14. Sulphur - Allotropic forms

SULPHUR (S)

Allotropic Forms Of Sulphur :

Sulphur forms numerous allotropes of which the yellow rhombic  (a - sulphur) and monoclinic (b - sulphur) forms are the most important. The stable forms at room temperature is rhombic sulphur, which transforms to monoclinic sulphur when heated above 369 K.

Rhombic sulphur  (a - sulphur) :

This allotrope is Syellow 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 CS₂. It is insoluble in water but dissolved  to some extent in benzene, alcohol and ether. It is readily soluble in CS₂ .

Monoclinic sulphur (b - sulphur) :

Its melting point is 393 K and specific gravity 1.98. It is soluble in CS₂. This form of 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 b - sulphur are formed. It is stable above 369 K and transforms into a - sulphur below it . Conversely,a  - sulphur is stable below 369 K and transforms into b - sulphur above this.  At 369 K both the forms are stable. This temperature is called transition temperature.

Both rhombic and  monoclinic sulphur have S₈ molecules these S₈ molecules are packed to give different crystal structures. The S₈ ring in both the forms is puckered and has a crown shape. The molecular dimensions are given in figure.

Several other modifications of sulphur containing 6-20 sulphur atoms per ring have been synthesised in the last two decades. In cyclo- S₆, the ring adopts the chair form and the molecular dimension are as shown in fig. (b).

Sulphur melts to form a mobile liquid. As the temperature is raised the colour darkens. At 160ºC C₈ rings break, and the diradicals so formed polymerize, forming long chains of up to a million atoms. The viscosity increases sharply, and continues to rise up to 200ºC. At higher temperatures chains break, and shorter chains and rings are formed, which makes the viscosity decrease upto 444ºC, the boiling point. The vapour at 200ºC consists mostly of S₈ rings, but contains 1-2% of S₂ molecules. At elevated temperature (~1000 K), S₂ is the dominant species and is paramagnetic like O₂, and presumably has similar bonding. S₂ gas is stable upto 2200ºC.