Elementary sulphur, selenium, and tellurium

At high temperatures the vapours of all three elements consist of diatomic molecules (in which the bond lengths are S=S, 1-89 A, Se=Se, 2-19 A, and Te=Te, 2 61 A), but at lower temperatures and in solution (e.g. in CS2) S and Se form Sg and Seg molecules respectively. Owing to the insolubility of Te there is no evidence for the formation of a similar molecule by this element. 

Sulphur is remarkable for the number of solid forms in which it can be obtained. These include at least four well-known normal polymorphs stable under atmospheric pressure, numerous high-pressure forms of which one is the fibrous form made from plastic sulphur, so-called amorphous forms characterized by small solubility in CS2, and coloured forms produced by condensing the vapour on surfaces cooled to the temperature of liquid nitrogen. Some at least of the amorphous sulphur preparations (e.g. milk of sulphur) give X-ray diffraction lines indicative of some degree of crystallinity, and it is perhaps preferable to use the term u-sulphur rather than amorphous sulphur.The term normal polymorph used above refers to the forms now known to consist of Sg or Sg molecules. In recent years cyclic molecules S (n = 7,9, 10, 12, 18, 20) have been prepared by special methods (see below). 

By the interaction of H2Sg and S4CI2 in CS2 a further crystalline form of 

---