The two important allotropes of sulphur are yellow rhombic (`alpha`-sulphur) and monoclinic (`beta`-sulphur). At room temperature, rhombic sulphur is stable which transforms to monoclinic sulphur when heated above 369K.
Rhombic sulphur (a-sulphur): This allotrope is yellow in colour, melting point 385.8 K and specific gravity 2.06.
Rhombic sulphur crystals are formed on evaporating the solution of roll sulphur in `CS_2`. It is insoluble in water but readily soluble in `CS_(2)` It dissolves in benzene upto some extent.
Monoclinic sulphur (`beta`-sulphur) : The monoclinic sulphur is prepared by melting rhombic sulphur in a dish and cooling, till crust is formed. Two holes are made in the crust and the remaining liquid poured out. On removing the crust, colourless needle shaped crystals of (`beta`-sulphur are formed.
The melting point is 393 K and specific gravity is 1.98. It is soluble in `CS_2`. It is stable above 369 K and transforms into a-sulphur below it. Conversely, `alpha`-sulphur is stable below 369 K and transforms into p-sulphur above this. At 369 K both the forms are stable. This temperature is called transition temperature.
`S_8` ring in rhombic sulphur and (b) `S_6` form
The rhombic and monoclinic sulphur have `S_8` molecules. These `S_8` molecules are packed to give different crystal structures. The `S_8` ring in both the forms is puckered and has a crown shape.
`S_2` like `O_2` is paramagnetic due to presence of unpaired electron in `pi^(*)` orbital. At temperature higher than 1000 K, `S_2` is dominant species. Several other modifications of sulphur containing 6-20 sulphur atoms per ring have been synthesised. In cyclo-`S_6`, the ring adopts the chair form.
