Final answer:
The order of σg(2pz) and πu(2px/2py) is reversed due to s-p mixing, which stabilizes the σg(2pz) molecular orbital, making it lower in energy than the πu(2px/2py) orbitals in certain diatomic molecules.
Step-by-step explanation:
The question you're asking pertains to molecular orbital theory in Chemistry and why the order of σg(2pz) and πu(2px/2py) molecular orbitals is reversed in certain cases. This reversal happens due to a phenomenon known as s-p mixing. S-p mixing is the interaction between s orbitals and p orbitals, it changes the energy levels of molecular orbitals without creating new ones. In molecules to the left of nitrogen on the periodic table, for example in O₂, Be₂, or B₂, the bonding interaction between s and p orbitals is significant when the atomic 2s and 2p orbitals are close in energy. This interaction leads to a stabilization of the σg(2pz) molecular orbital, making it lower in energy than the πu(2px/2py) orbitals. As a consequence, the σg(2pz) appears below the πu(2px/2py) in the molecular orbital energy level diagram for these molecules.