Draw a molecular orbital energy-level diagram for the gaseous heteronuclear diatomic molecule boron nitride, BN. How does it differ from that for C2?
The Correct Answer and Explanation is:
✅ Molecular Orbital (MO) Energy-Level Diagram for BN (Boron Nitride)
Molecular Formula: BN
- Boron (B): Atomic number 5
- Nitrogen (N): Atomic number 7
- Total electrons in BN: 5 (B) + 7 (N) = 12 electrons
🧬 MO Energy-Level Diagram for BN:
Because BN is a heteronuclear diatomic molecule, the energy levels of atomic orbitals from B and N differ (N is more electronegative, so its atomic orbitals are lower in energy). The MO diagram is based on the same general pattern as for B₂, C₂, and N₂, with some asymmetry due to different atomic orbital energies.
BN MO Diagram:
(Molecular orbitals filled with 12 electrons)
- σ(1s) – 2 electrons
- σ(1s)* – 2 electrons
- σ(2s) – 2 electrons
- σ(2s)* – 2 electrons
- π(2p_x) = π(2p_y) – 4 electrons
- No electrons in π(2p) or σ(2p_z)*
Therefore, the electron configuration is:
(σ1s)² (σ1s)² (σ2s)² (σ2s)² (π2p_x = π2p_y)⁴
🔁 How BN’s MO Diagram Differs from C₂
C₂ (Carbon Dimer):
- Each carbon atom contributes 6 electrons, so C₂ has 12 electrons—same as BN.
- However, because C₂ is homonuclear, the energy levels of the atomic orbitals are the same.
- In C₂, due to the mixing of s and p orbitals (sp mixing), the order of 2p MOs is:
π(2p_x) = π(2p_y) < σ(2p_z)
BN Difference:
- BN is heteronuclear → no perfect orbital symmetry.
- Nitrogen’s atomic orbitals are lower in energy due to higher electronegativity.
- Energy asymmetry leads to polar covalent bonding.
- The MO diagram is asymmetric, with more contribution from nitrogen to lower-energy orbitals.
- Bonding molecular orbitals are closer to nitrogen, antibonding closer to boron.
- This results in bond polarity, unlike the nonpolar C₂ molecule.
🧪 Summary:
- BN and C₂ have the same number of electrons (12), so they fill orbitals similarly.
- However, BN’s MOs are asymmetric due to differing atom electronegativities.
- This causes unequal sharing of electrons and partial ionic character in BN, unlike the symmetrical and purely covalent C₂.
🧮 Bond Order for BN = (8 bonding – 4 antibonding) ÷ 2 = 2
✅ Final Answer:
BN’s MO diagram resembles that of C₂ in orbital filling but differs due to the asymmetric energy levels caused by different electronegativities.
