Carbon and Its Allotropes
~8 min read
- Carbon: Atomic number 6. Forms 4 bonds (tetravalent). Catenates (forms chains/rings) — basis of organic chemistry.
- Allotropes: Same element, different forms: Diamond, Graphite, Fullerene, Carbon nanotubes, Amorphous carbon (coal, coke, charcoal).
- Differences: Diamond — hardest, insulator. Graphite — soft, conducts electricity. Fullerene (C₆₀, 'buckyball') — discovered 1985.
Carbon's ability to form many different bonding patterns gives it remarkable variety. NDA tests allotropes and their properties.
Why Carbon is Special
- Tetravalent: 4 outer electrons → forms 4 covalent bonds. Maximum bonding flexibility.
- Catenation: Carbon-carbon bonds are strong; chains can extend indefinitely. Unique among elements (silicon shows mild catenation).
- Bond types: Single, double, triple. Allows variety of structures.
- Forms with: H (hydrocarbons), O, N, halogens — endless organic molecules.
- Backbone of all life — ~25% of human body by mass is carbon (after water).
Diamond and Graphite
| Property | Diamond | Graphite |
|---|---|---|
| Structure | 3D tetrahedral lattice; each C bonded to 4 others | 2D hexagonal sheets stacked weakly |
| Hardness | Hardest natural substance (10 on Mohs) | Soft, slippery (1-2 on Mohs) |
| Electrical conductivity | Insulator (no free electrons) | Good conductor (delocalised electrons between sheets) |
| Density | 3.5 g/cm³ | 2.2 g/cm³ |
| Refractive index | 2.42 (very high — sparkle) | Opaque grey/black |
| Uses | Jewellery, cutting/drilling tools, abrasive | Pencils, lubricant, electrodes, batteries |
Same element — different structure → vastly different properties. This is the essence of allotropy.
Fullerenes and Carbon Nanotubes
- Fullerene (C₆₀): "Buckminster fullerene" or "buckyball" — discovered 1985 by Curl, Kroto, Smalley (Nobel 1996). Cage-like molecule of 60 carbon atoms in shape of soccer ball.
- Carbon nanotubes: Hollow cylindrical structures of carbon. Discovered 1991. Very strong (steel-strength at fraction of weight), good electrical conductors, semiconducting variants. Used in advanced materials, electronics research.
- Graphene: Single-atom-thick sheet of graphite. Discovered 2004 (Nobel 2010). Extraordinary properties — strongest material known, excellent conductor.
Amorphous Carbon — Coal, Coke, Charcoal
- Amorphous = no long-range crystalline order.
- Coal: Fossilised plant material. Types (in order of carbon content): peat → lignite → bituminous → anthracite. Anthracite has highest C content (~95%).
- Coke: Solid residue after heating coal in absence of air. Used in steel making (blast furnaces) as reducing agent.
- Charcoal: Made by heating wood without oxygen. Used as fuel, decolouriser (activated charcoal), in water filters.
- Carbon black (soot): Fine carbon powder from incomplete combustion. Used in tires, ink, paints.
- Activated charcoal: Charcoal with vastly increased surface area (~1000 m²/g). Used for adsorption — water purification, gas masks, decolouring sugar.
NDA PYQ Examples
Q: The hardest natural substance is:
(a) Steel (b) Quartz (c) Diamond (d) Iron
Answer: (c) Diamond — 10 on Mohs scale.
Q: Graphite conducts electricity because it has:
(a) Ionic bonds (b) Hydrogen bonds (c) Delocalised electrons between sheets (d) Metallic bonds
Answer: (c) Delocalised electrons between hexagonal carbon sheets.
Q: Fullerene (C₆₀) was discovered in:
(a) 1885 (b) 1945 (c) 1985 (d) 2005
Answer: (c) 1985.
Q: Activated charcoal is used because of its:
(a) Hardness (b) Conductivity (c) High adsorption surface area (d) Heat resistance
Answer: (c) High adsorption surface area.
Drill Carbon and Its Allotropes for NDA
NDA-pattern items on Carbon and Its Allotropes with answer keys and explanations.
Start Free Mock TestFrequently Asked Questions
Why is diamond hard but graphite soft despite both being pure carbon?
Diamond — each C atom bonded to 4 others in 3D lattice. To break it you must break millions of strong covalent bonds → hardness. Graphite — strong bonds within hexagonal sheets, but weak bonds between sheets. Sheets slide past each other easily → softness/lubrication.
Why does graphite conduct electricity but diamond doesn't?
Diamond — all 4 outer electrons of each C are in fixed covalent bonds → no mobile charges. Graphite — only 3 outer electrons in bonds; the 4th is delocalised between sheets, free to move → conducts electricity.
How is artificial diamond made?
Two main methods. (1) HPHT (High Pressure, High Temperature): subjecting graphite to ~5 GPa pressure and ~1500°C. (2) CVD (Chemical Vapour Deposition): growing diamond layer-by-layer from carbon-containing gas. Quality matches natural diamonds; cheaper.
What is coke used for?
Coke is residual carbon after coal is heated without air (driving off volatiles). Used in steel-making blast furnaces as a fuel and reducing agent — reduces iron oxide (ore) to iron metal: Fe₂O₃ + 3CO → 2Fe + 3CO₂.
Why are fullerenes called buckyballs?
Named after architect Buckminster Fuller (1895-1983), famous for geodesic dome buildings. C₆₀ has a similar pattern of hexagons and pentagons — same as a soccer ball. The full name 'buckminsterfullerene' was a tribute.