Chemistry of Diamond PYQs MCQs

Diamond MCQs
NEET, IIT-JEE, IIT-JAM, GATE, CSIR-NET

NEET 2024–2025 Most Expected

Q1: The hardest naturally occurring substance is:

1. Graphite
2. Diamond
3. Fullerene
4. Silicon carbide (Carborundum)

Correct Answer: B (Diamond)

Detailed Explanation:
• Diamond has Mohs hardness = 10 (maximum on scale).
• Reason: Each carbon is sp³ hybridized and covalently bonded to 4 other carbons forming a rigid 3D tetrahedral network.
• No weak van der Waals layers (unlike graphite).
• Silicon carbide (9.5) is very hard but not harder than diamond.

JEE Main Repeated

Q2: Hybridisation of carbon in diamond is:

1. sp
2. sp²
3. sp³
4. sp³d

Correct Answer: C (sp³)

Detailed Explanation:
• In diamond, each carbon forms 4 equivalent σ-bonds → tetrahedral geometry → sp³ hybridisation.
• Graphite → sp² (3 σ + 1 π), Fullerene → nearly sp².

JEE Advanced Classic

Q3: Each carbon atom in diamond is covalently bonded to how many other carbon atoms?

1. 2
2. 3
3. 4
4. 6

Correct Answer: C (4)

Detailed Explanation:
• sp³ hybridisation → 4 σ-bonds per carbon.
• Coordination number = 4 → forms giant covalent 3D network.
• Compare: Graphite → coordination number 3.

NEET Favourite

Q4: Diamond is a bad conductor of electricity because:

1. All valence electrons are used in covalent bonding
2. It has delocalised π-electrons
3. It has layered structure
4. It is soft

Correct Answer: A

Detailed Explanation:
• All 4 valence electrons of carbon are involved in strong σ-bonding.
• No free/mobile electrons or delocalised π-cloud → insulator.
• Graphite has delocalised π-electrons → conductor.

JEE Advanced 2023 Pattern

Q5: C–C bond length in diamond is:

1. 1.42 Å
2. 1.54 Å
3. 1.34 Å
4. 3.35 Å

Correct Answer: B (1.54 Å)

Detailed Explanation:
• Pure single bond (σ-bond) in sp³ carbon → 1.54 Å (same as in alkanes).
• Graphite (in-plane) → 1.42 Å (partial double bond character due to resonance).

GATE / CSIR-NET

Q6: Density of diamond is approximately:

1. 2.26 g/cm³
2. 3.51 g/cm³
3. 1.70 g/cm³
4. 3.00 g/cm³

Correct Answer: B (3.51 g/cm³)

Detailed Explanation:
• Diamond has very compact tetrahedral packing → higher density than graphite (2.26 g/cm³).
• More atoms per unit volume due to 3D network.

NEET 2025 Hot

Q7: Diamond is thermodynamically __________ stable than graphite at ordinary conditions.

1. More
2. Less
3. Equally
4. Cannot predict

Correct Answer: B (Less stable)

Detailed Explanation:
• Standard free energy of formation (ΔGf°) of graphite = 0 (most stable allotrope).
• ΔGf° of diamond = +2.9 kJ/mol → metastable.
• Diamond → Graphite is spontaneous but extremely slow (kinetic stability).

JEE Advanced Numerical Value

Q8: Coordination number of carbon in diamond structure is:

Correct Answer: 4

Detailed Explanation:
• Each carbon is surrounded by 4 other carbons in tetrahedral geometry.
• Same as ZnS (zinc blende) structure.

NEET Classic

Q9: Diamond has very high refractive index (2.42) due to:

1. Small covalent bond length and strong bonds
2. Layered structure
3. Free electrons
4. Ionic nature

Correct Answer: A

Detailed Explanation:
• Short C–C bonds + high electron density → strong interaction with light → high RI → brilliance & fire in gems.

JEE Advanced 2024 Expected

Q10: Which allotrope of carbon has the highest thermal conductivity at room temperature?

1. Graphite
2. Diamond
3. Fullerene
4. Amorphous carbon

Correct Answer: B (Diamond)

Detailed Explanation:
• Diamond: ~2000–2200 W/m·K (highest among all materials at RT).
• Reason: Strong, rigid covalent bonds + light carbon atoms → efficient phonon transfer.
• Graphite (in-plane): ~100–400 W/m·K.

JEE Advanced

Q11: Bond angle in diamond is:

1. 109°28'
2. 120°
3. 180°
4. 90°

Correct Answer: A (109°28')

Perfect tetrahedral angle due to sp³ hybridisation.

CSIR-NET

Q12: Diamond crystallises in:

1. Hexagonal
2. Face-centred cubic lattice with basis
3. Simple cubic
4. Body-centred cubic

Correct Answer: B

FCC lattice with 2 carbon atoms per unit cell (zinc blende type structure).

