, which adopts a puckered ring – or "crown" – structure. Two other polymorphs are known, also with nearly identical molecular structures. In addition to S₈, sulfur rings of 6, 7, 9–15, 18 and 20 atoms are known. At least five allotropes are uniquely formed at high pressures, two of which are metallic.
  The number of sulfur allotropes reflects the relatively strong S−S bond of 265 kJ/mol.
context_section_description: It is a bright yellow solid. Four (α-, β-, γ-, δ-) forms of cyclo-heptasulfur are known. Two forms (γ-, δ-)have been characterized. The cyclo-S₇ ring has an unusual range of bond lengths of 199.3–218.1 pm. It is said to be the least stable of all of the sulfur allotropes.
caption: Structure of S7.
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