ü  Sulphur (S): Sulphur is the 2nd element of oxygen family. Sulphur forms a large number of allotropes. Among these Yellow Rhombic (α-Sulphur) and Monoclinic (β -Sulphur) forms are the most important. The stable form at room temperature is rhombic sulphur, which transforms to monoclinic sulphur when heated above 369 K.

 

 

·       Rhombic Sulphur (a - Sulphur): This allotrope is yellow in colour, m.p. 385.8 K and specific gravity 2.06. Rhombic sulphur crystals are formed on evaporating the solution of roll sulphur in CS2. It is insoluble in water but dissolved to some extent in benzene, alcohol and ether. It is readily soluble in CS2.


·       Monoclinic (β -Sulphur): It is prepared by melting rhombic sulphur in a dish and cooling, till a crust is formed. Two holes are made in the crust and the remaining liquid is poured out. On removing the crust, colourless needle shaped crystals of β-sulphur are formed. It is stable above 369 K and transforms into α-sulphur below it. At 369 K both the forms are stable. This temperature is called transition temperature.

Both rhombic and monoclinic sulphur have S8 molecules. The S8 ring in both the forms is puckered and has a crown shape.