NEET

Q13: Synthetic diamonds are produced by:

1. HPHT method
2. CVD method
3. Both
4. None

Correct Answer: C

• HPHT (High Pressure High Temperature) → industrial diamonds.
• CVD (Chemical Vapour Deposition) → gem-quality & ultra-pure diamonds.

JEE Advanced

Q14: Pure diamond is transparent to:

1. X-rays
2. UV rays
3. Visible light
4. All of these

Correct Answer: A

Diamond is used in X-ray windows because X-rays pass through it easily (low atomic number).

GATE

Q15: Mohs hardness of diamond is:

1. 8
2. 9
3. 10
4. 7

Correct Answer: C (10)

Highest on Mohs scale → only diamond can scratch diamond.

NEET 2024 Repeated

Q16: Diamond is an example of:

1. Ionic solid
2. Molecular solid
3. Covalent (network) solid
4. Metallic solid

Correct Answer: C (Covalent network solid)

Detailed Explanation:
• Giant 3D covalent network extending throughout the crystal.
• No discrete molecules → extremely high melting point, insoluble, very hard.
• Examples: Diamond, SiO₂ (quartz), SiC.

JEE Main 2025 Expected

Q17: Diamond is used as a gemstone because of:

1. High refractive index and dispersion
2. High density
3. Electrical conductivity
4. Softness

Correct Answer: A

Detailed Explanation:
• Refractive index = 2.42 → total internal reflection → brilliance.
• High dispersion ("fire") → splits white light into colours.
• Proper cutting (57 facets) maximises these optical properties.

JEE Advanced 2023 Type

Q18: In diamond, carbon atoms occupy:

1. All octahedral voids of fcc lattice
2. 50% tetrahedral voids of fcc lattice
3. All tetrahedral voids
4. Body-centred positions

Correct Answer: B

Detailed Explanation:
• Diamond structure = fcc lattice of carbon atoms + another 4 carbon atoms in alternate tetrahedral voids.
• Total 8 carbon atoms per unit cell (4 from fcc + 4 from tetrahedral voids).
• Exactly similar to ZnS (zinc blende) structure.

NEET Classic

Q19: Which of the following has the highest melting point?

1. Graphite
2. Diamond
3. Fullerene (C₆₀)
4. Charcoal

Correct Answer: B (Diamond)

Detailed Explanation:
• Diamond sublimes at ~3550–4000°C.
• Graphite sublimes at ~3700°C, but diamond has stronger 3D bonding.
• Fullerene melts at ~600°C only.

GATE / CSIR-NET

Q20: Diamond turns black when strongly heated in air because:

1. It burns to CO₂
2. It converts to graphite
3. It absorbs moisture
4. It sublimes

Correct Answer: B

Detailed Explanation:
• Above 1500°C in air, diamond surface converts to graphite (graphitisation).
• Graphite is black → diamond appears black on surface.
• Pure combustion to CO₂ occurs only above 800°C in excess oxygen.

JEE Advanced Previous Year

Q21: Diamond is a bad conductor but becomes good conductor when doped with boron because:

1. It becomes p-type semiconductor
2. It becomes n-type semiconductor
3. Metallic bonding is introduced
4. It becomes ionic

Correct Answer: A

Detailed Explanation:
• Boron has 3 valence electrons → creates "holes" → p-type semiconductor.
• Used in high-temperature electronics (diamond semiconductors).

NEET 2025 Expected

Q22: The reason diamond is used in cutting tools and drills is:

1. Extreme hardness and high thermal conductivity
2. Electrical conductivity
3. Low density
4. Softness

Correct Answer: A

Detailed Explanation:
• Hardness → cuts any material.
• High thermal conductivity (~2000 W/m·K) → dissipates heat quickly → prevents tool from melting/softening.

JEE Main 2024 Shift

Q23: Number of nearest neighbours around each carbon atom in diamond is:

1. 3
2. 4
3. 6
4. 8

Correct Answer: B (4)

Tetrahedral arrangement → each carbon has exactly 4 nearest neighbours.

CSIR-NET

Q24: Lonsdaleite is:

1. Cubic diamond
2. Hexagonal diamond
3. Amorphous diamond
4. Graphite

Correct Answer: B

Detailed Explanation:
• Rare hexagonal polymorph of diamond (wurtzite-like structure).
• Formed in meteorite impacts → sometimes harder than cubic diamond.

JEE Advanced Final

Q25: Diamond does NOT conduct electricity in pure form because:

1. All valence electrons are localised in covalent bonds
2. It has free electrons like metals
3. It has mobile π-electrons
4. It has ionic lattice

Correct Answer: A

Detailed Explanation:
• All four valence electrons are used in strong σ-bonds → no mobile charge carriers.
• Band gap ~5.5 eV → perfect insulator.
• Graphite has one delocalised π-electron per carbon → conductor.

Must Read Chemistry of Diamond