Allotropy is the existence of an element in two or more forms with different physical properties but the same chemical nature. CBSE Class 10 and 11 and NEET test allotropes of carbon, sulphur and phosphorus most often.
Core Concepts
Carbon allotropes
Diamond — tetrahedral 3D network, hardest natural substance, non-conductor. Graphite — layered 2D sheets, soft, conductor along layers. Fullerene (C60) — soccer-ball-shaped cage. Carbon nanotubes and graphene — more recent.
Diamond in detail: Each carbon is sp3 hybridised, bonded to 4 other carbons in a rigid tetrahedral network. Bond angle = 109.5°. All electrons are in sigma bonds — none free to conduct electricity. The 3D network of strong C-C bonds (bond energy ~348 kJ/mol each) makes diamond the hardest natural material, with the highest thermal conductivity of any material.
Graphite in detail: Each carbon is sp2 hybridised, bonded to 3 other carbons in flat hexagonal sheets. Bond angle = 120°. The fourth electron (in the unhybridised p orbital) is delocalised across the entire sheet — this is why graphite conducts electricity along the layers. The sheets are held together by weak van der Waals forces and can slide over each other — this is why graphite is soft and used as a lubricant.
| Property | Diamond | Graphite |
|---|---|---|
| Hybridisation | sp3 | sp2 |
| Bond angle | 109.5° | 120° |
| Structure | 3D network | Layered sheets |
| Hardness | Hardest natural material | Very soft |
| Conductivity | Non-conductor | Conductor along layers |
| Density | 3.51 g/cm3 | 2.26 g/cm3 |
| Appearance | Transparent, brilliant | Opaque, grey-black |
| Uses | Cutting tools, jewellery | Pencils, lubricant, electrodes |
| Thermodynamic stability | Metastable (converts to graphite very slowly) | More stable at 298 K, 1 atm |
Fullerene (C60): Discovered in 1985 by Kroto, Curl, and Smalley (Nobel Prize 1996). 60 carbon atoms arranged in a truncated icosahedron — 12 pentagons and 20 hexagons, like a football. Each carbon is sp2 hybridised. Soluble in organic solvents (unlike diamond and graphite). Can trap atoms inside the cage — useful for drug delivery research.
Graphene: A single sheet of graphite — one atom thick. Isolated in 2004 by Geim and Novoselov (Nobel Prize 2010). It is the strongest material ever tested (about 200 times stronger than steel by weight), transparent, flexible, and an excellent conductor. Applications being researched: flexible screens, ultra-fast electronics, water purification membranes.
Carbon nanotubes (CNTs): Rolled-up graphene sheets, either single-walled or multi-walled. Exceptional strength-to-weight ratio. Can be metallic or semiconducting depending on how the sheet is rolled (chirality). Used in composite materials, electronics, and nanotechnology.
Sulphur allotropes
Rhombic (alpha-sulphur) — stable at room temperature, S8 puckered rings. Monoclinic (beta-sulphur) — stable above 96°C, also S8. Plastic sulphur — amorphous, rubbery, metastable.
The transition temperature: Rhombic sulphur is stable below 96°C. Above 96°C, it slowly converts to monoclinic. This temperature (96°C) is the transition temperature. Both forms consist of S8 rings but pack differently in the crystal.
| Allotrope | Crystal System | Stability | Key Feature |
|---|---|---|---|
| Rhombic (alpha) | Orthorhombic | Below 96°C | Most stable at room temperature |
| Monoclinic (beta) | Monoclinic | 96-119°C | S8 rings, different packing |
| Plastic | Amorphous | Metastable (reverts) | Rubbery, long chains (not rings) |
When sulphur is melted and heated further, the S8 rings break and form long chains — the liquid becomes viscous (thick). Above 200°C, the chains break into shorter fragments and viscosity decreases again. Pouring this hot liquid into cold water gives plastic sulphur (stretched chains frozen in place).
Phosphorus allotropes
White phosphorus — P4 tetrahedra, highly reactive, stored under water, glows in dark. Red phosphorus — polymeric chains, less reactive, safer. Black phosphorus — layered, most stable.
| Property | White | Red | Black |
|---|---|---|---|
| Structure | P4 tetrahedra (discrete) | Polymeric chains | Layered (like graphite) |
| Colour | White/yellow | Dark red | Black |
| Reactivity | Very high (ignites in air) | Much less reactive | Least reactive |
| Toxicity | Highly toxic | Non-toxic | Non-toxic |
| Storage | Under water | Normal conditions | Normal conditions |
| Solubility | Soluble in CS2 | Insoluble in CS2 | Insoluble in CS2 |
| Melting point | 44°C | 590°C (sublimes) | - |
| Glow in dark | Yes (chemiluminescence) | No | No |
| Bond angle (P-P-P) | 60° (strained) | Varies | - |
Why white phosphorus is so reactive: The P4 tetrahedron has P-P-P bond angles of only 60° — much less than the ideal 109.5° for sp3. This angular strain makes the bonds weak and the molecule eager to react and relieve the strain.
Why white phosphorus glows: Slow oxidation by air releases energy as visible light (chemiluminescence). The name “phosphorus” comes from Greek: “phos” (light) + “phorus” (bearer).
Why allotropes have different properties
Same atoms but different arrangements give different bonds, different structures, and different properties. Diamond and graphite are chemically identical (both pure C) but physically very different.
