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
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
The same idea in a real exam question:
NDA · Apr 2024 · Q68Moderate
Example 1 · Carbon and Its Compounds · Allotropes of Carbon
Match List-I (Allotrope of Carbon) with List-II (Property) and select the correct answer :
| List-I | List-II |
|---|---|
| A. Graphite | 1. Thinnest and strongest material |
| B. Diamond | 2. Hardest natural substance |
| C. Fullerene | 3. Very light and strong material |
| D. Graphene | 4. Soft and slippery material |
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
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
The same idea in a real exam question:
NDA · Apr 2020 · Q92Moderate
Example 2 · Carbon and Its Compounds · Allotropes of Carbon
Which one of the following statements is NOT correct?
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
Test yourself on Carbon and Its Compounds
15 past NDA questions from this chapter, timed at 15 minutes and marked the way the exam marks it. You see your score and every answer the moment you finish. Free to start.