Question
What is allotropy? Compare the allotropes of carbon, sulphur, and phosphorus. Explain why diamond and graphite — both pure carbon — have such vastly different properties.
(CBSE 12 + JEE Main + NEET pattern)
Solution — Step by Step
| Allotrope | Structure | Key properties |
|---|---|---|
| Diamond | Each C bonded to 4 others (sp, tetrahedral) | Hardest substance, insulator, transparent |
| Graphite | Layers of hexagonal rings (sp), weak van der Waals between layers | Soft, conducts electricity (delocalised electrons), lubricant |
| Fullerene (C) | Spherical cage of 60 C atoms | Molecular solid, used in nanotechnology |
| Graphene | Single layer of graphite | Strongest material per weight, excellent conductor |
| Allotrope | Structure | Key property |
|---|---|---|
| Rhombic sulphur (-S) | S ring, packed in rhombic crystal | Stable below 96 degrees C |
| Monoclinic sulphur (-S) | S ring, packed differently | Stable between 96-119 degrees C |
| Plastic sulphur | Long helical chains | Formed by quenching molten S, elastic |
Both rhombic and monoclinic are made of S rings — they differ only in how these rings pack in the crystal.
| Allotrope | Structure | Key property |
|---|---|---|
| White phosphorus (P) | Tetrahedral P molecules | Very reactive, glows in dark (chemiluminescence), poisonous |
| Red phosphorus | Polymeric chains of P atoms | Stable, non-toxic, used in matchboxes |
| Black phosphorus | Layered structure (like graphite) | Semiconductor, most stable allotrope |
White P is the most reactive because the P-P-P bond angle in the P tetrahedron is 60 degrees — far from the ideal 109.5 degrees — causing severe angle strain.
flowchart TD
A["Allotropy: same element, different structures"] --> B["Carbon"]
A --> C["Sulphur"]
A --> D["Phosphorus"]
B --> E["Diamond: sp³, 3D network"]
B --> F["Graphite: sp², layered"]
B --> G["Fullerene: C₆₀ cage"]
C --> H["Rhombic S₈ (stable < 96°C)"]
C --> I["Monoclinic S₈ (96-119°C)"]
D --> J["White P₄ (reactive, strained)"]
D --> K["Red P (polymeric, stable)"]
D --> L["Black P (layered, most stable)"]Why This Works
Allotropes exist because atoms of the same element can bond in different geometric arrangements, each giving a different crystal structure and different properties. Diamond and graphite are both 100% carbon, but their bonding differs completely:
In diamond, every carbon uses sp hybridisation and bonds to 4 neighbours in a 3D network. This rigid framework makes diamond extremely hard, with no free electrons (insulator).
In graphite, every carbon uses sp hybridisation and bonds to 3 neighbours in flat hexagonal layers. The fourth electron is delocalised across the layer (like a metal), enabling electrical conductivity. The layers are held by weak van der Waals forces, so they slide over each other easily — making graphite soft and a good lubricant.
Alternative Method — Thermodynamic Stability Comparison
For each element, the most stable allotrope at room temperature:
- Carbon: graphite (diamond is metastable — it does not convert to graphite because the activation energy is enormous)
- Sulphur: rhombic (below 96 degrees C)
- Phosphorus: black (thermodynamically, but red is kinetically stable and more common)
For NEET and JEE, remember that white phosphorus is stored under water (to prevent spontaneous ignition in air), while sodium is stored in kerosene. Do not mix these up — it is a classic MCQ trap. Also, the bond angle in P (60 degrees) causing strain is a frequently asked reason for white phosphorus’s high reactivity.
Common Mistake
Students write “diamond is the most stable form of carbon.” Graphite is actually thermodynamically more stable at room temperature and pressure ( of diamond is slightly positive relative to graphite). Diamond is metastable — it should theoretically convert to graphite, but the conversion is so astronomically slow at room temperature that diamonds are “forever” for all practical purposes. The phrase “diamonds are forever” is a kinetic statement, not a thermodynamic one.