Allotropes of Carbon — Revision Notes

⚡ 30-Second Revision

Allotropy — Same element, different forms.
Diamond — $sp^3$ hybridization, tetrahedral, 3D network, insulator, hardest, high density.
Graphite — $sp^2$ hybridization, trigonal planar, 2D layers, conductor, soft, slippery, lower density, thermodynamically more stable.
Fullerenes (C60) — $sp^2$ hybridization, cage-like molecules, soluble in organic solvents.
Graphene — Single layer of $sp^2$ carbon, strongest, best conductor.
Amorphous — Charcoal, coke, lamp black (no definite structure).
Key Distinction — Diamond (localized e-) vs. Graphite (delocalized e-).
Stability — Graphite > Diamond at STP.

2-Minute Revision

Carbon exhibits allotropy due to its ability to form different hybridization states ($sp^3$, $sp^2$) and extensive catenation. The main crystalline allotropes are diamond, graphite, fullerenes, graphene, and carbon nanotubes.

Diamond features $sp^3$ hybridized carbon atoms in a rigid 3D tetrahedral network. This structure makes it the hardest natural substance and an electrical insulator, as all valence electrons are localized in strong covalent bonds. It has a high density and melting point.

Graphite consists of $sp^2$ hybridized carbon atoms arranged in planar hexagonal layers. These layers are held by weak van der Waals forces, allowing them to slide, making graphite soft and slippery. The presence of delocalized pi electrons within each layer makes it an excellent electrical conductor. Graphite is thermodynamically more stable than diamond at standard conditions.

Fullerenes, like C60, are discrete molecular allotropes with $sp^2$ hybridized carbon atoms forming cage-like structures (pentagons and hexagons). Unlike diamond and graphite, they are soluble in organic solvents. Graphene is a single layer of graphite, known for extreme strength and conductivity. Amorphous forms like charcoal and coke lack long-range order and have specific industrial uses.

5-Minute Revision

Allotropy in carbon is a fascinating phenomenon where the same element exists in various structural forms, leading to vastly different properties. This is primarily driven by carbon's tetravalency, its strong catenation ability, and its capacity for $sp^3$ and $sp^2$ hybridization.

Crystalline Allotropes:

1
Diamond — Each carbon atom is $sp^3$ hybridized, forming four strong covalent bonds in a tetrahedral fashion, creating a vast 3D network. This structure imparts extreme hardness (the hardest natural substance), high density ($3.51,\text{g/cm}^3$), high melting point, and makes it an electrical insulator (due to localized electrons). Uses include cutting tools and jewelry.
2
Graphite — Carbon atoms are $sp^2$ hybridized, forming three covalent bonds within a plane, creating hexagonal rings arranged in layers. The fourth valence electron forms a delocalized pi electron cloud across the layer, making graphite an excellent electrical conductor. Weak van der Waals forces hold the layers together, making it soft, slippery, and useful as a lubricant and in pencil leads. It has a lower density ($2.25,\text{g/cm}^3$) and is thermodynamically more stable than diamond at standard conditions.
3
Fullerenes (e.g., C60) — These are discrete molecular allotropes, typically $sp^2$ hybridized, forming cage-like structures (e.g., 12 pentagons and 20 hexagons in C60). Their molecular nature allows them to be soluble in organic solvents, unlike diamond or graphite. They exhibit semiconducting or superconducting properties when doped.
4
Graphene — A single, one-atom-thick layer of $sp^2$ hybridized carbon atoms in a hexagonal lattice. It's renowned for its exceptional strength, flexibility, and superior electrical and thermal conductivity.
5
Carbon Nanotubes — Rolled-up sheets of graphene, possessing high tensile strength and conductivity.

Amorphous Allotropes: These lack a definite crystalline structure and include charcoal (from wood), coke (from coal, used as a reducing agent in metallurgy), and lamp black (from burning hydrocarbons, used as pigment). They often contain microcrystalline graphite-like regions.

