CDS Chemistry · Carbon and Its Compounds
Carbon and Its Allotropes
Diamond, graphite, fullerene and graphene: how each is built, why only graphite conducts, and what is not an allotrope.
Why this matters
Seven CDS questions, the largest page in the chapter. Almost all of them come back to one contrast: diamond is a hard insulator, graphite a soft conductor, and the reason is how many bonds each carbon makes.
Concept 1 of 2: The allotropes of carbon
Allotropes are different forms of the same element, made of the same atoms joined in different ways. Carbon has four that CDS asks about. Coal is not one of them: it is a rock that contains carbon along with other elements.
Definition
The four forms:
- Diamond: each carbon bonded to four others in a 3-D network. Hardest natural substance; insulator.
- Graphite: each carbon bonded to three others in flat hexagonal layers; one electron per carbon is free, so it conducts. Soft and slippery: used as a lubricant and in pencils.
- Fullerene (C₆₀, buckminsterfullerene): football-shaped molecules; not a good conductor.
- Graphene: a single layer of graphite, the thinnest material known, almost transparent, an excellent conductor. It is a zero band-gap semimetal, not a wide band-gap semiconductor.
- Coal is not an allotrope: it is a mixture containing carbon with hydrogen, sulphur, nitrogen and minerals.
- Carbon is a minor part of the Earth's crust, roughly 0.02% by mass, mostly as carbonates and fossil fuels.
| Form | Bonds per carbon | Conducts electricity? |
|---|---|---|
| Diamond | 4 (3-D network) | No |
| Graphite | 3 (flat layers) | Yes |
| Fullerene (C₆₀) | 3 (closed cage) | No |
| Graphene | 3 (one layer) | Yes, very wellQ CDS 2025 (II): 'a wide band-gap semiconductor' is the property graphene does NOT have. |
| Coal | Not an allotrope: a mixture | Not a single form of carbonQ CDS 2016 (II): coal is the one that is not an allotrope of carbon. |
Practice this conceptself-check · 4 quick reps
The same idea in a real exam question:
CDS · 2021 · CDS (II) 2021 — General Knowledge · Q8Moderate
Example 1 · Carbon and Its Compounds · Carbon and Its Allotropes
Which of the following carbon allotropes is/are good conductor(s) of electricity ?
1. Diamond
2. Graphite
3. Fullerene
Select the correct answer using the code given below :
The coal answer: a carbon-rich rock is not an allotrope
An allotrope is a pure form of one element. Coal contains hydrogen, sulphur, nitrogen and minerals, so it is not an allotrope. Diamond, graphite, graphene and fullerene are.
Graphene has no band gap
Graphene is a zero band-gap semimetal, which is why it conducts so well. Calling it a wide band-gap semiconductor is the false statement.
Concept 2 of 2: Diamond and graphite compared
Diamond and graphite are both pure carbon, so they have the same percentage of carbon and give only carbon dioxide when burnt. Every difference between them comes from structure: four bonds in three dimensions against three bonds in a flat sheet.
Definition
Point by point:
- Same: pure carbon; same percentage of carbon; both burn to CO₂.
- Diamond: rigid 3-D network, each C–C bond the same length (154 pm) in every direction; very hard; no free electrons, so an insulator.
- Graphite: each carbon bonded to three others in the same plane, forming a hexagonal array; layers held by weak forces, so soft and slippery; one free electron per carbon, so a conductor.
| Property | Diamond | Graphite |
|---|---|---|
| Structure | 3-D tetrahedral network | Flat hexagonal layers |
| Bonds per carbon | 4 | 3, all in one planeQ CDS 2019 (II): in graphite each carbon is bonded to three others in the same plane, giving a hexagonal array. |
| Hardness | Hardest natural substance | Soft, slippery |
| Electrical conduction | Insulator | Good conductor |
| Percentage of carbon | 100% (pure) | 100% (pure) |
Practice this concept3 quick reps
The same idea in a real exam question:
CDS · 2023 · CDS (II) 2023 — General Knowledge · Q40Easy
Example 2 · Carbon and Its Compounds · Carbon and Its Allotropes
Which one of the following statements is correct about diamond and graphite ?
Same element, different properties
'Diamond and graphite have similar physical and chemical properties' is false for physical properties: one is hard and insulating, the other soft and conducting. Their chemistry is the same because both are pure carbon.
Diamond's bonds are all the same length
Every C–C bond in diamond is the same length in every direction. A statement that diamond has 'different carbon to carbon distance in all directions' is untrue.
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)
The allotropes of carbon5 rows
| Form | Bonds per carbon | Conducts electricity? |
|---|---|---|
| Diamond | 4 (3-D network) | No |
| Graphite | 3 (flat layers) | Yes |
| Fullerene (C₆₀) | 3 (closed cage) | No |
| Graphene | 3 (one layer) | Yes, very wellQ CDS 2025 (II): 'a wide band-gap semiconductor' is the property graphene does NOT have. |
| Coal | Not an allotrope: a mixture | Not a single form of carbonQ CDS 2016 (II): coal is the one that is not an allotrope of carbon. |
Diamond and graphite compared5 rows
| Property | Diamond | Graphite |
|---|---|---|
| Structure | 3-D tetrahedral network | Flat hexagonal layers |
| Bonds per carbon | 4 | 3, all in one planeQ CDS 2019 (II): in graphite each carbon is bonded to three others in the same plane, giving a hexagonal array. |
| Hardness | Hardest natural substance | Soft, slippery |
| Electrical conduction | Insulator | Good conductor |
| Percentage of carbon | 100% (pure) | 100% (pure) |
Watch out for (4)
- The coal answer: a carbon-rich rock is not an allotrope→ The allotropes of carbon
- Graphene has no band gap→ The allotropes of carbon
- Same element, different properties→ Diamond and graphite compared
- Diamond's bonds are all the same length→ Diamond and graphite compared
Test yourself on Carbon and Its Compounds
10 past CDS questions from this chapter, timed at 10 minutes and marked the way the exam marks it. You see your score and every answer the moment you finish. Free to start.