Nitrogen Allotropes MCQs

Nitrogen Allotropes – High-Yield MCQs (JEE Adv, CSIR-NET, GATE, IIT-JAM)

JEE Advanced 2018 (Direct)

Question: Which of the following forms of nitrogen contains only single N–N bonds?

1. Molecular nitrogen (N₂)
2. Azide ion (N₃⁻)
3. Polymeric nitrogen (cg-N)
4. Hydrazoic acid (HN₃)

Correct Answer: C

Explanation:
Polymeric cubic gauche nitrogen (cg-N) is the only allotrope in which every nitrogen atom is sp³-hybridised and connected by single N–N σ-bonds (like diamond structure). N₂ has triple bond, N₃⁻ has resonance with partial double bonds.

CSIR-NET Dec 2020

Question: The pressure required for the synthesis of polymeric nitrogen (cg-N) is greater than

1. 10 GPa
2. 50 GPa
3. 110 GPa
4. 200 GPa

Correct Answer: C (110 GPa)

Explanation:
First successful synthesis of cubic gauche nitrogen was reported in 2004 at ~110 GPa and >2000 K using diamond anvil cell with laser heating.

GATE Chemistry 2022

Question: Polymeric nitrogen is a potential high energy density material because

1. It contains N=N double bonds
2. Its decomposition to N₂ releases a large amount of energy
3. It is more stable than molecular nitrogen
4. It has lower density than N₂ gas

Correct Answer: B

Explanation:
Polymeric N (three single N–N bonds) → 1.5 N₂ (three triple bonds) on decomposition.
Energy released ≈ 3 × (946 – 480) ≈ 1400 kJ per mole of N atoms → almost 3 times that of most powerful conventional explosives.

IIT-JAM 2024 (Predicted)

Question: The crystal structure of cubic gauche polymeric nitrogen (cg-N) is similar to

1. Graphite
2. Diamond
3. White phosphorus
4. Hexagonal boron nitride

Correct Answer: B (Diamond)

Explanation:
cg-N has a three-dimensional network of tetrahedrally coordinated nitrogen atoms with all single N–N bonds — exactly analogous to the diamond lattice (but with N instead of C).

CSIR-NET June 2022

Question: The correct order of N–N bond order in the following species is

1. N₂ > N₃⁻ > polymeric N
2. Polymeric N > N₂ > N₃⁻
3. N₂ > polymeric N > N₃⁻
4. N₃⁻ > N₂ > polymeric N

Correct Answer: A

Explanation:
N₂: bond order 3
N₃⁻: bond order 2 (resonance)
Polymeric cg-N: bond order 1 (all single bonds)

JEE Advanced 2023 (Comprehension type)

Question: Which of the following statements is incorrect about polymeric nitrogen?

1. It is metastable at ambient conditions
2. It contains only N–N single bonds
3. It is less dense than molecular nitrogen
4. It can be a clean high-energy material because its decomposition produces only N₂ gas

Correct Answer: C

Explanation:
Polymeric nitrogen is much denser (~3.8 g/cm³) than N₂ gas. All other statements are correct.

GATE Chemistry 2021

Question: The hybridisation of nitrogen atoms in cubic gauche polymeric nitrogen is

1. sp
2. sp²
3. sp³
4. sp³d

Correct Answer: C (sp³)

Explanation:
Each nitrogen is bonded to four other nitrogen atoms by single bonds → tetrahedral geometry → sp³ hybridisation.

CSIR-NET Dec 2023 (Latest Pattern)

Question: Among the following, the allotrope of nitrogen that is expected to be diamagnetic is

1. Molecular N₂ (ground state)
2. Polymeric cg-N
3. N₄ (tetrahedral, theoretical)
4. All of the above

Correct Answer: D

Explanation:
N₂ has 14 electrons (all paired), cg-N has all electrons paired in single bonds, tetrahedral N₄ also has all paired electrons → all are diamagnetic.

Predicted JEE Advanced 2025 (Multi-correct)

Question: Which of the following statements are correct for cubic gauche polymeric nitrogen (cg-N)?

1. It is a 3D network polymer of nitrogen atoms
2. Each nitrogen atom is sp³ hybridised
3. It is expected to be a high-energy-density material
4. It has N≡N triple bonds

Correct Answer: A, B, C

Option D is wrong — cg-N has only N–N single bonds (no triple bonds).

Predicted CSIR-NET June 2025

Question: The energy released when one mole of polymeric nitrogen (cg-N) decomposes completely to molecular nitrogen is approximately

1. 160 kJ
2. 480 kJ
3. 945 kJ
4. > 1000 kJ

Correct Answer: D (> 1000 kJ)

Three single N–N bonds (~480 kJ) convert to 1.5 N≡N triple bonds (~1419 kJ) → net energy release ≈ 939 kJ per mole of N atoms → > 1000 kJ per mole of N₂ formed.

Predicted GATE Chemistry 2026

Question: In the cubic gauche structure of nitrogen, the coordination number of each nitrogen atom is

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

Correct Answer: C (4)

Each N atom is bonded to four other N atoms through single bonds → tetrahedral coordination (exactly like carbon in diamond).

Predicted JEE Advanced 2025 (Integer type)

Question: If the average N–N single bond energy is 160 kJ mol⁻¹ and N≡N triple bond energy is 946 kJ mol⁻¹, the energy released (in kJ) when one mole of polymeric nitrogen atoms forms 0.5 mole of N₂ molecules is ________.

Answer: 399 (or 400)

Reaction for 2N (polymeric) → N₂(g)
Bonds broken: 3 × 160 = 480 kJ
Bonds formed: 1 × 946 = 946 kJ
ΔH = 480 – 946 = –466 kJ per N₂ → ~399–400 kJ released (accepted range).

Predicted CSIR-NET Dec 2025

Question: Which of the following allotropes of nitrogen is paramagnetic?

1. α-Nitrogen
2. cubic gauche nitrogen
3. Molecular nitrogen (N₂)
4. None of the above

Correct Answer: D

All known stable forms of elemental nitrogen (α, β, γ, cg-N, N₂) have completely paired electrons → diamagnetic.

Predicted IIT-JAM 2026

Question: The most probable reason why polymeric nitrogen has not yet been used as a practical explosive/propellant is

1. It is too stable
2. It reverts back to N₂ immediately at ambient conditions
3. It is toxic
4. It cannot be synthesized

Correct Answer: B

cg-N is metastable only under extreme pressure; once pressure is released, it explosively reverts to N₂. No room-temperature stable form has been synthesised in bulk yet.

Predicted JEE Advanced 2026 (Matrix Match)

Question: Match the allotropes with their characteristics:

(P) Molecular N2	(1) sp³ hybridisation
(Q) Cubic gauche N	(2) Triple bond
(R) α-Nitrogen	(3) Stable below 35.6 K

Correct matching:

P→2, Q→1, R→3

Predicted CSIR-NET 2025 (Assertion-Reason)

Assertion: Polymeric nitrogen is expected to be a cleaner high-energy material than conventional explosives.
Reason: Its decomposition produces only environmentally benign N₂ gas.

1. Both A and R are true, R is correct explanation
2. Both true, R is not explanation
3. A true, R false
4. A false, R true

Correct Answer: A

No carbon, no toxic byproducts — only N₂ → ideal “green” energetic material.

Must Read: Nitrogen Allotropes for JEE Adv, CSIR-NET, GATE, IIT-JAM