Sulphur And Its Allotropic Forms - Practice Questions & MCQ

Sulphur forms numerous allotropes of which the 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 (α-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 dissolves to some extent in benzene, alcohol and ether. It is readily soluble in CS₂

Monoclinic sulphur (β-sulphur)
Its m.p. 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 β-sulphur are formed. It is stable above 369 K and transforms into α-sulphur below it. Conversely, α-sulphur is stable below 369 K and transforms into β-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 forms is puckered and has a crown shape. The molecular dimensions are given in the figure below.

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 dimensions are as shown in the figure. At elevated temperatures (~1000K), S₂ is the dominant species and is paramagnetic like O₂.