Sulphur: Allotropic Forms (Rhombic and Monoclinic)

Sulphur, like phosphorus, shows extensive allotropy, and the two crystalline forms prescribed for

detailed study here -- rhombic ($\alpha$-) sulphur and monoclinic ($\beta$-) sulphur --

are an instructive case of two allotropes built from the same underlying molecular unit but

packed into two different crystal lattices.

Both forms are built from the puckered, crown-shaped $\text{S}_8$ ring: eight sulphur atoms,

each bonded to its two neighbours by ordinary S-S single bonds, arranged not in a flat octagon

but in a non-planar, zig-zag "crown" conformation that relieves angular strain (each internal

S-S-S angle is close to sulphur's preferred value, around $105^\circ$, rather than the much

larger angle a flat ring would require). This $\text{S}_8$ ring is the basic structural unit of

ordinary sulphur in all its common forms.

Rhombic sulphur is the thermodynamically stable form of sulphur at ordinary room temperature,

crystallising as pale yellow, octahedral-looking crystals in which $\text{S}_8$ rings pack

together in a particular repeating lattice arrangement. It is obtained by evaporating a solution

of roll sulphur in carbon disulphide, allowing well-formed crystals to grow slowly. Monoclinic sulphur, obtained by melting rhombic sulphur and allowing it to crystallise slowly, forms

long, needle-like prismatic crystals -- the same $\text{S}_8$ rings, but now packed in a

different, less densely ordered lattice geometry.

The two forms interconvert reversibly at a sharp transition temperature of $369\ \text{K}$ ($95.5^\circ\text{C}$): below $369\ \text{K}$, rhombic sulphur is the stable form and

monoclinic sulphur slowly converts into it; above $369\ \text{K}$ (and up to sulphur's melting

point around $392\ \text{K}$), monoclinic sulphur is the stable form and rhombic sulphur

converts into it. At exactly $369\ \text{K}$ the two forms are in equilibrium with each other,

which is why this temperature is called the transition temperature -- a textbook example of

enantiotropy (reversible interconversion between two solid forms, each stable over its own

temperature range, as opposed to monotropy, where one form is always the more stable one). Both …