Diamond: Preparation, Properties, Structure, Bonding and Uses
NEET • JEE Main • JEE Advanced • GATE • CSIR-NET • IIT-JAM • State PSC
1. Occurrence & Natural Sources
• Found in kimberlite pipes (South Africa, India – Panna, Golconda historically).
• India was the only source till 18th century (Golconda mines).
• India was the only source till 18th century (Golconda mines).
2. Preparation of Diamond
| Method | Description | Key Points (Exam Favourite) |
|---|---|---|
| HPHT Method (High Pressure High Temperature) |
• Graphite + metal solvent (Fe/Ni) subjected to 5–6 GPa & 1500–2000°C • Most common industrial method |
▪ Pressure: ~55,000 atm ▪ Temperature: ~1500°C ▪ Frequently asked in JEE & GATE |
| CVD Method (Chemical Vapour Deposition) |
• Hydrocarbon gas (CH₄ + H₂) decomposed on substrate at 800–1000°C & low pressure • Produces ultra-pure gem-quality diamonds |
▪ Modern method for lab-grown diamonds ▪ NEET 2024 asked about CVD |
| Historical (Old) | Moissan tried heating charcoal + iron → dissolved in acid → small crystals (actually SiC) | Not real diamond |
Most asked: “Synthetic diamonds are manufactured by HPHT and CVD methods.”
3. Structure of Diamond
- 3D giant covalent network structure
- Each carbon is sp³ hybridized
- Tetrahedral arrangement → bond angle = 109°28'
- Each carbon covalently bonded to 4 other carbons → coordination number = 4
- C–C bond length = 1.54 Å
- Crystal system: Cubic (face-centered cubic with basis)
- 8 carbon atoms per unit cell (4 from fcc + 4 in tetrahedral voids)
- Similar to zinc blende (ZnS) structure
JEE Advanced favourite: Diamond has FCC lattice with 50% tetrahedral voids occupied.
Figure 1: Structure of Diamond showing each carbon atom linked with other four carbon atom.
4. Bonding in Diamond
- Only strong σ-covalent bonds (no π-bonds)
- All 4 valence electrons involved in bonding → no free electrons
- No weak forces → extremely rigid 3D network
- Band gap ≈ 5.5 eV → insulator
5. Physical Properties of Diamond
| Property | Value / Nature | Reason (Must Know for Exams) |
|---|---|---|
| Appearance | Colourless, transparent, lustrous | High refractive index |
| Density | 3.51 g/cm³ | Compact tetrahedral packing |
| Melting point | ~3550°C (sublimes) | Strong 3D covalent network |
| Hardness | 10 on Mohs scale (hardest natural substance) | Rigid 3D bonding |
| Thermal conductivity | Highest known (~2000 W/m·K) | Strong bonds + light atoms |
| Electrical conductivity | Insulator | No mobile electrons |
| Refractive index | 2.42 | Strong covalent bonds → high RI → brilliance |
| Solubility | Insoluble in all solvents | Giant covalent structure |
NEET/JEE Repeated: Diamond has highest thermal conductivity but is electrical insulator.
6. Chemical Properties
- Highly inert due to strong bonding
- Burns in air/O₂ at 800–900°C → CO₂
- Reacts with fluorine at 700°C → CF₄
- Converted to graphite above 1500°C in absence of air (graphitisation)
- Thermodynamically less stable than graphite at RTP (ΔGf° = +2.9 kJ/mol)
Most asked: “Diamond is kinetically stable but thermodynamically unstable w.r.t. graphite.”
7. Uses of Diamond (Very High Weightage)
- Cutting tools, drills, dies – hardest substance
- Gemstones (jewellery) – high RI & dispersion
- Abrasives – polishing, grinding
- Heat sinks in electronics – highest thermal conductivity
- X-ray windows – transparent to X-rays
- Surgical knives, scalpels
- Semiconductors (boron-doped diamond) – high temperature resistant
- Anvils in high-pressure research
JEE Main 2024 asked: “Diamond is used in heat sinks due to its exceptionally high thermal conductivity.”
8. Quick Comparison: Diamond vs Graphite (Must for Exams)
| Property | Diamond | Graphite |
|---|---|---|
| Hybridisation | sp³ | sp² |
| Structure | 3D network | 2D layers |
| Coordination number | 4 | 3 |
| C–C bond length | 1.54 Å | 1.42 Å (in plane) |
| Density | 3.51 g/cm³ | 2.26 g/cm³ |
| Hardness | Hardest (10) | Soft (1–2) |
| Electrical conductivity | Insulator | Conductor (along layers) |
| Thermal conductivity | Very high | High (in plane) |
| Thermodynamic stability | Less stable | More stable |
Must Read Diamond MCQs asked in NEET ▪ JEE Main & Advanced ▪ JAM ▪ GATE ▪ CSIR-NET ▪ State PSC
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