Molecular Orbitals from 2p Atomic Orbitals The 2pz orbitals overlap in head-to-head fashion, so these bonds are... s bonds. -- the corresponding MOs are: s2p and s*2p z x y The other 2p orbitals (i.e., 2px and 2py) overlap in sideways fashion, so the bonds are... p bonds. -- the corresponding MOs are: p2p (two of these) and p*2p (two of these)

Rule 3 above suggests that, from low energy to high, the 2p MOs SHOULD follow the order: LOW ENERGY HIGH ENERGY s2p < p2p < p*2p < s*2p General energy-level diagrams for MOs of second-row homonuclear diatomic molecules... don’t fit on this slide. (And so, they’re on the next one… …if that’s all right with you.) (And if not, you can… …quit and go make pancakes.)

s*2p s*2p “Mr. B” p*2p p*2p s2p p2p p2p s2p same for both s*2s s*2s For B2, C2, and N2... For O2, F2, and “Ne2”... s*2p s*2p “Mr. B” p*2p p*2p (or Mr. C) s2p p2p (or Mr. N) p2p s2p same for both s*2s s*2s s2s s2s (1s MOs are down here) Here, the interaction between the 2s of one atom and the 2p of the other is strong. The orbital energy distribution is altered. Here, the interaction is weak. The energy distribution is as expected.

paramagnetism of liquid oxygen paramagnetism: describes the attraction of molecules with unpaired e– to a magnetic field diamagnetism: describes substances with no unpaired e– (“di-” = two; diamagnetic = “dielectron”) ~ -- such substances are VERY weakly (almost unnoticeably) repelled by a magnetic field Use the energy diagrams above to tell if diatomic species are paramagnetic or diamagnetic. paramagnetism of liquid oxygen

Paramagnetic or diamagnetic? s2s s*2s p2p p*2p s2p s*2p s1s s*1s B2 C2 N2 O2 F2 O2+ O22– C22– (10) (12) (14) (16) (18) (15) P D