# How can bonding orbitals in diatomic oxygen be described?

Jan 25, 2014

The bonding orbitals in dioxygen are σ, σ^★, π, and π^★ molecular orbitals (this editor will not allow me to use an asterisk symbol, so I will use a star instead).

Each O atom has the electron configuration [He] 2s² 2px² 2py 2pz.

When two O atoms approach along the z-axis, the atomic orbitals overlap to form molecular orbitals.

1s + 1s → σ(1s) and σ^★(1s); 2s + 2s → σ(2s) and σ^★(2s)
2pz + 2pz → σ(2pz) and σ^★(2pz)
2px + 2px → π(2px) and π^★(2px); 2py + 2py → π(2py) and π^★(2py)

The order of energies is

σ(2s) < σ^★(2s) < σ(2pz) < π(2px) = π(2py) < π^★(2px) = π^★(2py) < σ^★(2pz).

In the following diagram, the atomic orbitals of the O atoms are on either side, and the molecular orbitals of the O₂ molecule are in the middle. We place the valence electrons in the atomic orbitals and the molecular orbitals and get the diagram below. We write the electron configuration of O₂ as
σ(1s)² σ^★(1s)² σ(2s)² σ^★(2s)² σ(2pz)² π(2px)² π(2py)² π^★(2px) π^★(2py).