The thermal and photochemical stereochemical reactivity of norbornene derivatives (Diels-Alder, retro-Diels-Alder, Cope rearrangement) can be rationalized through the Bent Bond /Antiperiplanar Hypothesis (BBAH) orbital model at the molecular level. This new orbital model considers unsaturations as Pauling τ bonds and electronic delocalization is expressed through pyramidal radical resonance structures interacting in antiperiplanar fashion with the antibonding orbital of τ bonds.