NDA Chemistry · Carbon and Its Compounds
Allotropes of Carbon
The same element, carbon, crystallises in different structures — diamond, graphite, fullerene and graphene — each with its own bonding and dramatically different properties.
Why this matters
The single highest-yield subtopic in the chapter — 15 PYQs, recurring almost every year. The bank tests it two ways: a straight 'match the allotrope to its property' table, and a 'which statement is NOT correct' trap where one allotrope's property is quietly swapped (diamond made a conductor, graphite made the second-hardest). Learn the structure→property chain for each and the traps fall away.
Concept 1 of 2
Crystalline allotropes — structure, hybridisation and properties
Intuition
Allotropes are different structural forms of the same element. For carbon, the bonding pattern decides everything: a 3-D tetrahedral network gives the hardest natural substance (diamond); flat sheets that slide give a soft conductor (graphite); a closed cage gives the purest form (fullerene); a single sheet gives the thinnest, strongest material (graphene). Same element, four completely different materials.
Definition
The four crystalline allotropes the bank tests:
- Diamond — each carbon is sp³, bonded to four others in a rigid 3-D tetrahedral network. Result: hardest natural substance, an electrical insulator (no free electrons), high refractive index. Its structure is isomorphous with crystalline silicon.
- Graphite — each carbon is sp², bonded to three others in flat hexagonal sheets; the sheets are held by weak forces and slide. Result: soft and slippery (a lubricant and pencil 'lead'), a good conductor (delocalised electrons), and the thermodynamically most stable form of carbon.
- Fullerene (e.g. C₆₀, buckminsterfullerene) — sp², a closed cage that looks like a football. It is regarded as the purest form of carbon.
- Graphene — a single one-atom-thick sheet of graphite; the thinnest and strongest known material.
| Allotrope | Structure / hybridisation | Key property | Use / identity |
|---|---|---|---|
| Diamond | 3-D tetrahedral network, sp³ | Hardest natural substance; electrical insulator | Cutting/abrasives; isomorphous with silicon Diamond does NOT conduct electricity — all four electrons are locked in covalent bonds. |
| Graphite | Flat hexagonal sheets, sp² | Soft, slippery; good conductor; most stable form | Pencil lead, lubricant, electrodes |
| Fullerene (C₆₀) | Closed cage (football), sp² | Purest form of carbon | Nanotechnology, lubricants |
| Graphene | Single one-atom-thick sheet, sp² | Thinnest and strongest material | Electronics, composites |
Diamond = hardest + insulator; Graphite = soft + conductor + most stable; Fullerene = purest; Graphene = thinnest & strongest.
Practice this conceptself-check · 6 quick reps
From the bank · past-year question
Match List-I (Allotrope of Carbon) with List-II (Property) and select the correct answer :
List-I: A. Graphite B. Diamond C. Fullerene D. Graphene
List-II: 1. Thinnest and strongest material 2. Hardest natural substance 3. Very light and strong material 4. Soft and slippery material
[Q68 · Apr · 2024]
Graphite is sp², diamond is sp³
A statement that 'the hybridisation of each carbon in graphite is sp³' is NOT true — graphite is sp² (three bonds per carbon, in sheets). Diamond is the sp³ one.
Only fullerene is cage-like
Of diamond, graphite and fullerene, only fullerene has a cage (football) structure. Diamond is a network and graphite is layered sheets.
Concept 2 of 2
Property traps and impure forms of carbon
Intuition
Most allotrope questions are phrased as 'which statement is NOT correct'. The bank takes a true property of one allotrope and swaps it onto another — diamond made a conductor, graphite made the second-hardest, graphite's weak interlayer forces called covalent bonds. It also slips in fly ash, which sounds carbon-y but is a combustion residue, not an allotrope.
Definition
The false claims to recognise, and the truth behind each:
- 'Diamond is a good conductor' → False: diamond is an insulator.
- 'Graphite is the second-hardest substance' → False: graphite is soft and slippery.
- 'Graphite layers are held together by carbon–carbon single bonds' → False: held by weak van der Waals forces (that is why the layers slide).
- 'Diamond and graphite differ in both physical AND chemical properties' → False: same element, so the chemistry is identical; only physical properties differ.
- 'Fly ash is an allotrope of carbon' → False: fly ash is a combustion residue, not an allotrope.
| Common false claim | The truth |
|---|---|
| Diamond conducts electricity | Insulator — no free electrons |
| Graphite is the second-hardest substance | Soft and slippery (a lubricant) |
| Graphite layers held by covalent single bonds | Held by weak van der Waals forces |
| Diamond and graphite differ chemically | Same element → same chemistry; only physical properties differ Allotropes are the SAME element, so they always share chemical properties — only physical properties change. |
| Fly ash is an allotrope of carbon | A combustion residue, not an allotrope |
Practice this concept4 quick reps
From the bank · past-year question
Which one of the following statements is NOT correct?
[Q92 · Apr · 2020]
Allotropes share chemistry, not physics
Because allotropes are the same element, their chemical properties match — both diamond and graphite burn to give CO₂. The statement that they differ in *both* physical and chemical properties is the wrong one.
Summary — formulas & gotchas at a glance
A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.
Reference tables (2)
Crystalline allotropes — structure, hybridisation and properties4 rows
| Allotrope | Structure / hybridisation | Key property | Use / identity |
|---|---|---|---|
| Diamond | 3-D tetrahedral network, sp³ | Hardest natural substance; electrical insulator | Cutting/abrasives; isomorphous with silicon Diamond does NOT conduct electricity — all four electrons are locked in covalent bonds. |
| Graphite | Flat hexagonal sheets, sp² | Soft, slippery; good conductor; most stable form | Pencil lead, lubricant, electrodes |
| Fullerene (C₆₀) | Closed cage (football), sp² | Purest form of carbon | Nanotechnology, lubricants |
| Graphene | Single one-atom-thick sheet, sp² | Thinnest and strongest material | Electronics, composites |
Diamond = hardest + insulator; Graphite = soft + conductor + most stable; Fullerene = purest; Graphene = thinnest & strongest.
Property traps and impure forms of carbon5 rows
| Common false claim | The truth |
|---|---|
| Diamond conducts electricity | Insulator — no free electrons |
| Graphite is the second-hardest substance | Soft and slippery (a lubricant) |
| Graphite layers held by covalent single bonds | Held by weak van der Waals forces |
| Diamond and graphite differ chemically | Same element → same chemistry; only physical properties differ Allotropes are the SAME element, so they always share chemical properties — only physical properties change. |
| Fly ash is an allotrope of carbon | A combustion residue, not an allotrope |
Watch out for (3)
- Graphite is sp², diamond is sp³→ Crystalline allotropes — structure, hybridisation and properties
- Only fullerene is cage-like→ Crystalline allotropes — structure, hybridisation and properties
- Allotropes share chemistry, not physics→ Property traps and impure forms of carbon
Drill every past-year question on this subtopic
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