Molecular Orbital Energy Levels

Q.
What is the sequence of molecular orbitals from lower to higher energies? How do you find the bonding order in a small diatomic molecule?
A.
The energy sequence is:
σ_1s2, σ_1s*2, σ_2s2, σ_2s*2, π_2py2, π_2pz2, σ_2p2, π_2py*2, π_2pz*2, σ_2p*2
See diagram in
Molecular Orbital Theory
To find the bonding order:
1. Place all the electrons in the diatomic molecule in orbitals following the above sequence (maximum of 2 per orbital)
2. Count the total number of electrons in NONstar molecular orbitals.
3. Count the total number of electrons in star orbitals.
Bonding order = (nonstar total - star total) / 2