The key principle: structure determines properties. The same element can behave as an insulator (diamond), a conductor (graphite), or a semiconductor (carbon nanotubes), depending purely on how the atoms are arranged.
Practical uses
Diamond in cutting tools and jewellery. Graphite in pencils, lubricants and electrodes. Red phosphorus in match striking surfaces. Rhombic sulphur in vulcanising rubber.
Match head chemistry: The match head contains KClO3 (oxidiser) and antimony trisulphide (fuel). The striking surface contains red phosphorus and powdered glass. When you strike, friction converts a tiny amount of red phosphorus to white phosphorus (which ignites easily), which then ignites the head.
Worked Examples
In graphite each C is bonded to only 3 others, leaving one electron per atom delocalised in a pi system — free to conduct. In diamond every C is bonded to 4 others with all electrons localised in sigma bonds.
White phosphorus is slowly oxidised by air, releasing energy as visible light. This is chemiluminescence. The name ‘phosphorus’ literally means ‘light bearer’.
At room temperature and 1 atm, graphite has lower Gibbs free energy than diamond. The conversion of diamond to graphite is thermodynamically favourable () but kinetically extremely slow (the activation energy barrier is enormous). This is why diamonds exist — they are kinetically stable despite being thermodynamically metastable.
Heat white phosphorus at 250°C in an inert atmosphere (no oxygen). The strained P4 tetrahedra break and reform into polymeric chains (red phosphorus). The process is irreversible under normal conditions.
Graphite conducts electricity (delocalised electrons) and is chemically inert at moderate temperatures. These two properties make it ideal for electrodes in electrochemical cells and electric arc furnaces. Diamond cannot conduct and would be extremely expensive.
Common Mistakes
Saying allotropes are isotopes. Allotropes differ in arrangement of atoms; isotopes differ in number of neutrons.
Writing that graphite is harder than diamond. The opposite is true.
Confusing white and red phosphorus. White is reactive and toxic; red is stable and relatively safe.
Saying diamond is the most stable form of carbon. Graphite is more stable thermodynamically at standard conditions. Diamond is metastable — it persists because the conversion to graphite is infinitely slow at room temperature.
Writing that graphite conducts in all directions. It conducts along the layers (in-plane) but poorly perpendicular to them (between layers), because the delocalised electrons are confined to each layer.
Exam Weightage and Revision
Allotropy carries 1-2 NEET questions per year, usually on carbon allotropes or phosphorus. CBSE Class 10 and 11 boards ask comparison tables (diamond vs graphite) and uses. JEE may ask about fullerene structure or graphene properties.
| Question Type | NEET Frequency | Difficulty |
|---|---|---|
| Diamond vs graphite comparison | Every year | Easy |
| White vs red phosphorus | Most years | Easy |
| Fullerene/graphene facts | Occasional | Medium |
| Sulphur allotropes | Rare | Easy |
| Conductivity reasoning | Most years | Medium |
The guaranteed question: “Why does graphite conduct electricity but diamond does not?” Answer in two sentences: sp2 hybridisation in graphite leaves one delocalised electron per carbon; sp3 hybridisation in diamond localises all electrons in sigma bonds. No free electrons = no conductivity.
Practice Questions
Q1. Why is white phosphorus stored under water?
White phosphorus ignites spontaneously in air (its ignition temperature is about 30°C — close to room temperature). Storing it under water prevents contact with oxygen and thus prevents spontaneous combustion. Water does not react with white phosphorus at room temperature.
Q2. What is the hybridisation of carbon in (a) diamond (b) graphite (c) fullerene?
(a) Diamond: sp3 (4 sigma bonds, tetrahedral). (b) Graphite: sp2 (3 sigma bonds + 1 delocalised pi, trigonal planar). (c) Fullerene (C60): approximately sp2 (each C is bonded to 3 others), but with slight deviation due to curvature of the cage.
Q3. Why is red phosphorus used in matchboxes while white phosphorus is not?
White phosphorus is extremely toxic (lethal dose ~50 mg) and ignites spontaneously in air — far too dangerous for consumer products. Red phosphorus is non-toxic and does not ignite in air. In the match striking surface, friction converts a tiny amount of red P to white P locally, which then ignites and sets off the match head. This design is safe because the red phosphorus alone does not catch fire.
FAQs
Can diamonds be made artificially?
Yes. High-pressure high-temperature (HPHT) synthesis mimics the conditions under which natural diamonds form (about 5 GPa, 1500°C). Chemical vapour deposition (CVD) grows diamond from methane gas. Synthetic diamonds are used extensively in cutting tools and increasingly in jewellery.
Is graphene really a single atom thick?
Yes. Graphene is exactly one atom thick — about 0.34 nm. It is the thinnest material possible (you cannot have anything thinner than one atom). Despite being so thin, it is remarkably strong because of the strong C-C bonds in the hexagonal lattice.
Do other elements show allotropy?
Yes. Oxygen has two allotropes: O2 (dioxygen, the gas we breathe) and O3 (ozone, which absorbs UV in the stratosphere). Tin has two: white tin (metallic) and grey tin (non-metallic, forms below 13°C — called “tin pest”). Iron has allotropes at different temperatures (BCC alpha-iron, FCC gamma-iron).
Make a three-row table — element, allotrope, key property, one use. That covers the chapter.
Allotropy is the clearest example that structure determines properties. Same atoms, different geometry, very different outcomes.