Key Differences for NEET: Focus on the hybridization ($sp^3$ vs. $sp^2$), the resulting geometry (tetrahedral vs. trigonal planar), the nature of bonding (localized vs. delocalized electrons), and how these factors influence properties like hardness, electrical conductivity, and solubility. Remember that graphite is thermodynamically more stable than diamond at STP, and fullerenes are molecular, not network solids.

Prelims Revision Notes

Allotropes of Carbon: NEET Quick Recall

1. Definition of Allotropy:

Existence of an element in two or more forms in the same physical state.
Different atomic arrangements lead to different properties.
Carbon exhibits allotropy due to tetravalency, catenation, and variable hybridization ($sp^3, sp^2$).

2. Crystalline Allotropes:

* Diamond: * Hybridization: $sp^3$ * Structure: Each C atom bonded to 4 other C atoms in a tetrahedral, 3D giant covalent network. * Bond length: $1.54,\text{Å}$ * Properties: Hardest natural substance, high melting point ($>3500^circ\text{C}$), high density ($3.

51, ext{g/cm}^3$), electrical insulator (localized electrons), good thermal conductor, transparent. * **Uses:** Cutting tools, abrasives, jewelry. * **Graphite:** * **Hybridization:**$sp^2$ * Structure: Each C atom bonded to 3 other C atoms in a trigonal planar, hexagonal 2D layer.

Layers held by weak van der Waals forces. * Bond length: C-C within layer $1.42,\text{Å}$, interlayer $3.40,\text{Å}$. * Properties: Soft, slippery (layers slide), good electrical conductor (delocalized $pi$ electrons), good thermal conductor (along layers), opaque, lower density ($2.

25, ext{g/cm}^3$). * **Stability:** Thermodynamically more stable than diamond at STP. * **Uses:** Lubricants, electrodes, pencil leads, nuclear moderator. * **Fullerenes (e.g., C60 - Buckminsterfullerene):** * **Hybridization:**$sp^2$ * Structure: Discrete cage-like molecules (e.

g., C60 has 12 pentagons and 20 hexagons, like a soccer ball). * Properties: Molecular solid, soluble in organic solvents, semiconductors/superconductors (when doped). * Uses: Drug delivery, catalysts, electronics.

* Graphene: * Hybridization: $sp^2$ * Structure: Single layer of graphite, 2D hexagonal lattice. * Properties: Strongest material, excellent electrical and thermal conductor, transparent, flexible.

* Uses: Future electronics, composites. * Carbon Nanotubes (CNTs): * Hybridization: $sp^2$ * Structure: Rolled-up sheets of graphene (single or multi-walled). * Properties: High tensile strength, high conductivity.

3. Amorphous Allotropes:

Lack definite crystalline structure (microcrystalline graphite-like regions).
Examples: — Charcoal (from wood, adsorbent), Coke (from coal, reducing agent in metallurgy), Lamp black (from hydrocarbons, pigment), Carbon black (filler in rubber).

4. Key Distinctions for NEET:

Hybridization: — Diamond ($sp^3$), Graphite/Fullerenes/Graphene ($sp^2$).
Conductivity: — Diamond (insulator), Graphite (conductor).
Hardness: — Diamond (hardest), Graphite (softest).
Stability (STP): — Graphite > Diamond.
Solubility: — Fullerenes (soluble), Diamond/Graphite (insoluble).

Vyyuha Quick Recall

Diamond Has Insulating Tetrahedral Structure ($sp^3$). Graphite Conducts Softly in Layers ($sp^2$). Fullerenes Make Spheres Soluble ($sp^2$).

(D - Diamond, H - Hard, I - Insulator, T - Tetrahedral, S - sp3) (G - Graphite, C - Conductor, S - Soft, L - Layers, S - sp2) (F - Fullerenes, M - Molecular, S - Spheres, S - Soluble, S - sp2)