Carbon — Allotropes and Bond Structures
~9 min read
- Tetravalency: Carbon has 4 valence electrons and forms 4 covalent bonds.
- Catenation: Self-linking ability — basis of millions of organic compounds.
- Allotropes: Diamond (3D tetrahedral, hardest), Graphite (2D layered, conducts), Fullerene C60, Graphene (single layer), Carbon nanotubes.
Carbon writes more compounds than all other elements combined. CDS/OTA tests its tetravalency, catenation and the dramatic contrast between diamond and graphite.
Tetravalency and Catenation
- Carbon has 4 valence electrons (2,4) — needs 4 more for octet.
- Shares electrons rather than gaining or losing — forms four covalent bonds.
- Catenation: ability to form long chains and rings by C-C bonds. Strongest in carbon (small atom, high bond strength).
- Result — > 10 million organic compounds known.
- Can form single (-C-C-), double (C=C) and triple (C≡C) bonds with itself and other atoms.
Crystalline Allotropes
| Allotrope | Structure | Properties | Uses |
|---|---|---|---|
| Diamond | 3D tetrahedral; each C bonded to 4 others | Hardest natural substance; non-conductor; high MP (~3550 °C) | Cutting, drilling, polishing, jewellery |
| Graphite | 2D layers of hexagons; layers held by weak forces | Soft, slippery; conducts electricity (free electrons) | Pencil lead, electrodes, lubricant, moderator in nuclear reactors |
| Fullerene (C60) | Soccer-ball cage of 60 C atoms | Discovered 1985; semiconducting | Drug delivery, lubricants, electronics research |
| Graphene | Single layer of graphite | Extremely strong, flexible, best conductor | Flexible electronics, batteries, sensors |
| Carbon nanotubes (CNT) | Rolled-up graphene cylinder | ~100× stronger than steel by weight | Composite materials, semiconductors |
Amorphous Forms
- Coal: Anthracite (90-95% C), Bituminous (70-80%), Lignite (~30%). Fossil fuel.
- Coke: heating coal in absence of air. Used in metallurgy and to make synthesis gas.
- Charcoal: from wood. Activated charcoal absorbs gases and dyes (used in gas masks and water filters).
- Lampblack, carbon black: incomplete combustion of hydrocarbons; used in tyres, ink.
Functional Groups (Brief)
| Functional group | Formula | Family | Example |
|---|---|---|---|
| Hydroxyl | -OH | Alcohol | Ethanol C₂H₅OH |
| Aldehyde | -CHO | Aldehyde | Formaldehyde HCHO |
| Ketone | >C=O | Ketone | Acetone CH₃COCH₃ |
| Carboxyl | -COOH | Carboxylic acid | Acetic acid CH₃COOH |
| Amino | -NH₂ | Amine | Methylamine CH₃NH₂ |
CDS/OTA PYQ Examples
Q: Hardest natural substance is:
(a) Iron (b) Granite (c) Diamond (d) Quartz
Answer: (c) Diamond.
Q: Graphite conducts electricity because:
(a) It has ionic bonds (b) Each carbon has a free electron in the delocalised π system (c) It is metallic (d) It contains hydrogen
Answer: (b) Free delocalised electrons within graphite layers.
Q: Tetravalency of carbon means it forms:
(a) 2 bonds (b) 3 bonds (c) 4 bonds (d) 8 bonds
Answer: (c) 4 covalent bonds.
Q: Fullerene C60 is a:
(a) Diamond polymorph (b) Hollow cage of 60 carbon atoms (c) Type of coal (d) Form of graphite
Answer: (b) Hollow soccer-ball cage.
Q: Activated charcoal is used in water purifiers because:
(a) It kills bacteria (b) It absorbs dissolved impurities by adsorption (c) It softens water (d) It adds chlorine
Answer: (b) Adsorption of organic impurities and odours.
Drill Carbon — Allotropes and Bond Structures for CDS/OTA
CDS/OTA-pattern items on Carbon — Allotropes and Bond Structures with answer keys and explanations.
Start Free Mock TestFrequently Asked Questions
Why is diamond hard but graphite soft if both are pure carbon?
Diamond — 3D tetrahedral covalent network, very rigid. Graphite — 2D sheets held by weak forces; sheets slide easily, making it soft and slippery.
Why does graphite conduct electricity but diamond does not?
Each carbon in graphite uses only 3 of its 4 valence electrons in bonding; the 4th electron is delocalised in π-system, free to conduct. In diamond, all 4 are tied up in σ bonds.
Why is carbon called the most versatile element?
Tetravalency + catenation + ability to form single/double/triple bonds with itself, H, O, N, halogens — creates millions of organic compounds, including those of life.