Sulphur — Allotropic Forms

Sulphur — Allotropic Forms

Sulphur exhibits numerous allotropes — different structural forms of the same element — of which the yellow rhombic ($\alpha$-sulphur) and monoclinic ($\beta$-sulphur) forms are by far the most important. At ordinary room temperature, rhombic sulphur is the thermodynamically stable form; on heating above 369 K it converts to monoclinic sulphur.

Rhombic Sulphur (α-Sulphur)

Rhombic sulphur is yellow, melts at 385.8 K, and has a specific gravity of 2.06. Its crystals form when a solution of roll sulphur in carbon disulphide ($CS_2$) is allowed to evaporate. It is insoluble in water, only partly soluble in benzene, alcohol and ether, but readily soluble in $CS_2$.

Monoclinic Sulphur (β-Sulphur)

Monoclinic sulphur melts at 393 K and has a specific gravity of 1.98; like the rhombic form it dissolves in $CS_2$. It is made starting from rhombic sulphur: it is melted in a dish and left to cool until a solid crust forms over the top; two holes are then made in the crust and the liquid sulphur still underneath is poured out. Removing the crust reveals colourless, needle-shaped crystals of β-sulphur.

β-sulphur is stable above 369 K and reverts to α-sulphur below that temperature; conversely, α-sulphur, stable below 369 K, converts to β-sulphur above it. At exactly 369 K — the transition temperature — both forms coexist and are simultaneously stable.

Molecular Structure

Both rhombic and monoclinic sulphur are built from $S_8$ molecules, which simply pack differently in the two crystal lattices. In both forms, the eight-membered $S_8$ ring is puckered into a characteristic crown shape, illustrated in part (a) of the figure, with an S–S bond length of 204 pm and an internal bond angle of about 107°. …