Bonding in Some Homonuclear Diatomic Molecules

Full Notes

This section applies the principles of molecular orbital theory (MOT) to real homonuclear diatomic species.

It shows how the electronic configurations, bond orders, and magnetic properties of these molecules can be deduced using the energy-level diagrams of molecular orbitals. The method provides a quantum mechanical basis for explaining molecular stability and behaviour.

Molecular Orbital Configurations and Bond Orders

Here are several examples of key homonuclear diatomic molecules:

Hydrogen Molecule (H₂)

• Atomic Number: 1 per H atom → 2 electrons total
• Configuration: σ(1s)²
• Bond order = 1/2 (2 − 0) = 1
• Exists as a stable molecule

Helium Molecule (He₂)

• Atomic Number: 2 per He atom → 4 electrons total
• Configuration: σ(1s)² σ*(1s)²
• Bond order = 1/2 (2 − 2) = 0
• Bond order is zero → Does not exist as a stable molecule
• Note: He₂⁺ does exist with a bond order of 1/2.

Lithium Molecule (Li₂)

• Atomic Number: 3 → Total electrons = 6
• Configuration: σ(1s)² σ*(1s)² σ(2s)²
• Bond order = 1/2 (4 − 2) = 1
• Exists and is stable

Carbon Molecule (C₂)

• Atomic Number: 6 → Total electrons = 12
• Configuration: σ(1s)² σ*(1s)² σ(2s)² σ*(2s)² π(2p_y)² π(2p_z)²
• Bond order = 1/2 (8 − 4) = 2
• All electrons are paired → Diamagnetic

Oxygen Molecule (O₂)

• Atomic Number: 8 → Total electrons = 16
• Configuration (without s–p mixing): σ(1s)² σ*(1s)² σ(2s)² σ*(2s)² σ(2p_x)² π(2p_y)² π(2p_z)² π*(2p_y)¹ π*(2p_z)¹
• Bond order = 1/2 (10 − 6) = 2
• Paramagnetic due to two unpaired electrons in π* orbitals
• This is consistent with experimental data (O₂ is attracted to a magnetic field)

Summary

• MO configurations predict bond order, stability, and magnetism for homonuclear diatomics.
• H₂ and Li₂ have bond order 1 and are stable.
• He₂ has bond order 0 and does not exist as a stable molecule.
• C₂ has bond order 2 and is diamagnetic.
• O₂ has bond order 2 and is paramagnetic due to two unpaired electrons.