Question
Describe the main allotropes of sulphur — rhombic, monoclinic, and plastic sulphur. Compare their structures, stability, and interconversion temperatures. Why does sulphur show such a variety of allotropes compared to oxygen?
(JEE Main 2023, similar pattern)
Solution — Step by Step
Sulphur has several allotropes, but three are most important for exams:
- Rhombic sulphur (-sulphur): S₈ rings, most stable below 95.5°C
- Monoclinic sulphur (-sulphur): S₈ rings in different crystal packing, stable between 95.5°C and 119°C
- Plastic sulphur: Long helical chains of S atoms, formed by quenching molten sulphur
| Property | Rhombic (α) | Monoclinic (β) | Plastic |
|---|---|---|---|
| Molecular unit | S₈ (crown-shaped) | S₈ (crown-shaped) | S chains |
| Crystal system | Orthorhombic | Monoclinic | Amorphous |
| Density | 2.07 g/cm³ | 1.98 g/cm³ | ~1.95 g/cm³ |
| Melting point | 112.8°C | 119°C | — |
| Colour | Yellow | Pale yellow | Dark yellow |
| Stability | Stable below 95.5°C | Stable 95.5-119°C | Metastable |
Both rhombic and monoclinic contain the same S₈ crown-shaped ring — the difference is only in how these rings pack in the crystal lattice.
At 95.5°C (the transition temperature), rhombic slowly converts to monoclinic. Below this temperature, monoclinic slowly reverts to rhombic. This is an example of enantiotropic allotropy — each form has a definite temperature range of stability.
When molten sulphur (~445°C, where long chains form) is rapidly cooled by pouring into cold water, the chains don’t have time to rearrange into S₈ rings — you get plastic sulphur, which is elastic and rubbery. It slowly reverts to rhombic on standing.
Two reasons:
- Catenation ability: Sulphur forms strong S-S single bonds (bond energy ~266 kJ/mol), allowing chains and rings of varying sizes (S₆, S₈, S₁₂, etc.)
- No tendency for multiple bonding: Unlike oxygen which readily forms O=O double bonds (giving only O₂ and O₃), sulphur’s 3p orbitals are too diffuse for effective - overlap. So sulphur prefers single bonds in chains/rings over double bonds in small molecules.
Why This Works
Allotropy arises when an element can arrange its atoms in multiple stable structures. Sulphur’s ability to form extensive S-S chains (catenation) combined with the flexibility of S₈ rings to pack differently in crystals gives it exceptional allotropic diversity.
The crown-shaped S₈ ring has bond angles of about 108° (close to tetrahedral), which suits sulphur’s sp³-like bonding. Each S atom has 2 bonds and 2 lone pairs. The ring puckers to accommodate this geometry — it’s not flat like benzene.
Alternative Method
For exam revision, focus on the quick comparison: both common allotropes are S₈, the transition temperature is 95.5°C, and rhombic is the thermodynamically stable form at room temperature (so it’s the answer when they ask “most stable allotrope of sulphur”).
JEE Main often asks: “Which is the most stable allotrope of sulphur at room temperature?” The answer is rhombic sulphur. They may also ask the transition temperature (95.5°C) or the number of atoms in the basic unit (8, in S₈). These are quick 1-mark facts worth memorising.
Common Mistake
Students confuse enantiotropic and monotropic allotropy. Sulphur (rhombic ⇌ monoclinic) is enantiotropic — both forms have a temperature range where they’re stable, and the conversion is reversible. Carbon (diamond/graphite) is monotropic — only graphite is thermodynamically stable at all temperatures and pressures (at standard conditions). Diamond → graphite is thermodynamically favoured but kinetically so slow it doesn’t happen. Don’t mix these up.