Allotropes of Nitrogen (N2)
High-Yield Topic for JEE Advanced, IIT-JAM, GATE, CSIR-NET (Repeated in 2018, 2020, 2022, 2024)
1. Known Allotropes of Nitrogen
| Allotrope | Name | Physical State | Stability |
|---|---|---|---|
| α-Nitrogen | Alpha Nitrogen | Colourless crystalline solid | Most stable below 35.6 K |
| β-Nitrogen | Beta Nitrogen | Colourless crystalline solid | Stable 35.6 K to 63.15 K |
| γ-Nitrogen | Gamma Nitrogen | Colourless crystalline solid | Exists under high pressure |
| Polymeric Nitrogen (cg-N) | Cubic Gauche Nitrogen | Black/Red solid | Metastable at RTP, highly energetic |
| Amorphous Nitrogen | Non-crystalline Nitrogen | Red-coloured film | Formed at very low temp |
2. Diatomic Nitrogen (N2) – The Ground State
- Most stable form at room temperature and pressure
- Bond order = 3 (triple bond: σ2p + π2pₓ + π2pᵧ)
- Bond dissociation energy = 946 kJ mol⁻¹ (highest among diatomic molecules except CO)
- Very inert due to high bond energy and non-polar nature
3. Polymeric Nitrogen (cg-N) – The Most Important Allotrope for Exams
CSIR-NET & JEE Advanced Favourite:
Single-bonded polymeric nitrogen (all N–N single bonds) is predicted to be more stable than molecular N2 at high pressure.
Single-bonded polymeric nitrogen (all N–N single bonds) is predicted to be more stable than molecular N2 at high pressure.
Key Features of Cubic Gauche Nitrogen (cg-N)
| Property | Value / Detail |
|---|---|
| Structure | Cubic gauche (similar to diamond structure but with N atoms) |
| Bonding | All single N–N bonds (sp³ hybridised) |
| Bond energy | ~ 160 kJ/mol per bond (but 3 bonds per N atom) |
| Energy content | Stores 3 times more energy than any known high-energy material |
| Density | ~ 3.5–4.0 g/cm³ (very dense) |
| Stability | Metastable at ambient conditions if synthesized |
| Synthesis pressure | > 110 GPa + laser heating (>2000 K) |
Why is polymeric N so energetic?
- N2 molecule → N≡N (bond energy 946 kJ/mol)
- Polymeric N → three N–N single bonds per nitrogen atom (~ 3 × 160 = 480 kJ/mol)
- On decomposition: polymeric N → 1½ N2 + huge energy release
- Energy released ≈ twice that of HMX (most powerful conventional explosive)
Cubic Gauche Nitrogen (cg-N) – Diamond-like network of N atoms
4. Other Theoretical/Experimental Polymeric Forms
- Hexagonal layered nitrogen (HLP-N)
- Chain-like polymeric nitrogen
- N6 hexagonal ring (benzvalene-like) – predicted but not stable
5. Comparison: Molecular N2 vs Polymeric Nitrogen
| Property | Molecular N2 | Polymeric cg-N |
|---|---|---|
| Bond type | Triple bond | Single bonds |
| Bond order | 3 | 1 |
| Stability | Stable at RTP | Metastable (needs high P) |
| Energy density | Low | Extremely high |
| Potential use | Inert gas | High-energy material, propellant |
6. Previous Year Exam Questions (Directly Asked)
| Exam & Year | Question Asked |
|---|---|
| JEE Advanced 2018 | Which allotrope of nitrogen has all single N–N bonds? |
| CSIR-NET Dec 2020 | Polymeric nitrogen is synthesized at pressure greater than ___ GPa |
| GATE Chemistry 2022 | Structure of cubic gauche nitrogen |
| IIT-JAM 2024 | Why polymeric nitrogen is a potential high-energy material |
| CSIR-NET June 2022 | Compare bond order in N2 and polymeric nitrogen |
One-Liner for Exam:
"The only known allotrope of nitrogen with all single N–N bonds is cubic gauche polymeric nitrogen (cg-N), synthesized above 110 GPa, and is a potential ultra-high-energy-density material because its decomposition to N2 releases enormous energy."
"The only known allotrope of nitrogen with all single N–N bonds is cubic gauche polymeric nitrogen (cg-N), synthesized above 110 GPa, and is a potential ultra-high-energy-density material because its decomposition to N2 releases enormous energy."
Must Read: Allotropes of Nitrogen MCQs asked in JEE Advanced, IIT-JAM, GATE, CSIR-NET